ALBERT

Description: Oxidative aging of soot particles in the atmosphere may contribute significantly to their climate and human health effects. With their large specific surface area, soot particles are an ideal substrate for heterogeneous reactions. However, these reactions are typically of limited relevance due to deactivation of the surface. Here we show that light in the UV-A and visible (Vis) range significantly enhances the reactivity of soot with ozone. In these laboratory experiments, soot from a propane burner was first “passivated” with ozone (ozone mixing ratio of 300 pbb) for 15 h before ozone uptake was measured in presence of UV radiation. Passivated soot was shown to be reactivated during irradiation with a 4 times higher steady state ozone uptake rate as compared to dark conditions. The uptake increased even by a factor of 15 when humidity (95% relative humidity) was introduced into the system. Possible mechanisms of ozone uptake are discussed. Further analysis using near edge X-ray absorption spectroscopy and contact angle measurements showed a decrease of oxygenated functional groups in the soot upon exposure to ozone and ozone in combination with light.

Description: The properties of equilibrated tropical convection are studied using a cloud-resolving model with large-scale dynamics parameterized by the weak temperature gradient (WTG) approximation. Model integrations are performed in both 2-D and 3-D geometries. The target profile toward which horizontal mean free tropospheric temperature is relaxed is held fixed, while sea surface temperature (SST) is varied. Consistent with previous studies, large-scale ascent and precipitation increase under WTG as the SST is increased, but more rapidly in two dimensions than in three dimensions. This is related to greater extremes of near-surface buoyancy in two dimensions as well as a lower gross moist stability, and perhaps also to weaker entrainment. In both two and three dimensions, the vertical profiles of large-scale vertical velocity are top heavy and remarkably self-similar in shape as SST is increased. When all integrations are analyzed together, precipitation increases with column-integrated relative humidity once the latter reaches a threshold, as in observations and other models. However, within each integration, the two quantities are correlated negatively, albeit over a very narrow range.

Description: Fossil carbon in particulate matter comes from anthropogenic use and combustion of fossil fuels, while nonfossil carbon may originate from both biogenic (e.g., pollen, plant debris, fungal spores, and biogenic secondary organic aerosol (SOA)) and anthropogenic sources (e.g., cooking and residential wood combustion). We investigated the relative contributions of fossil and nonfossil sources to fine carbonaceous aerosols in five European cities by radiocarbon analysis of aerosol samples collected at four types of sites in 2002–2004. The average fraction of nonfossil carbon was 43 ± 11%, with the lowest fraction, 36 ± 7%, at urban curbside sites and the highest fraction, 54 ± 11%, at rural background sites, farthest away from the impact of man-made emissions. Generally, fossil carbon concentrations at urban curbside sites are elevated in comparison to background sites, which is expected because of their proximity to vehicular emissions. Contrary to what might be expected, the concentration of nonfossil carbon is also higher at curbside than at background sites. This may be attributable to differences between site categories in levels of primary biological aerosols, brake and tire wear in resuspended road dust, biofuels, emissions from cooking and residential wood combustion, or processes such as anthropogenic enhancement of biogenic SOA and increased partitioning of semivolatile compounds into the aerosol phase at urban sites. The exact causes should be investigated by future detailed source analyses.

Description: The response to wave forcing of finite duration comprises a transient forerunner and the steady state signal (or simply the signal). It is the latter that carries information on the spectral content of the forcing, and the signal velocity is the velocity at which wave energy flows. To the extent that group velocity is a good measure of the energy flow velocity, the ray-tracing formalism is a valid description of signal propagation. We have examined vertical group velocities as a measure of vertical energy flow velocity for gravity and acoustic waves propagating into the dissipative lower thermosphere. We find that the effects of dissipation on gravity waves can cause group velocity to become a meaningless measure of the energy flow velocity. When certain terms originating in the diffusion of heat and momentum are neglected, the validity of group velocity can be extended to F region altitudes. For acoustic waves, group velocity can be a good measure of energy flow velocity throughout the lower thermosphere because acoustic waves are far less subject to dissipation.

Description: Cloud top and tropopause relationships are examined using cloud top observations from the Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) cloud data and National Centers for Environmental Prediction Global Forecast System (GFS) tropopause data. Statistical analyses of cloud top occurrence in tropopause and jet referenced relative altitude coordinates are performed on a global scale using 4 years (2006–2010) of CALIPSO 5 km resolution cloud layer data. The results show that the thermal tropopause appears to be a significant constraint for the cloud top. The zonal vertical distribution of cloud tops in tropopause-relative coordinates shows a maximum at the tropopause level for both the tropics (20°S–20°N) and midlatitudes (40°–60°S, 40°–60°N) for all four seasons. Occurrence of cloud tops above the tropopause is examined and quantified. The results show that with the consideration of tropopause height uncertainty, the data do not provide sufficient evidence of significant presence of cloud tops above the tropopause in the midlatitudes. In the tropics, the significant occurrences of cloud top above the thermal tropopause are found in regions known for seasonal deep convection. In most cases, the occurrence is up to 24% in 2° × 3° latitude-longitude bins with isolated higher frequencies in the western Pacific during the northern hemispheric winter season. The vertical distributions show that these events are mostly up to 2.5 km above the lapse rate tropopause, which is comparable to the differences between the lapse rate and the cold point tropopause in regions of active convection. We speculate that this separation may be responsible for a significant fraction of the cloud tops that do occur above the lapse rate tropopause in our analyses. It is also important to note that our results are limited by the CALIPSO twice-daily sampling with local equator crossing times of 0130 and 1330. The data therefore do not provide a good representation of convection over land, which is known to have maxima in afternoon local times. The tropopause determination is a significant component of this type of studies, and errors in the tropopause height may lead to significantly different conclusions. Our analyses show that the tropopause product from the GFS model is in better agreement with radiosonde measurements. The Goddard Earth Observing System Model Version 5 tropopause product, given as the ancillary data in the CALIPSO data file, shows a much larger uncertainty, primarily because the tropopause is identified at the model grid levels.

Description: High-resolution simulations from the Advanced Research Weather Research and Forecasting (ARW-WRF) model, coupled to an urban canopy model (UCM), are used to investigate impacts of soil moisture, sea surface temperature (SST), and city of Houston itself on the development of a stagnant wind event in the Houston-Galveston (HG) area on 30 August 2000. Surface and wind profiler observations are used to evaluate the performance of WRF-UCM. The model captures the observed nocturnal urban-heat-island intensity, diurnal rotation of surface winds, and the timing and vertical extent of sea breeze and its reversal in the boundary layer remarkably well. Using hourly SST slightly improves the WRF simulation of offshore wind and temperature. Model sensitivity tests demonstrate a delicate balance between the strength of sea breeze and prevailing offshore weak flow in determining the duration of the afternoon-evening stagnation in HG. When the morning offshore flow is weak (3–5 m s−1), variations (1°–3°C) in surface temperature caused by environmental conditions substantially modify the wind fields over HG. The existence of the city itself seems to favor stagnation. Extremely dry soils increase daytime surface temperature by about 2°C, produced more vigorous boundary layer and faster moving sea breeze, favoring stagnation during late afternoon. The simulation with dry soils produces a 3 h shorter duration stagnation in the afternoon and 4 h longer duration in the evening, which may lead to more severe nighttime air pollution. Hourly variations of SST in shallow water in the Galveston Bay substantially affect the low-level wind speed in HG.

Description: Direct and semidirect radiative effects of biomass burning aerosols from southern African fires during July–October are investigated using 20 year runs of the Community Atmospheric Model (CAM) coupled to a slab ocean model. Aerosol optical depth is constrained using observations in clear skies from Moderate Resolution Imaging Spectroradiometer (MODIS) and for aerosol layers above clouds from Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). Over the ocean, where the aerosol layers are primarily located above cloud, negative top of atmosphere (TOA) semidirect radiative effects associated with increased low cloud cover dominate over a weaker positive all-sky direct radiative effect (DRE). In contrast, over the land where the aerosols are often below or within cloud layers, reductions in cloud liquid water path (LWP) lead to a positive semidirect radiative effect that dominates over a near-zero DRE. Over the ocean, the cloud response can be understood as a response to increased lower tropospheric stability (LTS) which is caused both by radiative heating in overlying layers and surface cooling in response to direct aerosol forcing. The marine cloud changes are robust to changes in the cloud parameterization (removal of the hard-wired dependence of clouds on LTS), suggesting that they are physically realistic. Over land, decreased LWP is consistent with weaker convection driven by increased static stability. Over the entire region the overall TOA radiative effect from the biomass burning aerosols is almost zero due to opposing effects over the land and ocean. However, the surface forcing is strongly negative, which leads to a reduction in precipitation and also a reduction in sensible heat flux. The former is primarily realized through reductions in convective precipitation on both the southern and northern flanks of the convective precipitation region spanning the equatorial rain forest and the Intertropical Convergence Zone (ITCZ) in the southern Sahel. The changes are consistent with the low-level aerosol-forced cooling pattern. The results highlight the importance of semidirect radiative effects and precipitation responses for determining the climatic effects of aerosols in the African region.

Description: The role of the redistribution of tropical upper tropospheric humidity in the formation of tropical cirrus is studied using three years (June 2006 to December 2008) of observations with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) as well as the Microwave Limb Sounder (MLS) onboard the Aura satellite. The National Centers for Environmental Prediction and Atmospheric Research reanalysis data are also used. Results show that the redistribution of upper tropospheric humidity from a highly convective zone to the Indian peninsular region leads to the formation of the tropical cirrus. Advection of upper layer humidity is caused by the tropical easterly jet (TEJ) associated with the Asian summer monsoon (ASM). Thus the present analysis brings out, for the first time, the role of the TEJ in the redistribution of upper tropospheric humidity and consequently in the formation of tropical cirrus. As little observational evidence exists in support of the generative mechanisms of the cirrus, the present results can be useful in quantifying the formation process of these clouds, which have implications for Earth's radiation budget and improving global climate models.

Description: A combined lidar-photometer method that permits the retrieval of vertical profiles of ash and non-ash (fine-mode) particle mass concentrations is presented. By using a polarization lidar, the contributions of non-ash and ash particles to total particle backscattering and extinction are separated. Sun photometer measurements of the ratio of particle volume concentration to particle optical thickness (AOT) for fine and coarse mode are then used to convert the non-ash and ash extinction coefficients into respective fine-mode and ash particle mass concentrations. The method is applied to European Aerosol Research Lidar Network (EARLINET) and Aerosol Robotic Network (AERONET) Sun photometer observations of volcanic aerosol layers at Cabauw, Netherlands, and Hamburg, Munich, and Leipzig, Germany, after the strong eruptions of the Icelandic Eyjafjallajökull volcano in April and May 2010. A consistent picture in terms of photometer-derived fine- and coarse-mode AOTs and lidar-derived non-ash and ash extinction profiles is found. The good agreement between the fine- to coarse-mode AOT ratio and non-ash to ash AOT ratio ( 15 μm.

Description: There continue to be surprises in the identification of the terrestrial nightglow components. Comparison of the nightglow as measured in astronomical sky spectra and Optical Spectrograph and Infrared Imager System (OSIRIS)/Odin orbiter data with that on FeO emission in meteor trains and laboratory experiments demonstrates that this emission is an important part of the nightglow, appearing as a quasi-continuum between 540 and 680 nm, with maximum intensity at 595 nm. In order to study the FeO emission, account must be taken of other nearby emissions, including the 589 nm sodium emission lines, and the underlying 8−2 OH Meinel band. The temporal behavior of the three emissions is normally closely related, but strong excursions can occur, which could provide valuable information on atmospheric processes. The temporal behavior of the FeO and OH emissions was compared to model results from the one-dimensional and time-resolved model FeMOD, which describes the iron chemistry in the mesosphere/lower thermosphere. Further observations and comparisons with satellite and lidar data as well as models will make it possible to improve our understanding of mesospheric chemistry, meteor ablation, and the role of iron in the atmosphere.

Description: The role of cold season climate variability on lakes and wetlands in the Great Lakes region of the United States was examined over a period of 91 years (1917–2007) using the variable infiltration capacity (VIC) land surface hydrologic model. Statistically significant trends in observed cold season precipitation and air temperature indicated that both have significantly increased during the last 91 years. Results also showed that despite the significant increase in the cold season (December–May) precipitation, snowfall is significantly decreased during the period 1917–2007, suggesting the change in the distribution of the cold season precipitation. Both total runoff and evapotranspiration during the spring (March–May) season are increased; however, the trend associated with evapotranspiration was significant. These changes in cold season precipitation and temperature resulted in an increasing trend in domain-averaged fractional inundation extent during the spring season. The inundation extent of lakes and wetlands during the spring season showed sensitivity to periods of extreme climate. Driest years and those with the lowest snowfall resulted in the lowest inundation extents, while years with highest snowfalls resulted in the greatest inundation extents. Five year composites of extreme dry; wet; cold; warm; low snow; high snow; low snow, high temperature; and high snow, low temperature showed the mean domain average fractional inundation extent in spring to be 0.17, 0.22, 0.23, 0.20, 0.17, 0.24, 0.21, and 0.22, respectively. The fractional inundation extent in spring was significantly correlated with snowfall, the amount of snowmelt in the cold season, and the total runoff (surface runoff + base flow) in spring. Spring inundation extent was negatively correlated with the cold season air temperature, suggesting that higher air temperature could lead to lower inundation.

Description: This first paper of the two-part series describes the objectives of the community efforts in improving the Noah land surface model (LSM), documents, through mathematical formulations, the augmented conceptual realism in biophysical and hydrological processes, and introduces a framework for multiple options to parameterize selected processes (Noah-MP). The Noah-MP's performance is evaluated at various local sites using high temporal frequency data sets, and results show the advantages of using multiple optional schemes to interpret the differences in modeling simulations. The second paper focuses on ensemble evaluations with long-term regional (basin) and global scale data sets. The enhanced conceptual realism includes (1) the vegetation canopy energy balance, (2) the layered snowpack, (3) frozen soil and infiltration, (4) soil moisture-groundwater interaction and related runoff production, and (5) vegetation phenology. Sample local-scale validations are conducted over the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site, the W3 catchment of Sleepers River, Vermont, and a French snow observation site. Noah-MP shows apparent improvements in reproducing surface fluxes, skin temperature over dry periods, snow water equivalent (SWE), snow depth, and runoff over Noah LSM version 3.0. Noah-MP improves the SWE simulations due to more accurate simulations of the diurnal variations of the snow skin temperature, which is critical for computing available energy for melting. Noah-MP also improves the simulation of runoff peaks and timing by introducing a more permeable frozen soil and more accurate simulation of snowmelt. We also demonstrate that Noah-MP is an effective research tool by which modeling results for a given process can be interpreted through multiple optional parameterization schemes in the same model framework.

Description: This paper focuses on blowing snow and its effect, through thermodynamic forcing, on anticyclogenesis and cyclolysis. A triple-moment blowing snow model (PIEKTUK-T) is coupled to an atmospheric model (MC2), and this system is used to simulate an anticyclogenesis event. For comparison, an uncoupled version of MC2 is used to model the same event. The coupled model (CPL) showed colder low-level temperatures in regions where blowing snow occurred. This cooling contributes to a rise in sea level pressure relative to the uncoupled simulation. A potential vorticity (PV) diagnostic is then applied to quantify how this microphysical cooling affects the geopotential height and balanced wind fields. Surface potential temperature differences between the coupled and uncoupled runs were used as lower boundary conditions for the inversion. The results showed that blowing snow has only a small cooling effect over the anticyclogenesis region in CPL and moderate cooling over Baffin Island, where a decaying cyclone was moving northward. The cooling induces positive geopotential height and anticyclonic flow perturbations extending up to 500 mbar over the cyclone region. The averaged inverted geopotential height anomaly at 1000 mbar level over the cooling region is up to 4.6 dam in 72 h. Surface cooling is demonstrated to play a role in the cyclolysis. The CPL run allows the relative humidity with respect to ice in the blowing snow module to remain supersaturated and includes the heat release from the supersaturated water vapor deposition. Another experiment was carried out, in which supersaturated vapor was not allowed in the blowing snow module. The sensitivity experiment results indicated that blowing snow cooling effects over Baffin Island will be much reduced.

Description: The Polar version 3.1.1 of the Weather Research and Forecasting model (WRF), a high-resolution regional scale model, is used to simulate conditions for the year December 2006 to November 2007. The goal is to compare model output of near-surface and tropospheric variables to observational data sets. The domain mirrors that of the Arctic System Reanalysis (ASR), an assimilation of model fields with Arctic observations being conducted partly by the Polar Meteorology Group of the Byrd Polar Research Center at Ohio State University. A key development in this Polar WRF study is the extension of the seasonal progression of sea ice albedo to the entire Arctic Ocean. The boundary conditions are specified by the NCEP Final global gridded analysis archive (FNL), a 1° × 1° global grid updated every 6 h. The simulations are performed in 48 h increments initialized daily at 0000 UTC, with the first 24 h discarded for model spin-up of the hydrologic cycle and boundary layer processes. Model large-scale variables of atmospheric pressure and geopotential height show good agreement with observations. Spatial distribution of near-surface air temperatures compares well with ERA-Interim despite a small negative bias in the station analysis. Surface dewpoint temperatures and wind speeds show small biases, but model skill is modest for near-surface winds. Tropospheric temperatures and wind speeds, however, agree well with radiosonde observations. This examination provides a benchmark from which to improve the model and guidance for further development of Polar WRF as ASR's primary model.

Description: Backscattering and extinction properties of various snow particle models are studied for three typical cloud radar frequency ranges, namely Ku band, Ka band, and W band, both in terms of their individual scattering properties as well as averaged over size distributions. Models studied include soft spheres, randomly oriented pristine nonspherical particles and complex aggregates, as well as horizontally aligned spheroids. It is shown that the concurrent use of Ku/Ka band and Ka/W band dual wavelength ratios (DWR) allows for a separation of different snow particle habits. It is further shown that triple-frequency approaches constrain the slope parameter of exponential size distributions more tightly than conventional single DWR approaches can. Uncertainties introduced by unknown mass-size relations for different snow particle habits remain a challenge when mass-related quantities are to be derived. Attenuation by snow, especially at W band, is found to potentially alter these results, albeit moderately, without affecting the general conclusions. Sensitivity studies performed with respect to cutoffs in the simulated size distribution highlight potential benefits of including larger particles in future scattering databases.

Description: Climate metrics are becoming a more widespread tool used in global circulation model (GCM) evaluation as well as climate change projections. In their more simple form they provide a quick overview of the performance of a large ensemble of GCM simulations such as the CMIP3 archive of coupled ocean-atmosphere GCMs. Most existing metrics focus on the comparison of fields at each grid point. We present here a complementary metric which targets structures as a whole (patterns). The methodology is based on a pattern matching technique used previously in numerical weather prediction and has been modified for the analysis of mean climate fields. The resulting error decomposition allows for a more detailed assessment of the field structure with regard to errors in placement, rotation, volume, and pattern. The technique is applied to two observational rainfall data sets and GCM simulations from the CMIP3 archive for seasonal rainfall structures over the South Pacific Convergence Zone.

Description: A climatology of cold air outbreaks (CAOs) over North America is presented on the basis of a 50 year simulation of the Whole Atmosphere Community Climate Model (WACCM). This climatology is compared to a similar CAO climatology based on 45 years (1957–2002) of European Centre for Medium-Range Weather Forecasts 40 Year Re-Analysis Project (ERA-40) data. A CAO is identified at a given grid point if the following criteria are met: (1) the surface temperature is lower than 1.5 standard deviations below the 31 day climatological running mean, (2) the standard deviation in temperature is greater than 2 K, and (3) conditions 1 and 2 are satisfied over a contiguous area of ∼5° longitude by 5° latitude. WACCM and ERA-40 comparisons are shown for CAO frequency, temperature anomaly from 31 day climatological mean, geographical location, minimum temperature, and areal extent. Overall, CAOs in WACCM occur ∼30% less frequently than in ERA-40 but cover ∼30% greater area and are 1–2 K lower. In midwinter, WACCM CAOs form at lower latitudes and penetrate to lower latitudes compared to CAOs in ERA-40.

Description: The long-term temperature profile data sets obtained by Rayleigh lidars at three different northern latitudes within the Network for the Detection of Atmospheric Composition Change were used to derive the middle atmosphere temperature trend and response to the 11 year solar cycle. The lidars were located at the Mauna Loa Observatory, Hawaii (MLO, 19.5°N); the Table Mountain Facility, California (TMF, 34.4°N); and the Observatoire de Haute Provence, France (OHP, 43.9°N). A stratospheric cooling trend of 2–3 K/decade was found for both TMF and OHP, and a trend of ≤0.5 ± 0.5 K/decade was found at MLO. In the mesosphere, the trend at TMF (3–4 K/decade) was much larger than that at both OHP and MLO (

Description: The augmented Noah land surface model described in the first part of the two-part series was evaluated here over global river basins. Across various climate zones, global-scale tests can reveal a model's weaknesses and strengths that a local-scale testing cannot. In addition, global-scale tests are more challenging than local- and catchment-scale tests. Given constant model parameters (e. g., runoff parameters) across global river basins, global-scale tests are more stringent. We assessed model performance against various satellite and ground-based observations over global river basins through six experiments that mimic a transition from the original Noah LSM to the fully augmented version. The model shows transitional improvements in modeling runoff, soil moisture, snow, and skin temperature, despite considerable increase in computational time by the fully augmented Noah-MP version compared to the original Noah LSM. The dynamic vegetation model favorably captures seasonal and spatial variability of leaf area index and green vegetation fraction. We also conducted 36 ensemble experiments with 36 combinations of optional schemes for runoff, leaf dynamics, stomatal resistance, and the β factor. Runoff schemes play a dominant and different role in controlling soil moisture and its relationship with evapotranspiration compared to ecological processes such as the β factor, vegetation dynamics, and stomatal resistance. The 36-member ensemble mean of runoff performs better than any single member over the world's 50 largest river basins, suggesting a great potential of land-based ensemble simulations for climate prediction.

Description: The annual and interannual characteristics of nitrous oxide (N2O) emissions from conventional vegetable fields are poorly understood. We carried out 4 year measurements of N2O fluxes from a conventional vegetable cultivation area in the Yangtze River delta. Under fertilized conditions subject to farming practices, approximately 86% of the annual total N2O release occurred following fertilization events. The direct emission factors (EFd) of the 12 individual vegetable seasons investigated ranged from 0.06 to 14.20%, with a mean of 3.09% and a coefficient of variation (CV) of 142%. The annual EFd varied from 0.59 to 4.98%, with a mean of 2.88% and an interannual CV of 74%. The mean value is much larger than the latest default value (1.00%) of the Intergovernmental Panel on Climate Change. Occasional application of lagoon-stored manure slurry coupled with other nitrogen fertilizers, or basal nitrogen addition immediately followed by heavy rainfall, accounted for a substantial portion of the large EFds observed in warm seasons. The large CVs suggest that the emission factors obtained from short-term observations that poorly represent seasonality and/or interannual variability will inevitably yield large uncertainties in inventory estimation. The results of this study indicate that conventional vegetable fields associated with intensive nitrogen addition, as well as occasional applications of manure slurry, may substantially account for regional N2O emissions. However, this conclusion needs to be further confirmed through studies at multiple field sites. Moreover, further experimental studies are needed to test the mitigation options suggested by this study for N2O emissions from open vegetable fields.

Description: We evaluate climatologies of upper tropospheric ozone and nitric acid retrieved from two satellite instruments (ACE-FTS and OSIRIS) with long-term in situ measurements from aircraft (MOZAIC, CR-AVE, PRE-AVE, PEM Tropics, and TC4) and ozonesondes. A global chemical transport model (GEOS-Chem) is used to guide the evaluation and to relate sparse in situ measurements with the satellite retrievals. Both satellite retrievals generally reproduce broad ozone features in the upper troposphere such as summer enhancements in the northern subtropics and larger concentrations over the tropical Atlantic versus the tropical Pacific. These comparisons indicate biases in annual, tropical mean ozone concentrations from both ACE-FTS (10–13%) and OSIRIS (5%) relative to aircraft and ozonesonde observations. More uncertain evidence suggests that nitric acid from ACE-FTS has a positive mean bias of 15%. We demonstrate that an upper limit on the ozone production efficiency in the upper troposphere can be determined using ACE-FTS satellite measurements of O3 and HNO3. The resulting value of 196 (+34, −61) mol/mol is in broad agreement with model simulations. Higher OPE values inferred from ACE-FTS over the tropical Pacific (249 (+21, −68) mol/mol) than the tropical Atlantic (146 (+16, −41) mol/mol) reflect increasing ozone production efficiency with decreasing pollution. This analysis indicates a new capability of satellite observations to provide insight into ozone production in the tropical troposphere.

Description: The consistency of tropical tropospheric temperature trends with climate model expectations remains contentious. A key limitation is that the uncertainties in observations from radiosondes are both substantial and poorly constrained. We present a thorough uncertainty analysis of radiosonde-based temperature records. This uses an automated homogenization procedure and a previously developed set of complex error models where the answer is known a priori. We perform a number of homogenization experiments in which error models are used to provide uncertainty estimates of real-world trends. These estimates are relatively insensitive to a variety of processing choices. Over 1979–2003, the satellite-equivalent tropical lower tropospheric temperature trend has likely (5–95% confidence range) been between −0.01 K/decade and 0.19 K/decade (0.05–0.23 K/decade over 1958–2003) with a best estimate of 0.08 K/decade (0.14 K/decade). This range includes both available satellite data sets and estimates from models (based upon scaling their tropical amplification behavior by observed surface trends). On an individual pressure level basis, agreement between models, theory, and observations within the troposphere is uncertain over 1979 to 2003 and nonexistent above 300 hPa. Analysis of 1958–2003, however, shows consistent model-data agreement in tropical lapse rate trends at all levels up to the tropical tropopause, so the disagreement in the more recent period is not necessarily evidence of a general problem in simulating long-term global warming. Other possible reasons for the discrepancy since 1979 are: observational errors beyond those accounted for here, end-point effects, inadequate decadal variability in model lapse rates, or neglected climate forcings.

Description: The Palmer Drought Severity Index (PDSI) has been widely used to study aridity changes in modern and past climates. Efforts to address its major problems have led to new variants of the PDSI, such as the self-calibrating PDSI (sc_PDSI) and PDSI using improved formulations for potential evapotranspiration (PE), such as the Penman-Monteith equation (PE_pm) instead of the Thornthwaite equation (PE_th). Here I compare and evaluate four forms of the PDSI, namely, the PDSI with PE_th (PDSI_th) and PE_pm (PDSI_pm) and the sc_PDSI with PE_th (sc_PDSI_th) and PE_pm (sc_PDSI_pm) calculated using available climate data from 1850 to 2008. Our results confirm previous findings that the choice of the PE only has small effects on both the PDSI and sc_PDSI for the 20th century climate, and the self-calibration reduces the value range slightly and makes the sc_PDSI more comparable spatially than the original PDSI. However, the histograms of the sc_PDSI are still non-Gaussian at many locations, and all four forms of the PDSI show similar correlations with observed monthly soil moisture (r = 0.4–0.8) in North America and Eurasia, with historical yearly streamflow data (r = 0.4–0.9) over most of the world's largest river basins, and with GRACE (Gravity Recovery and Climate Experiment) satellite-observed water storage changes (r = 0.4–0.8) over most land areas. All the four forms of the PDSI show widespread drying over Africa, East and South Asia, and other areas from 1950 to 2008, and most of this drying is due to recent warming. The global percentage of dry areas has increased by about 1.74% (of global land area) per decade from 1950 to 2008. The use of the Penman-Monteith PE and self-calibrating PDSI only slightly reduces the drying trend seen in the original PDSI. The percentages of dry and wet areas over the global land area and six select regions are anticorrelated (r = −0.5 to −0.7), but their long-term trends during the 20th century do not cancel each other, with the trend for the dry area often predominating over that for the wet area, resulting in upward trends during the 20th century for the areas under extreme (i.e., dry or wet) conditions for the global land as a whole (∼1.27% per decade) and the United States, western Europe, Australia, Sahel, East Asia, and southern Africa. The recent drying trends are qualitatively consistent with other analyses and model predictions, which suggest more severe drying in the coming decades.

Description: To reduce the Earth's radiation budget uncertainty related to cloud types' changes, and better understand the climate constraints resulting from long-term clouds' variability, frequent and finer (than actually existing) observations are necessary. This is one of the aims of future satellite programs such as the Global Change Observation Mission-Climate (GCOM-C) satellite, to be launched by the Japan Aerospace Exploration Agency (JAXA). To facilitate the transition from past to future observations, the actual state of climate variables (e.g., cloud types) needs to be evaluated. This evaluation is attempted in the present work with the analysis of long-term cloud types' distribution and amounts. The data set used for this study is 25 years (1982–2006) of global daytime cloud properties observed by the National Oceanic and Atmospheric Administration-Advanced Very-High-Resolution Radiometer (NOAA-AVHRR) satellites sensors. Though various calibrations have been applied on NOAA-AVHRR data, the effects of the orbit drift experienced by these satellites need to be corrected. A signal processing decomposition method allowing the filtering of the cloud types' amount trend affected by the orbit drift is used to perform the necessary corrections. The results obtained show a quantifiable improvement of the cloud amount estimation and trends of the individual NOAA satellites initial observations, at the global and regional scales. The corrected global cloud amount shows a slight decrease in its linear trend. The driving factors of this trend are the decrease in mid and low clouds overwhelming the increase in high clouds (+0.04% cloud amount/yr). A comparison with other cloud climatology studies such as the International Cloud Satellite Climatology Project (ISCCP) data set shows that the global cloud decrease noticed in NOAA-AVHRR's data is smaller. And, contrary to the NOAA-AVHRR's data, the driving force of the ISCCP linear trend is a sharp decrease in low clouds (−0.20% cloud amount/yr).

Description: Large-scale climate data for the north tropical Atlantic (NTA) region show that air temperatures have increased during the past 50 years (1955–1959 to 2000–2004) with moderate warming near the Caribbean islands to considerable heating in the northern region. This pattern may be driven by sea surface temperature anomalies in the same region of study that follow relatively small changes in the Caribbean basin to stronger anomalies in the northeast. These changes might be associated with changes in the long-term pattern of the NTA high-pressure system that drives climate in the region. A series of mesoscale numerical experiments were designed to study the regional impacts these large-scale changes have on the hydrological cycle of the island of Puerto Rico. Results indicate that increased easterly surface winds for the 1950–2000 time frame disrupts a pattern of inland moisture advection and convergence zone, increasing cloud base heights and reducing the total column liquid water content over high elevations. This combination of factors produces a reduction in precipitation over the central and eastern mountains of Puerto Rico.

Description: In the laboratory, we have investigated the growth and composition of frost flowers. Their ionic composition has shown little difference from those of field measurements. Young frost flowers grown on sea ice are saline, leading us to speculate that wicking occurs continually during their growth on sea ice. The surface area of frost flowers is only a little larger than the area of ice underneath, consistent with recent field measurements from the Arctic. Time-lapse photography has allowed us to observe the extreme mobility of freshly forming sea ice, at the stage at which the mush has become rather solid, and continuing while the flowers grow. This mobility results in new brine being expelled to the surface, which therefore remains wet. During various stages of frost flower growth, we observed their freshly formed dendritic parts rapidly diminishing in size after contacting the surface, consistent with repeated wicking. Frost flowers proved to be very stable in the presence of wind, such that no aerosol was observed when wind was blown across them in the laboratory chamber. This is consistent with recent field observations of frost flowers coexisting with wind-blown snow.

Description: The relative contributions of Southeast Asian convective source regions during boreal summer to water vapor in the tropical stratosphere are examined using Lagrangian trajectories. Convective sources are identified using global observations of infrared brightness temperature at high space and time resolution, and water vapor transport is simulated using advection-condensation. Trajectory simulations are driven by three different reanalysis data sets, GMAO MERRA, ERA-Interim, and NCEP/NCAR, to establish points of consistency and evaluate the sensitivity of the results to differences in the underlying meteorological fields. All ensembles indicate that Southeast Asia is a prominent boreal summer source of tropospheric air to the tropical stratosphere. Three convective source domains are identified within Southeast Asia: the Bay of Bengal and South Asian subcontinent (MON), the South China and Philippine Seas (SCS), and the Tibetan Plateau and South Slope of the Himalayas (TIB). Water vapor transport into the stratosphere from these three domains exhibits systematic differences that are related to differences in the bulk characteristics of transport. We find air emanating from SCS to be driest, from MON slightly moister, and from TIB moistest. Analysis of pathways shows that air detrained from convection over TIB is most likely to bypass the region of minimum absolute saturation mixing ratio over the equatorial western Pacific; however, the impact of this bypass mechanism on mean water vapor in the tropical stratosphere at 68 hPa is small (

Description: Ferrel Circulation variability in the Southern Hemisphere and its linkages with the tropical and subtropical sea surface temperature (SST) for the period 1979–2009 are investigated. Two dominant spatiotemporal patterns of Ferrel Circulation variability are identified, one showing opposing change between the poleward and equatorward portion of ∼48°S of the Ferrel Circulation (EOF1) and the other showing a rather uniform change of the Ferrel Circulation (EOF2). An increasing trend is found for the EOF1, indicating the strengthening (weakening) of the poleward (equatorward) portion of the Ferrel Circulation for the past three decades, although the Ferrel Circulation strength index suggests an overall weakening of the Ferrel Circulation. Distinctly different forms of tropical and subtropical ocean variability are connected to the two dominant modes of Ferrel Circulation variability. Statistical analysis suggests that the EOF1 has a close association with the warming trend of the tropical and subtropical oceans for the past three decades, whereas the EOF2 is significantly correlated with SST variability in the Pacific that resembles the La Niña Modoki.

Description: The response of brightness temperatures at different microwave frequencies to overland precipitation is investigated by using the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and Microwave Imager (TMI) data. The Spearman correlation coefficients between observations at TMI channels or channel combinations and PR-measured near-surface rain are computed using 3 years of TRMM data. The results showed that the brightness temperature combinations from 19 and 37 GHz, that is, V19-V37 (the letter V denotes vertical polarization, and the numbers denote frequency in GHz) or V21-V37, can explain ∼10% more variance of near-surface rainfall rate than can the V85 brightness temperature. Also, the global distribution of the above correlation revealed that over almost all of the tropical land area covered by TRMM satellite, the V19-V37 channel has a closer response to the overland rainfall than does the V85 channel. This result is somewhat counterintuitive, because it has been long believed that the dominant signature of overland rainfall is the brightness temperature depression caused by ice scattering at high microwave frequencies (e.g., 85 GHz). To understand the underlying physics of this better low-frequency response, data analysis and radiative transfer modeling have been conducted to assess the influence on brightness temperatures from clouds with different ice and liquid water partitions. The results showed that under the condition of low frozen water and medium liquid water in the atmospheric column, the signal from the V19-V37 channel responded better to rainfall rate than did the one from the V85 channel. A plausible explanation to this result is that in addition to ice scattering signature, the V19-V37 channel contains liquid water information as well, which is more directly related to surface rain than to ice water aloft. At heavy rainfall conditions, the V19-V37, V37, and V85 channels all are correlated with near-surface rain reasonably well, and the V37 or V21 channel becomes the top responder to surface rain as the amount of hydrometeors in the atmospheric column reaches very high values. Additionally, it is found that land surface type and 2 m air temperature have significant skills in characterizing rain cloud types, so that the V19-V37 channel is more sensitive to surface rainfall for more vegetated warm surface, while the V85 channel is more sensitive to cold bare land. This finding implies that the above two parameters may be used to prioritize satellite observations at different channels, so that the channel that has the best rainfall sensitivity under a given condition receives the highest weight in retrieval algorithms.

Description: The Pearl River Delta (PRD) region in South China is one of the most economically developed regions in China while also noted for its severe air pollution, especially in the urban environments. In order to understand in depth the aerosol chemistry and the emission sources in PRD, an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed at an urban site in the Hong Kong–Shenzhen metropolitan area between 25 October and 2 December 2009. Ten minute–resolved measurement data were analyzed, and an average mass concentration of 44.5 ± 34.0 μg m−3 was calculated for the entire campaign. On average, organic matter was the most abundant PM1 component accounting for 39.7% of the total mass, followed by sulfate (24.5%), black carbon (measured by aethalometer, 14.0%), ammonium (10.2%), nitrate (10.0%), and chloride (1.6%). Moreover, organic matter comprised an increasing fraction of the PM1 loading as the PM1 loading increased, denoting its key role in particulate pollution in this region. Calculations of organic elemental composition based on the high-resolution organic mass spectra obtained indicated that C, H, O, and N on average contributed 33.8%, 55.1%, 10.2%, and 0.9%, respectively, to the total atomic numbers of organic aerosol (OA), which corresponded to an OM/OC ratio (the ratio of organic matter mass/organic carbon mass) of 1.57 ± 0.08. Positive matrix factorization analysis was then conducted on the high-resolution organic mass spectral data set. Four OA components were identified, including a hydrocarbon-like (HOA), a biomass burning (BBOA), and two oxygenated (LV-OOA and SV-OOA) components, which on average accounted for 29.5%, 24.1%, 18.8%, and 27.6%, respectively, of the total organic mass. The HOA was found to have contributions from both fossil fuel combustion and cooking emissions, while the BBOA was well correlated with acetonitrile, a known biomass burning marker. The LV-OOA and SV-OOA corresponded to more aged and fresher secondary organic aerosol, respectively. The diurnal variations of the LV-OOA and SV-OOA showed significant increase in concentration in the daytime, denoting their substantial photochemical formation. Back trajectory analysis indicated that the short-range regional transport from the northeast was the key factor leading to severe submicron aerosol pollution in this area. The HR-ToF-AMS measurement results in this campaign are completely compared with a previous paper that reports the HR-ToF-AMS measurement results at a rural site in PRD in the same season, based on which the regional pollution characteristics of submicron particle in PRD were analyzed.

Description: The migrating diurnal tide is one of the dominant dynamical features in the low latitudes of the Earth's mesosphere and lower thermosphere (MLT) region, representing the atmospheric response to the largest component of solar forcing. Ground-based observations of the tide have resolved short-term variations attributed to nonlinear interactions between the tide and planetary waves that are also in the region. Using the NCAR Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM), we simulate a quasi 2 day wave (QTDW) event under late-January conditions. In this case, sideband sum and difference child waves are resolved, indicating that a nonlinear interaction is occurring between the QTDW and the tide. The migrating diurnal tide in the MLT displays local amplitude decreases of 20–40%, as well as a shortening of vertical wavelength by roughly 4 km. Examining the physical mechanisms driving the interaction, nonlinear advection is found to result in amplification of the tide in some regions and damping in others, manifesting as increased smoothing of the tidal structure when the QTDW is present in the MLT. Additionally, the QTDW also enhances the easterly summer mean wind jet that can also account for changes in tidal amplitude and vertical wavelength. We find that QTDW-induced background atmosphere changes in TIME-GCM can drive tidal variability at levels greater than nonlinear advection, a possibility not previously considered.

Description: A cloud-to-ground lightning stepped leader has been recorded with a slitless spectrograph at a recording rate of 10,000 images per second at a distance of 0.6 km. Five sequential images of the leader spectra were recorded with an exposure (integration) time of 99.6 μs each over a spectral range from 600 to 1050 nm. These are the first stepped leader spectra covering the range 600 to 1050 nm. The last three spectra, obtained immediately before the return stroke, were analyzed at an altitude of between 108 and 122 m above a struck vehicle. The spectral emissions in the near infrared are dominated by neutral nitrogen and oxygen emissions, and Hα, with only a few emission lines from singly ionized nitrogen. A singly ionized nitrogen line at 661.1 nm is present in the first analyzed image, but not in the two subsequent images at the same height, which suggests a cooling of the channel. The emissions are integrated over a 99.6 μs exposure time and therefore show no evidence of stepping. The ensuing negative return stroke was detected by the National Lightning Detection Network and had an estimated peak current of −15.2 kA. One subsequent stroke was outside the field of view of the spectrograph. The flash occurred on 11 September 2009 near New Underwood, South Dakota, and the exact location of the first stroke is known because it struck a car traveling on Interstate 90. The stepped leader two-dimensional speed increased in the last four steps from 1.53 × 105 to 2.42 × 105 m/s with an average of 2.03 × 105 m/s.

Description: A physically based snow albedo model (PBSAM), which can be used in a general circulation model, is developed. PBSAM calculates broadband albedos and the solar heating profile in snowpack as functions of snow grain size and concentrations of snow impurities, black carbon and mineral dust, in snow with any layer structure and under any solar illumination condition. The model calculates the visible and near-infrared (NIR) albedos by dividing each broadband spectrum into several spectral subbands to simulate the change in spectral distribution of solar radiation in the broadband spectra at the snow surface and in the snowpack. PBSAM uses (1) the look-up table method for calculations of albedo and transmittance in spectral subbands for a homogeneous snow layer, (2) an “adding” method for calculating the effect of an inhomogeneous snow structure on albedo and transmittance, and (3) spectral weighting of radiative parameters to obtain the broadband values from the subbands. We confirmed that PBSAM can calculate the broadband albedos of single- and two-layer snow models with good accuracy by comparing them with those calculated by a spectrally detailed radiative transfer model (RTM). In addition, we used radiation budget measurements and snow pit data obtained during the two winters from 2007 to 2009 at Sapporo, Hokkaido, Japan, for simulation of the broadband albedos by PBSAM and compared the results with the in situ measurements. A five-layer snow model with one visible subband and three NIR subbands were necessary for accurate simulation. Comparison of solar heating profiles calculated by PBSAM with those calculated by the spectrally detailed RTM showed that PBSAM calculated accurate solar heating profiles when at least three subbands were used in both the visible and NIR bands.

Description: In the first part of this paper, the rigorous theory describing the electromagnetic field radiated by a lightning return stroke over a two-layered conducting ground was presented and the exact expressions for the lightning electromagnetic fields were developed and discussed. In this part of the paper, the theory along with its time domain numerical evaluation algorithm is used for the assessment of the validity of simplified approaches proposed in the literature for the vertical electric and horizontal magnetic field components. The simplified approaches are based on the concept of ground surface impedance and its corresponding attenuation function. It is shown that the results obtained using the simplified approaches are in excellent agreement with exact results in both near (50 m) and intermediate (1000 m) distance range. However, since the vertical electric and azimuthal magnetic field components are not appreciably affected by the ground finite conductivity, they can also be evaluated assuming the ground as a perfectly conducting ground. On the other hand, the horizontal electric field above a horizontally stratified ground is very much affected by the ground electrical parameters. Its waveform is characterized by an early negative excursion due to the currents flowing into the ground followed by a late time positive excursion which is due to the elevation of the observation point from the ground level. The magnitude of the negative peak is sharper for subsequent return strokes than first return strokes and is higher for lower conducting grounds. A new formula is proposed for the evaluation of the horizontal electric field at a given height above the air-ground interface. The formula can be viewed as the generalization of the Cooray-Rubinstein formula for the case of a two-layer ground. We show that the new formula is able to reproduce in a satisfactory manner the horizontal electric field above a two-layer ground. The proposed formulation is, however, less accurate at distances as close to 10 m from the channel base and for very poor ground conductivity (0.0001 S/m).

Description: Two methods, based on different assumptions, have been investigated for computing the uncertainty in the trend of 960 mesopause region temperature profiles taken over 20 years. In addition to the trend, the fitting model includes annual and semiannual variations, a solar effect, and an episodic term that can be associated with the Mount Pinatubo volcanic eruption. In our standard procedure we assume that the unknown uncertainties of each measurement, and thus the weights of the observations, are constant with time and are determined by assuming that we have a good model. In addition we investigate including second derivatives along with first derivatives of the Hessian matrix. In the Monte Carlo bootstrap method one resamples the data with replacement to obtain new data sets which are fit to the model and yield a distribution of fitting parameters. The standard deviation of the distribution for a parameter is taken to be the uncertainty in the parameter. A possible assumption to justify this method is that the data are “independent and identically distributed.” Although we have not been able to justify the assumptions of either method, the two methods support each other since they give essentially the same results.

Description: A coupled atmosphere-ocean model is used to examine the climatic effects of Southeast Asian deforestation. On the deforested grid cells surface temperatures rise, and precipitation is reduced after deforestation. Regional moisture convergence and convection increase and lead to strongly enhanced rainfall. The easterlies and associated oceanic surface currents are reduced in the Southeast Asian realm. As a result, the upwelling of cold ocean water weakens, and surface temperatures rise. This acts as a strong positive feedback mechanism for the enhanced regional moisture convergence. Besides the regional effects, we find strong teleconnections to the tropics and to the high latitudes of both hemispheres. The signal from Southeast Asia propagates to the extratropics with the high-level winds in the form of sensible heat and potential energy. Possibly because of strong albedo–sea ice feedbacks, some regions in the high latitudes amplify relatively small initial climate changes and exhibit strong remote effects. The excitation of atmospheric waves (probably Rossby waves) is the underlying mechanism that causes the signal propagation to the extratropics.

Description: In this paper an improved antenna theory (AT) model with nonlinearly varying resistive loading and fixed inductive loading is used to electromagnetically simulate lightning strikes to tall structures. Measurement data captured from Toronto's CN tower are used to verify the validity of the new model. Both the return stroke channel (RSC) and the tower are modeled by straight thin conducting wires. The wire model of the channel is assumed to have distributed nonlinear resistive elements as a function of current and time, adopted from the numerical models of a spark channel and consequent shockwave from a lightning discharge, yielding a varying value of the channel radius from the base to the cloud along the RSC. Such distributed elements are used to take into account the current attenuation while propagating along the channel and varying propagation speeds lower than the speed of light. RSC current distribution and radiated electromagnetic fields in near, intermediate, and far range distances predicted by the proposed model are compared with those obtained from the measurement data and with those of the original AT model and the AT with fixed inductive loading (ATIL-F) model. Current wave propagation speed profile in RSC and tower is investigated as a function of height as well. The effects of applying different tower geometry models are also studied. It is shown that the new model is able to reproduce one of the characteristic features of the electromagnetic fields radiated by lightning, namely, the far-field inversion of polarity with a zero crossing occurring in the tens of microseconds range. We have also investigated the effect of nonlinearity of the channel assumed in the new model. It is shown that among the electromagnetic models, distributed nonlinear resistance along the channel leads to a zero crossing in the tens of microseconds range even for large values of resistance. It is also shown that decreasing the nonlinearity results in the predictions asymptotically converging to those of the ATIL-F model in which a uniformly distributed resistance along the channel is used. Further, the profile of the return stroke current attenuation and speed and removed charge as a function of height are investigated. Finally, the effect of lossy ground is analyzed using Wait-Cooray convolution formula in the time domain to verify the validity of PEC assumption in the proposed model.

Description: Anecdotes from local residents and modeling studies suggest that deforestation may delay the onset of the rainy season (O) in western Brazil, but detection studies using climatological time series are not available. Here we investigate trends in O in the state of Rondonia, Brazil, a region that has been continuously deforested since the 1970s. Daily rainfall data from 16 station time series, spanning periods of at least 25 years, with five covering more than 30 years, are used. We define O as the first day after 1 September with rainfall greater than 20 mm d−1. A t test indicates that for stations that lie inside the major deforested area, O has significantly shifted to, on average, 11 days (and up to 18 days) later in the year over the last three decades. However, for stations that lie in areas that have not been heavily deforested, O has not shifted significantly. Nonparametric and parametric trend analyses all gave similar results for the change of O with time, and all of the statistically significant results indicated a delay in O. Twenty-five percent (four) of the stations analyzed showed a marked shift in timing of O: these stations are located inside deforested areas, primarily near the BR 364 highway that crosses Rondonia. Delayed onset may be a result of land use change, and this signal may strengthen in future: current delaying trends may be as great as 0.6 days per year, and after 30 years of deforestation the onset of the rainy season is expected to be 18 days later.

Description: The central Pacific Coast of the United States is one of the few regions in North America where precipitation exhibited a high proportion of variance at decadal time scales (10 to 20 years) during the last century. We use a network of tree ring-width records to estimate the behavior of the observed decadal pattern in regional winter precipitation during the last three and a half centuries. The pattern was most vigorous during the mid and late 20th century. Between A.D. 1650 and 1930, proxy estimates show a limited number of events separated by longer intervals of relatively low variance. The multicentennial perspective offered by tree rings indicates the energetic decadal pattern in winter precipitation is a relatively recent feature. Until a physical mechanism can be identified that explains the presence of this decadal rhythm, as well as its inconsistency during the period of record, we cannot rule out the possibility that this behavior may cease as abruptly as it began.

Description: Measurements from the High Resolution Dynamics Limb Sounder (HIRDLS) on NASA's Aura satellite are used to quantify gravity wave momentum fluxes generated from monsoon regions across the globe over the 3 years 2005–2007. Momentum fluxes in the altitude range 16–60 km over eight individual monsoon regions in both hemispheres are examined and compared to convective data from National Oceanic and Atmospheric Administration's Outgoing Longwave Radiation time series and precipitation data from the Tropical Rainfall Monitoring Mission. Good correlations are observed over seven of these eight regions, the exception being the North American monsoon region. An increase in measured momentum flux of approximately 50% is observed over the 10°N–30°N latitude band during the boreal monsoon period, with around half of the annual momentum flux for this latitude range measured during this period; however, the limited correspondence in the North American region suggests that at least some of this increase may be due to other processes.

Description: Using remote sensing data from multiple satellites and numerical simulations we addressed the hypothesis that the Western Ghats tropical montane wet forests in southwest India are hydrometeorologically stressed and lowland deforestation has decreased cloud cover and precipitation. We generated water cloud and Leaf Area Index climatologies for a 6 year period and answered the following questions: (1) what are the diurnal and seasonal variations of water clouds, (2) what is the relationship between cloud cover and topography, (3) where are potential locations of the tropical montane wet forests, (4) what are the current hydrometeorological conditions over the identified tropical montane wet forests, and (5) what is the sensitivity of cloud cover and precipitation over the montane wet forests to lowland deforestation? The study found that (1) strong correlations between climatological precipitation and cloud frequency were present thereby indicating that higher spatial and temporal resolution cloud frequency is a good indicator of hydrometeorological conditions; (2) cloud frequency was lower in the dry season (10–25%) and higher in the monsoon seasons (40–45%); (3) a morning-afternoon pattern in cloud cover exists during both the dry and wet seasons; (4) morning cloud cover decreased from October to January with afternoon cloud cover similarly decreasing but with higher values than the morning cloud cover; (5) cloud cover generally increased with elevation; (6) 39%, 70%, 47% and 42% of the clouds intersected the mountains during March and May 2007 and February and April 2008, respectively; (7) our maps showing regions with conditions suitable to sustain tropical montane wet forests successfully identified known montane biodiversity hot spots; and (8) simulations showed an increase of afternoon cloud cover and dry season precipitation but reduced cloud immersion over the montane forests with lowland deforestation.

Description: The salient features of the daytime cloud radiative effect (CRE, also known as cloud radiative forcing) corresponding to various cloud regimes or weather states are examined. The analysis is based on a 24 year long data set from the International Satellite Cloud Climatology Project (ISCCP) for three distinct geographical zones covering most of the Earth's surface area. Conditional sampling and averaging of the ISCCP cloud fraction and CRE in 2.5° grid cells is performed for each weather state, and the state's radiative importance expressed as the relative contribution to the total CRE of its geographical zone is explained in terms of dominant cloud type, cloud fraction, and frequency of occurrence. Similarities and differences within and between geographical zones in the cloud fraction and CRE characteristics of the various weather states are identified and highlighted. By providing an exposition of the radiative energy characteristics of different cloud type mixtures, we facilitate the meteorological situation-dependent evaluation of radiation budget effects due to clouds in climate models.

Description: The Tropical Warm Pool–International Cloud Experiment (TWP-ICE) provided extensive observational data sets designed to initialize, force, and constrain atmospheric model simulations. In this first of a two-part study, precipitation and cloud structures within nine cloud-resolving model simulations are compared with scanning radar reflectivity and satellite infrared brightness temperature observations during an active monsoon period from 19 to 25 January 2006. Seven of nine simulations overestimate convective area by 20% or more leading to general overestimation of convective rainfall. This is balanced by underestimation of stratiform rainfall by 5% to 50% despite overestimation of stratiform area by up to 65% because of a preponderance of very low stratiform rain rates in all simulations. All simulations fail to reproduce observed radar reflectivity distributions above the melting level in convective regions and throughout the troposphere in stratiform regions. Observed precipitation-sized ice reaches higher altitudes than simulated precipitation-sized ice despite some simulations that predict lower than observed top-of-atmosphere infrared brightness temperatures. For the simulations that overestimate radar reflectivity aloft, graupel is the cause with one-moment microphysics schemes whereas snow is the cause with two-moment microphysics schemes. Differences in simulated radar reflectivity are more highly correlated with differences in mass mean melted diameter (Dm) than differences in ice water content. Dm is largely dependent on the mass-dimension relationship and gamma size distribution parameters such as size intercept (N0) and shape parameter (μ). Having variable density, variable N0, or μ greater than zero produces radar reflectivities closest to those observed.

Description: We present the results of a validation of atmospheric inversions of CO2 fluxes using four transport models. Each inversion uses data primarily from surface stations, combined with an atmospheric transport model, to estimate surface fluxes. The validation (or model evaluation) consists of running these optimized fluxes through the forward model and comparing the simulated concentrations with airborne concentration measurements. We focus on profiles from Cape Grim, Tasmania, and Carr, Colorado, while using other profile sites to test the generality of the comparison. Fits to the profiles are generally worse than to the surface data from the inversions and worse than the expected model-data mismatch. Thus inversion estimates are generally not consistent with the profile measurements. The TM3 model does better by some measures than the other three models. Models perform better over Tasmania than Colorado, and other profile sites bear out a general improvement from north to south and from continental to marine locations. There are also errors in the interannual variability of the fit, consistent in time and common across models. This suggests real variations in sources visible to the profile but not the surface measurements.

Description: The surface radiation budget (SRB) is modulated by smoke aerosols that attenuate solar and emit thermal infrared radiation. Direct radiative impacts of smoke depend on several factors that lead to large uncertainties in assessing how wildfires influence climate. To quantify the impact of smoke on the SRB, evaluations of the longwave (LW) as well as the shortwave (SW) radiative forcing efficiencies (RFElw and RFEsw) are needed. Radiative forcing efficiency (RFE) is defined as the change in net irradiance per unit increase in aerosol optical depth at 500 nm (AOD500). An opportunity to evaluate RFElw and RFEsw of smoke presented itself on 6–7 September 2010, when a wildfire burned west of Boulder, Colorado. Smoke passed over sites where SRB, atmospheric state, and AOD measurements were being made. Values of RFE were derived empirically from coincident SRB and AOD measurements made over a range of Sun angles. RFEsw ranged between −65 and −194 Wm−2AOD500−1 as solar zenith angles decreased from 73° to 34° (at solar noon). RFElw averaged ∼10 (±7) Wm−2AOD500−1 throughout the daylight hours. During the event, the diurnally integrated value of net RFE was −51.5 Wm−2AOD500−1, revealing the dominance of SW cooling over LW warming attributed to the smoke. In response, the surface under the smoke plume cooled by 2°–5°C during the day, depending on the smoke's opacity. No evaluation of nighttime effects were possible, although very slight warming may have occurred owing to thermal emissions from the plume.

Description: The Pearl River Delta (PRD) region in southeastern China is one of the most polluted industrial and metropolitan areas in the world. We report single-particle measurements of the refractory black carbon (rBC) component in ambient aerosol in the region, and we interpret the results to improve understanding of the importance of rBC for radiative forcing in this region and others like it. A Single-Particle Soot Photometer (SP2) provided rBC mass loadings, size distributions, and mixing state information. Average rBC loadings are approximately an order of magnitude higher than those measured near Houston, Texas, a major U.S. metropolitan area. The rBC in the PRD is also more often internally mixed with nonrefractory materials than is rBC around Houston. Localized atmospheric heating from rBC solar absorption reached an average rate of 0.4 K d−1. A one-dimensional discrete ordinates radiative transfer (DISORT) model coupled with the average measured rBC is used to illustrate the potential local contribution of rBC mass and associated scattering aerosol to the radiative forcing. Solar energy deposition rates in the atmosphere at the sampling site due to rBC absorption are estimated to have an average value of 17 W m−2 and be largest in the first 1 km above the ground. These results indicate that rBC has significant potential to influence local meteorology and climate parameters in the PRD region.

Description: Thin cirrus that frequently form in the tropical tropopause layer (TTL) are important for vertical transport through the TTL, regulation of stratospheric humidity, and the Earth's radiation budget. Here, we use a three-dimensional cloud-resolving model to investigate the impact of circulations driven by TTL cirrus radiative heating on the cloud evolution. We use observations of TTL environmental conditions (thermal stability and wind shear) and TTL cirrus microphysical properties (ice crystal sizes and concentrations) to constrain the simulations. We show that with ice crystal sizes consistent with available observations (effective radii ≥ 12 μm), typical thermal stability, and moderate wind shear, the ice cloud sediments to lower levels before radiative heating can drive a circulation to maintain the cloud and before small-scale convection builds up. In this case, the cloud lifetime is controlled by sedimentation of ice crystals into subsaturated air below the initial cloud level, followed by sublimation. Strong wind shear (〉10 m s−1 km−1) tends to hasten the cloud dissipation. With relatively weak thermal stability, small-scale convection builds up rapidly, resulting in mixing at cloud top and extension of the cloud lifetime. We also consider the impact of synoptic-scale and mesoscale temperature variability on cloud lifetime. Using TTL trajectories with small-scale wave temperature perturbations superimposed, we show that TTL cirrus will often dissipate within 12–24 h simply as a result of background temperature variability.

Description: In this study, a data assimilation scheme is developed on the basis of the ensemble Kalman filter algorithm and the common land model (CoLM); soil moisture and model parameters are simultaneously optimized to improve the estimates of turbulent fluxes. Land surface temperature (LST) is retrieved from geostationary operational environmental satellites (GOES) data, validated, and then assimilated into the model. The data assimilation results are validated at six observation sites in the United States that include grassland, cropland, and forestland cover types. Data assimilation results indicate that in addition to improvements in the prediction of turbulent fluxes, model uncertainties are also reduced as a result of the assimilation of GOES LST retrieval data. The average reductions in root mean square error (RMSE) values are 47.5 and 31.1 W m−2. The effects of simultaneous optimization of soil moisture and model parameters are compared with those resulting from separate optimization; simultaneous optimization is found to yield smaller RMSE values. Further, in this study, the effects of Moderate Resolution Imaging Spectroradiometer (MODIS) and GOES temporal resolution data on data assimilation results are studied. The assimilation results indicate that the average RMSE values for GOES temporal resolution data are smaller than that for MODIS temporal resolution data. During the assimilation time period, the soil moisture obtained from assimilation closely agrees with the observed values, and the four vegetation parameters show distinct seasonal variations. However, the lack of sufficient information makes it difficult to estimate the true value of these variables and parameters.

Description: This study combines a conceptual modeling approach and a physical process–based modeling approach to diagnose and understand the equilibrium state(s) of coupled biosphere-atmosphere models using the NCAR CAM3-CLM3-DGVM model as an example, with a focus on the West Africa and South America regions. First, a conceptual model is parameterized according to results from offline simulations using CAM3-CLM3 and CLM3-DGVM. The resulting conceptual model is then used to predict the potential biosphere-atmosphere equilibrium state(s) for the coupled CAM3-CLM3-DGVM model. Finally, simulations using the coupled CAM3-CLM3-DGVM model itself are carried out to verify the results of the conceptual model. Only one equilibrium state is found in both the conceptual model and the physically based numerical model. The CLM3 model features excessively high soil evaporation and very low plant transpiration, which leads to high bare-ground ET and low sensitivity of ET to vegetation cover changes in the land model. Despite the high sensitivity of precipitation to evapotranspiration changes in the atmospheric model, the land model deficiency leads to a high amount of precipitation in the absence of vegetation cover and a low sensitivity of precipitation to vegetation changes in CAM3-CLM3, two conditions that are found to anchor a coupled biosphere-atmosphere model to a single equilibrium state. This statement also holds for the up-to-date version of the models using CLM4. This study provides an example for the importance of land surface processes in determining the potential equilibrium states of fully coupled biosphere-atmosphere models.

Description: To evaluate the wintertime regional brown haze in northern China, trace gases and aerosols were measured at an urban site between 9 and 20 November 2009. Ion chromatography and transmission electron microscopy (TEM) were used to investigate soluble ions in PM2.5 and the mixing state of individual particles. The contrasts between clear and hazy days were examined in detail. Concentrations of the primary gases including NO (55.62 ppbv), NO2 (54.86 ppbv), SO2 (83.03 ppbv), and CO (2.07 ppmv) on hazy days were 2 to 6 times higher than those on clear days. In contrast, concentrations of O3 remained low (5.71 ppbv) on hazy days. Mass concentrations of PM2.5 (135.90 μg m−3) and BC (7.85 μg m−3) were 3 times higher on hazy days than on clear days. Based on the estimations from TEM analysis, fractions of both ammoniated sulfate (AS)–soot (20%) and AS-soot/organic matter/fly ash (20%) were larger on hazy days than on clear days (13% and 12%), implying that coagulation is an important mixing process in the polluted air. The SO2 emissions from coal combustion for power plants, industrial activities, and household heating led to high concentrations. Also, high concentrations of secondary sulfates significantly formed in the haze. Therefore, high concentrations of acidic gases contributed to the increased mass and number of secondary aerosols. Our study indicates that metal-catalyzed oxidation in the aqueous phase is a major pathway of sulfate formation. The mixtures of aerosol particles, together with MODIS images, suggest that the hazes covered not only the industrial cities, but extended into the neighboring rural regions.

Description: This paper evaluates global mean radiatively important properties of chemistry climate models (CCMs). We evaluate stratospheric temperatures and their 1980–2000 trends, January clear sky irradiances, heating rates, and greenhouse gas radiative forcings from an offline comparison of CCM radiation codes with line-by-line models, and CCMs' representation of the solar cycle. CCM global mean temperatures and their change can give an indication of errors in radiative transfer codes and/or atmospheric composition. Biases in the global temperature climatology are generally small, although five out of 18 CCMs show biases in their climatology that likely indicate problems with their radiative transfer codes. Temperature trends also generally agree well with observations, although one model shows significant discrepancies that appear to be due to radiation errors. Heating rates and estimated temperature changes from CO2, ozone, and water vapor changes are generally well modeled. Other gases (N2O, CH4, and CFCs) have only played a minor role in stratospheric temperature change, but their heating rates have large fractional errors in many models. Models that do not account for variations in the spectrum of solar irradiance cannot properly simulate solar-induced variations in stratospheric temperature. The combined long-lived greenhouse gas global annual mean instantaneous net radiative forcing at the tropopause is within 30% of line-by-line models for all CCM radiation codes tested. Problems remain in simulating radiative forcing for stratospheric water vapor and ozone changes with errors between 3% and 200% compared to line by line models. The paper makes recommendations for CCM radiation code developers and future intercomparisons.

Description: High-altitude ozone measurements at the Jungfraujoch observatory, Switzerland (JFJ, 3850 asl), covering the period 1990–2008 are investigated in this study. Anthropogenic ozone precursor emissions decreased over Europe and North America since the early 1990s. However, ozone concentrations at JFJ over the 19 year period show significant positive trends in the 1990s and no significant trends after 1999. Ozone trends were further studied with respect to air mass origin using 20 day back trajectories. The ozone increase during winter was particularly large in air masses with recent contact with the European planetary boundary layer (PBL), most probably in response to the steady decrease in European NO emissions leading to less ozone titration. On the other hand, the corresponding summer ozone increase was small, possibly attributable to the balancing effects of decreasing European ozone precursor emissions and increasing baseline ozone concentrations. Ozone from all source regions other than European PBL has a similar temporal pattern, exhibiting an increase for about the first 10 years, then leveling off to either no trend or insignificant decrease. This suggests that the physical processes determining the trend are beyond the time scale of the used backward trajectories or not described adequately by the simplified transport description in individual trajectories.

Description: The depiction of water vapor in the upper troposphere in Geophysical Fluid Dynamics Laboratory (GFDL) climate model and ERA-40 reanalysis is evaluated through a model-to-radiance approach. Brightness temperatures of High-Resolution Infrared Radiation Sounder (HIRS) 6.7 μm channel, Special Sensor for Microwave Water Vapor Profiler (SSM/T-2) 183.31 ± 1 GHz channel, and Microwave Sounding Unit (MSU) 60 GHz channel simulated with data from the GFDL climate model and ERA-40 reanalysis show a distinct cold and moist bias in the upper troposphere compared to satellite observations, particularly over the subtropics. Temperature biases are a common feature in many climate models and complicate the interpretation of radiance-based comparisons with satellite data. We introduce a new method for evaluating the water vapor distribution which combines both HIRS 6.7 μm and SSM/T-2 183.31 ± 1 GHz channels and is much less sensitive to tropospheric temperature biases. Using this method, we show that GFDL climate model has a more humid upper troposphere over dry subtropical area than ERA-40 reanalysis. The geographical distribution of the humidity bias is found to exhibit a close association with differences in the 500 hPa vertical pressure velocity, suggesting that much of the bias in tropical upper tropospheric relative humidity can be attributed to errors in simulating the intensity of large-scale tropical circulation. Given the strong dependence of upper tropospheric water vapor on the large-scale circulation, these results suggest that long-term monitoring of upper tropospheric water vapor from satellites may also offer insight into variations in the large-scale atmospheric circulation.

Description: Projected changes in the Earth system will likely be manifested in changes in reflected solar radiation. This paper introduces an operational Observational System Simulation Experiment (OSSE) to calculate the signals of future climate forcings and feedbacks in top-of-atmosphere reflectance spectra. The OSSE combines simulations from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report for the NCAR Community Climate System Model (CCSM) with the MODTRAN radiative transfer code to calculate reflectance spectra for simulations of current and future climatic conditions over the 21st century. The OSSE produces narrowband reflectances and broadband fluxes, the latter of which have been extensively validated against archived CCSM results. The shortwave reflectance spectra contain atmospheric features including signals from water vapor, liquid and ice clouds, and aerosols. The spectra are also strongly influenced by the surface bidirectional reflectance properties of predicted snow and sea ice and the climatological seasonal cycles of vegetation. By comparing and contrasting simulated reflectance spectra based on emissions scenarios with increasing projected and fixed present-day greenhouse gas and aerosol concentrations, we find that prescribed forcings from increases in anthropogenic sulfate and carbonaceous aerosols are detectable and are spatially confined to lower latitudes. Also, changes in the intertropical convergence zone and poleward shifts in the subsidence zones and the storm tracks are all detectable along with large changes in snow cover and sea ice fraction. These findings suggest that the proposed NASA Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission to measure shortwave reflectance spectra may help elucidate climate forcings, responses, and feedbacks.

Description: Between May 2008 and September 2009 the cloud condensation nuclei (CCN) number concentration, NCCN, was measured at the high alpine site Jungfraujoch, which is located in the free troposphere most of the time. Measurements at 10 different supersaturations (0.12%–1.18%) were made using a CCN counter (CCNC). The monthly median NCCN values show a distinct seasonal variability with ∼5–12 times higher values in summer than in winter. The major part of this variation can be explained by the seasonal amplitude of total aerosol number concentration (∼4.5 times higher values in summer), but it is further amplified (factor of ∼1.1–2.6) by a shift of the particle number size distribution toward slightly larger sizes in summer. In contrast to the extensive properties, the monthly median of the critical dry diameter, above which the aerosols activate as CCN, does not show a seasonal cycle (relative standard deviations of the monthly median critical dry diameters at the different supersaturations are 4–9%) or substantial variability (relative standard deviations of individual data points at the different supersaturations are less than 18–37%). The mean CCN-derived hygroscopicity of the aerosol corresponds to a value of the hygroscopicity parameter $\kappa$ of 0.20 (assuming a surface tension of pure water) with moderate supersaturation dependence. NCCN can be reliably predicted throughout the measurement period with knowledge of the above-mentioned averaged $\kappa$ value and highly time-resolved (∼5 min) particle number size distribution data. The predicted NCCN was within 0.74 to 1.29 times the measured value during 80% of the time (94,499 data points in total at 10 different supersaturations).

Description: Wind, temperature, and humidity observations from radiosonde and aircraft are the main sources of upper air information for meteorology. For mesoscale meteorology, the horizontal coverage of radiosondes is too sparse. Aircraft observations through Aircraft Meteorological Data Relay (AMDAR) sample an atmospheric profile in the vicinity of airports. However, not all aircraft are equipped with AMDAR or have the system activated. Observations inferred from an enhanced tracking and ranging (TAR) air traffic control radar can fill this gap. These radars follows all aircraft in the airspace visible to the radar for air traffic management. The TAR radar at Schiphol airport in Netherlands has a range of 270 km. This Mode-S radar contacts each aircraft every 4 s on which the transponder in the aircraft responds with a message that contains information on flight level, direction, and speed. Combined with the ground track of an aircraft, meteorological information on temperature and wind can be inferred from this information. Because all aircraft are required to respond to the TAR radar, the data volume is extremely large, being around 1.5 million observations per day. Note that there are no extra costs for this data link. The quality of these observations is assessed by comparison to numerical weather prediction (NWP) model information, AMDAR observations, and radiosonde observations. A preprocessing step is applied to enhance the quality of wind and temperature observations, albeit with a reduced time frequency of one observation of horizontal wind vector and temperature per aircraft per minute. Nevertheless, the number of observations per day is still very large. In this paper it is shown that temperature observations from Mode-S, even after corrections, are not very good; an RMS which is twice as large as AMDAR is observed when compared to NWP. In contrast to the temperature observations, the quality found for wind after correction and calibration is good; it is comparable to AMDAR, slightly worse than radiosonde but certainly very valuable for mesoscale NWP.

Description: Large-scale aspects of the Australian tropical climate are analyzed in the CMIP3 models, including means and seasonal variations of temperature, mean sea level pressure, winds, and precipitation as well as interannual variability of precipitation and its association with El Niño–Southern Oscillation (ENSO). A notable characteristic is an enormous range in model skill, but with biases in a number of variables averaging out to a surprisingly skilful representation by the ensemble area average mean. There are some significant geographical biases, with mean precipitation extending too far southward into the continental interior. The model ensemble shows seasonal reversals of low-level easterlies into westerlies and the reverse aloft, and reasonable skill in the location, orientation, and seasonal progression of the low-level monsoon “shear line.” Broadscale features of winds between the equator and the continent in the Australian region are generally too weak, particularly those immediately north and northeast of the continent. Models with greater/lesser wet season precipitation over the continent have stronger/weaker low-level westerlies and more/less intense regional convection. An inverse relationship is also found between precipitation amounts and collocated continental surface temperature. Precipitation biases in models are related to differences in occurrence of convection/suppressed vertical motion, and to related precipitation amounts. Interannual variability in precipitation is too weak in the model ensemble (although model range in variability is considerable). This is not, however, the result of weak correlations with ENSO, which are found to be slightly too strong in most models.

Description: This study evaluates the sensitivity of long-range transport of black carbon (BC) from midlatitude and high-latitude source regions to the Arctic to aging, dry deposition, and wet removal processes using the Geophysical Fluid Dynamics Laboratory (GFDL) coupled chemistry and climate model (AM3). We derive a simple parameterization for BC aging (i.e., coating with soluble materials) which allows the rate of aging to vary diurnally and seasonally. Slow aging during winter permits BC to remain largely hydrophobic throughout transport from midlatitude source regions to the Arctic. In addition, we apply surface-dependent dry deposition velocities and reduce the wet removal efficiency of BC in ice clouds. The inclusion of the above parameterizations significantly improves simulated magnitude, seasonal cycle, and vertical profile of BC over the Arctic compared with those in the base model configuration. In particular, wintertime concentrations of BC in the Arctic are increased by a factor of 100 throughout the tropospheric column. On the basis of sensitivity tests involving each process, we find that the transport of BC to the Arctic is a synergistic process. A comprehensive understanding of microphysics and chemistry related to aging, dry and wet removal processes is thus essential to the simulation of BC concentrations over the Arctic.

Description: The Bremen Optimal Estimation differential optical absorption spectroscopy (DOAS) (BESD) algorithm for satellite based retrievals of XCO2 (the column-average dry-air mole fraction of atmospheric CO2) has been applied to Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) data. It uses measurements in the O2-A absorption band to correct for scattering of undetected clouds and aerosols. Comparisons with precise and accurate ground-based Fourier transform spectrometer (FTS) measurements at four Total Carbon Column Observing Network (TCCON) sites have been used to quantify the quality of the new SCIAMACHY XCO2 data set. Additionally, the results have been compared to NOAA's assimilation system CarbonTracker. The comparisons show that the new retrieval meets the expectations from earlier theoretical studies. We find no statistically significant regional XCO2 biases between SCIAMACHY and the FTS instruments. However, the standard error of the systematic differences is in the range of 0.2 ppm and 0.8 ppm. The XCO2 single-measurement precision of 2.5 ppm is similar to theoretical estimates driven by instrumental noise. There are no significant differences found for the year-to-year increase as well as for the average seasonal amplitude between SCIAMACHY XCO2 and the collocated FTS measurements. Comparison of the year-to-year increase and also of the seasonal amplitude of CarbonTracker exhibit significant differences with the corresponding FTS values at Darwin. Here the differences between SCIAMACHY and CarbonTracker are larger than the standard error of the SCIAMACHY values. The difference of the seasonal amplitude exceeds the significance level of 2 standard errors. Therefore, our results suggest that SCIAMACHY may provide valuable additional information about XCO2, at least in regions with a low density of in situ measurements.

Description: Latitudinal and interhemispheric differences of model results on trends in mesospheric ice layers and background conditions are analyzed. The model nudges to European Centre for Medium-Range Weather Forecasts data below ∼45 km. Greenhouse gas concentrations in the mesosphere are kept constant. Temperature trends in the mesosphere mainly come from shrinking of the stratosphere and from dynamical effects. Water vapor increases at noctilucent cloud (NLC) heights and decreases above due to increased freeze drying caused by temperature trends. There is no tendency for ice clouds in the Northern Hemisphere for extending farther southward with time. Trends of NLC albedo are similar to satellite measurements, but only if a time period longer than observations is considered. Ice cloud trends get smaller if albedo thresholds relevant to satellite instruments are applied, in particular at high polar latitudes. This implies that weak and moderate NLC is favored when background conditions improve for NLC formation, whereas strong NLC benefits less. Trends of ice cloud parameters are generally smaller in the Southern Hemisphere (SH) compared to the Northern Hemisphere (NH), consistent with observations. Trends in background conditions have counteracting effects on NLC: temperature trends would suggest stronger ice increase in the SH, and water vapor trends would suggest a weaker increase. Larger trends in NLC brightness or occurrence rates are not necessarily associated with larger (more negative) temperature trends. They can also be caused by larger trends of water vapor caused by larger freeze drying, which in turn can be caused by generally lower temperatures and/or more background water. Trends of NLC brightness and occurrence rates decrease with decreasing latitude in both hemispheres. The latitudinal variation of these trends is primarily determined by induced water vapor trends. Trends in NLC altitudes are generally small. Stratospheric temperature trends vary differently with altitude in the NH and SH but add up to similar trends at mesospheric cloud heights.

Description: Changes in the galactic cosmic ray (GCR) flux due to variations in solar activity may provide an indirect connection between the Sun's and the Earth's climates. Epoch superpositional (composite) analyses of high-magnitude GCR fluctuations, known as Forbush decrease (FD) events, have been widely used to test this hypothesis, with varied results. This work provides new information regarding the interpretation of this approach, suggesting that FD events do not isolate the impacts of GCR variations from those of solar irradiance changes. On average, irradiance changes of ∼0.4 W m−2 outside the atmosphere occur around 2 days in advance of FD-associated GCR decreases. Using this 2 day gap to separate the effects of irradiance variations from GCR variations on cloud cover, we demonstrate small, but statistically significant, anomalous cloud changes occurring only over areas of the Antarctic plateau in association with the irradiance changes, which previous workers had attributed to GCR variations. Further analysis of the sample shows that these cloud anomalies occurred primarily during polar darkness, precluding the possibility of a causal link to a direct total solar irradiance effect. This work suggests that previous FD-based studies may have ineffectively isolated the impacts of GCR variations on the Earth's atmosphere.

Description: Scores of modeling studies have shown that increasing greenhouse gases in the atmosphere impact the global hydrologic cycle; however, disagreements on regional scales are large, and thus the simulated trends of such impacts, even for regions as large as the tropics, remain uncertain. The present investigation attempts to examine such trends in the observations using satellite data products comprising Global Precipitation Climatology Project precipitation and International Satellite Cloud Climatology Project cloud and radiation. Specifically, evolving trends of the tropical hydrological cycle over the last 20–30 years were identified and analyzed. The results show (1) intensification of tropical precipitation in the rising regions of the Walker and Hadley circulations and weakening over the sinking regions of the associated overturning circulation; (2) poleward shift of the subtropical dry zones (up to 2° decade−1 in June-July-August (JJA) in the Northern Hemisphere and 0.3–0.7° decade−1 in June-July-August and September-October-November in the Southern Hemisphere) consistent with an overall broadening of the Hadley circulation; and (3) significant poleward migration (0.9–1.7° decade−1) of cloud boundaries of Hadley cell and plausible narrowing of the high cloudiness in the Intertropical Convergence Zone region in some seasons. These results support findings of some of the previous studies that showed strengthening of the tropical hydrological cycle and expansion of the Hadley cell that are potentially related to the recent global warming trends.

Description: We use acoustic (3.3–500 Hz) arrays to locate local (

Description: Vertical dust flux parameterizations were assessed by implementing three different dust emission schemes, namely, those of Marticorena and Bergametti (1995), Lu and Shao (1999), and Shao (2004) (hereinafter referred to as MB, LS, and S04 schemes, respectively) in Weather Research and Forecasting with Chemistry (WRF/Chem). Through sensitivity tests, the scattering of vertical dust fluxes resulting from different parameterizations was shown even under the condition of same horizontal sand flux. The difference between the estimated vertical dust fluxes of three emission schemes ranges from the order of 101 for sand to the order of 102 for clay. The MB scheme generally produces higher dust emissions than the LS and S04 schemes, and the difference is the greatest for clay because the MB scheme considers vertical dust flux to be related to clay content, while the LS and S04 schemes consider it to be inversely proportional to surface hardness. To investigate the performance of each dust emission scheme in the simulation of Asian dust events, a case study was carried out for a severe Asian dust event that took place between 30 March and 1 April 2007. Simulation results reproduced the outbreak and transport pattern of dust plumes satisfactorily. However, the estimated dust emission amounts in each scheme differed greatly, particularly in loamy soil. The total dust emission amounts averaged for the main dust source region in this Asian dust event for five consecutive days are 84 Tg, 149 Tg, and 532 Tg for the LS, S04, and MB schemes, respectively.

Description: Moisture budget components over a rectangular region defined by the longitudes 6.0°W–36.0°E and latitudes 30.0°N to 45.0°N, with an area of about 6.08 × 106 km2 over the Mediterranean (Med) Basin, are studied by the use of the Japan Meteorological Agency super-high-resolution (20 km) GCM monthly mean data. The research time periods are 1979–2007 for current run and 2075–2099 for future run. Six rainy months of October to March with a total of 168 months for the current run and 144 months for the future run were selected. The rain months have been categorized into five groups of months based on the mean monthly rainfall amounts where the five groups are P 〈 1.0, 1.0 ≤ P 〈 1.5, 1.5 ≤ P 〈 2.0, 2.0 ≤ P 〈 2.5, and 2.5 mm/d ≤ P. We found that generally, over the Mediterranean, the outflow-inflow is balancing the independently calculated evaporation-precipitation quite well with a correlation coefficient of about 0.89. The present seasonal (October-March) precipitation simulated from the 20 km GCM showed a quite reasonable agreement with the CRU. The seasonal area mean precipitation and evaporation are 1.85 mm/d and 2.44 mm/d, respectively. The largest two precipitation categories contribute over 50% of the total seasonal rainfall. The evaporation varies positively with the precipitation for all precipitation categories. Also, the relatively high mean recycling ratio (55%) indicates that the local Med evaporation has a central role in the local precipitation. Another important finding is that the decreasing trend of recycling ratio with the rising of the precipitation category implies that the outside moisture inflow role increases with the increase of the precipitation category. For all the precipitation categories, the total outflow is larger than the total inflow, indicating that the Med area is an important source of moisture. Individual boundary moisture flux shows that the main moisture comes from the west boundary and contributes 59% of the total inflow, while the main outflow is through east boundary and is responsible for 46% of total outflow. Analysis of monthly precipitation indicates that the October and November have the two largest amount of precipitation over the research region. The moisture budget study separated for the east and the west Med shows that the area mean precipitation for the east and the west Med are 2.14 and 2.29 mm/d, while the evaporation are 4.48 and 3.59 mm/d. The plausible reason for the differences between these two basins has been discussed. The moisture supplies to the east Med is mainly from the west boundary, while for the west Mediterranean, the north boundary inflow also plays an important role along with the west boundary. The future moisture budget components over Med suggest that the precipitation is decreasing from 1.85 to 1.62 mm/d and the evaporation is increasing from 2.44 to 2.56 mm/d between current and future. Another finding is that the largest precipitation number of months decreases from 12% to only 6% of the total number of months, while the intensity of the precipitation in this category enhances in the future.

Description: This study presents a global assessment of the sensitivity of droplet number to diabatic activation (i.e., including effects from entrainment of dry air) and its first-order tendency on indirect forcing and autoconversion. Simulations were carried out with the NASA Global Modeling Initiative (GMI) atmospheric and transport model using climatological metereorological fields derived from the former NASA Data Assimilation Office (DAO), the NASA Finite volume GCM (FVGCM) and the Goddard Institute for Space Studies version II' (GISS) GCM. Cloud droplet number concentration (CDNC) is calculated using a physically based prognostic parameterization that explicitly includes entrainment effects on droplet formation. Diabatic activation results in lower CDNC, compared to adiabatic treatment of the process. The largest decrease in CDNC (by up to 75%) was found in the tropics and in zones of moderate CCN concentration. This leads to a global mean effective radius increase between 0.2–0.5 μm (up to 3.5 μm over the tropics), a global mean autoconversion rate increase by a factor of 1.1 to 1.7 (up to a factor of 4 in the tropics), and a 0.2–0.4 W m−2 decrease in indirect forcing. The spatial patterns of entrainment effects on droplet activation tend to reduce biases in effective radius (particularly in the tropics) when compared to satellite retrievals. Considering the diabatic nature of ambient clouds, entrainment effects on CDNC need to be considered in GCM studies of the aerosol indirect effect.

Description: In this paper we examine the variations of the boreal summer season sea breeze circulation along the Florida panhandle coast from relatively high resolution (10 km) regional climate model integrations. The 23 year climatology (1979–2001) of the multidecadal dynamically downscaled simulations forced by the National Centers for Environmental Prediction–Department of Energy (NCEP-DOE) Reanalysis II at the lateral boundaries verify quite well with the observed climatology. The variations at diurnal and interannual time scales are also well simulated with respect to the observations. We show from composite analyses made from these downscaled simulations that sea breezes in northwestern Florida are associated with changes in the size of the Atlantic Warm Pool (AWP) on interannual time scales. In large AWP years when the North Atlantic Subtropical High becomes weaker and moves further eastward relative to the small AWP years, a large part of the southeast U.S. including Florida comes under the influence of relatively strong anomalous low-level northerly flow and large-scale subsidence consistent with the theory of the Sverdrup balance. This tends to suppress the diurnal convection over the Florida panhandle coast in large AWP years. This study is also an illustration of the benefit of dynamic downscaling in understanding the low-frequency variations of the sea breeze.

Description: This study investigates the existence of a dipole mode in the sea surface temperatures (SST) over the South Atlantic Ocean (SAO), using observational and reanalysis data sets from 1950 to 2008. Our results demonstrate that an opposite SST mode, the SAO dipole (SAOD) occurs in the SAO as the anomalous surface waters in the northeastern part; that is, the Atlantic Niño sector and the southwestern part off the Argentina-Uruguay-Brazil coast are consistently anticorrelated in all months. A typical SAOD episode has a life cycle of about eight months, although the peak intensity in which the SST anomalies are evidently coupled to atmospheric circulation and precipitation anomaly fields lasts for four months during the austral winter (May–August). This coupled atmosphere-ocean interaction mechanism appears to be unique, distinct from the classical Atlantic Niño and independent of the direct influence of the Pacific Ocean-based El Niño or global SST variability. The SAOD may provide a useful framework for investigating climate variability and for improved predictions especially over parts of Africa and the Americas, and some preliminary results are already indicated, e.g., the SAOD is widely related to precipitation anomalies in these regions particularly during the austral winter.

Description: The perennial (September) Arctic sea ice cover exhibits large interannual variability, with changes of over a million square kilometers from one year to the next. Here we explore the role of changes in Arctic cyclone activity, and related factors, in driving these pronounced year-to-year changes in perennial sea ice cover. Strong relationships are revealed between the September sea ice changes and the number of cyclones in the preceding late spring and early summer. In particular, fewer cyclones over the central Arctic Ocean during the months of May, June, and July appear to favor a low sea ice area at the end of the melt season. Years with large losses of sea ice are characterized by abnormal cyclone distributions and tracks: they lack the normal maximum in cyclone activity over the central Arctic Ocean, and cyclones that track from Eurasia into the central Arctic are largely absent. Fewer storms are associated with above-average mean sea level pressure, strengthened anticyclonic winds, an intensification of the transpolar drift stream, and reduced cloud cover, all of which favor ice melt. It is also shown that a strengthening of the central Arctic cyclone maximum helps preserve the ice cover, although the association is weaker than that between low cyclone activity and reduced sea ice. The results suggest that changes in cyclone occurrence during late spring and early summer have preconditioning effects on the sea ice cover and exert a strong influence on the amount of sea ice that survives the melt season.

Description: Measurements of black carbon (BC) and organic carbon (OC) were conducted in Bangkok during 2007–2008. Annual trends of BC and OC show strong seasonality with lower and higher concentrations during wet and dry seasons, respectively. Flow of cleaner air, wet removal, and negligible biomass burning resulted in the lowest concentrations of aerosols in the wet season. In addition to anthropogenic sources, long-range transport and biomass burning caused higher concentrations in the dry and hot seasons, respectively. Despite extensive biomass burning in the hot season, moderate levels of aerosols were due to the mixing with air masses from the Pacific Ocean. Diurnal distributions exhibit peaks during rush hour marked by minima in the OC/BC ratio and stagnant wind flow. The lowest concentrations in the afternoon hours could be due to deeper planetary boundary layer and reduced traffic. Overall, the concentrations of both BC and OC decrease with the increase in wind speed. The weekend effects, due to reduced emission during weekends, in the concentrations of both BC and OC were significant. Therefore, stricter abatement in vehicular emissions could substantially reduce pollution. A slope of ΔBC/ΔCO of 9.8 ngm−3 ppbv−1 for the wet season represents the emission ratio from vehicular sources. The highest of ΔOC/ΔBC (3 μg μg−1) in the hot season was due to the predominant influence of biomass burning and significant formation of secondary OC. The levels of BC and OC in Bangkok fall within the ranges of their concentrations measured in the major cities of East Asia.

Description: An obstacle to the simultaneous use of near-infrared (NIR) and thermal infrared (TIR) observations from the Measurements of Pollution in the Troposphere (MOPITT) instrument has been a lack of understanding of NIR radiance errors. Retrieval uncertainties produced by optimal estimation-based retrieval algorithms used for satellite instruments like MOPITT are only meaningful if radiance error statistics are accurately quantified in the measurement error covariance matrix. MOPITT's gas correlation radiometers are subject to a unique form of “geophysical noise” due to the combined effects of (1) translational motion of the instrumental field of view during a single observation and (2) fine-scale spatial variability of surface radiative properties. We describe and demonstrate a new method for quantifying this source of error for each observation. Both TIR and NIR radiance errors due to this effect are highly variable, especially over land, but are qualitatively consistent with the variability of Moderate Resolution Imaging Spectroradiometer radiances in similar spectral bands. In addition, retrieval algorithm modifications are described which adjust the trade-off between smoothing error and retrieval noise within the optimal estimation framework. These modifications are necessary to fully exploit the information in MOPITT's NIR channels. A case study based on MOPITT observations over Minnesota demonstrates significant improvement in retrieval performance as the result of the retrieval algorithm modifications.

Description: The present study investigates the interdecadal change in winter (January-February-March, or JFM) rainfall over South China and in the South China JFM rainfall–sea surface temperature (SST) relationship by using station observations for the period of 1958–2002, the Met Office Hadley Centre's SST data for the period of 1900–2008, and the ERA-40 reanalysis for the period of 1958–2002. It is found that the relationship between South China JFM rainfall and SST experienced an obvious interdecadal change around the year 1978. The analyses show that the JFM rainfall anomalies during 1960–1977 and 1978–2002 were closely associated with the South China Sea (SCS) SST and El Niño–Southern Oscillation (ENSO), respectively. Moreover, southwesterly anomalies at 700 hPa dominate over the SCS for positive SCS SST anomaly years during 1960–1977 and for El Niño years during 1978–2002, respectively. These wind anomalies, which are associated with the enhancement of the western Pacific subtropical high, transport more moisture into South China, favoring increases in rainfall.

Description: Past studies have suggested that possible climate change would be most pronounced and perhaps most easily detected in the arctic regions of the globe. Furthermore, since these changes would be amplified during the winter months, it follows that their signature could appear in extreme cold arctic air masses. To explore this likely possibility, we analyzed National Centers for Environmental Prediction reanalysis surface temperature data spanning a 60 year time period over the source regions for the formation of arctic air masses in North America, and we were able to find variability within the coldest arctic air masses on a decadal timescale. Extreme cold arctic air masses have warmed at least in terms of surface temperature since the beginning of the data record. Extreme cold air arctic outbreaks have decreased in frequency and duration over time. In addition, we have identified criteria that could be used to classify cold air masses in terms of depth and strength, such as lowest kilometer temperature change and inversion height. The 850 and 700 mbar results confirm that inversions are deeper with colder surface temperatures, and the strongest depth indicators are the lapse rates at least in the lowest kilometer; the lapse rates are stronger with colder surface temperatures. Differences between air masses with strong and weak lapse rates in the lowest kilometer are visible on the synoptic scale on sea level pressure maps. No significant relationship between the depth of cold air masses and wind shear was discovered. Finally, a cold bias in the reanalysis data over observations was found in the lowest kilometer of the atmosphere.

Description: Recent observations and theoretical work suggest that the 2 day planetary wave in the summertime mesosphere is composed of multiple superposed zonal wave numbers. Here we use EOS Aura Microwave Limb Sounder (MLS) temperature data to determine the component zonal wave numbers of the 2 day wave in the mesosphere at latitudes of 70°S to 70°N from 2004 to 2009. We consider the effect of aliasing between different wave numbers and note that significant aliasing can occur and result in spurious signals, particularly at high latitudes in winter. The seasonal evolution of the different wave numbers is investigated and found to be very different between the Northern and Southern Hemispheres. In both hemispheres the wave is dominated by westward traveling waves of zonal wave number 3 and 4 (W3 and W4). However, in the Southern Hemisphere the wave is dominated by the W3 component, but in the Northern Hemisphere the W3 component is smaller and the W4 component is often of similar or larger amplitude. A small-amplitude westward traveling zonal wave number 2 (W2) wave is also evident in both hemispheres. In the Northern Hemisphere, the W2 amplitudes never exceed 3 K, the W3 amplitudes can reach 3.5 K, and the W4 can be the largest component, reaching amplitudes of 4 K. In the Southern Hemisphere, the W2 amplitudes can reach up to 3.5 K, the W3 amplitudes can be much larger, reaching 12 K, and the W4 amplitudes are smaller than in the Northern Hemisphere, in 4 out of 5 years not exceeding 3 K. The Northern Hemisphere W4 can reach large amplitudes in August when the W3 is small, which means that the late summer Northern Hemisphere quasi-2 day wave is usually a W4 oscillation rather than the familiar W3. In contrast, in the Southern Hemisphere, the W3 is often larger than the W4 around the summer solstice, and there are no episodes observed where the wave becomes dominated by the W4 for an extended period of time. A high degree of interannual variability is evident, particularly in the Southern Hemisphere, where the W3 peak amplitudes vary from 12 K in January 2006 to 3 K in January 2009. The height-latitude structure of the W4 suggests that this wave is a (4, 0) Rossby-gravity wave.

Description: Airborne measurements of aerosol and cloud condensation nuclei (CCN) were conducted aboard the National Oceanic and Atmospheric Administration WP-3D platform during the 2006 Texas Air Quality Study/Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS). The measurements were conducted in regions influenced by industrial and urban sources. Observations show significant local variability of CCN activity (CCN/CN from 0.1 to 0.5 at s = 0.43%), while variability is less significant across regional scales (∼100 km × 100 km; CCN/CN is ∼0.1 at s = 0.43%). CCN activity can increase with increasing plume age and oxygenated organic fraction. CCN measurements are compared to predictions for a number of mixing state and composition assumptions. Mixing state assumptions that assumed internally mixed aerosol predict CCN concentrations well. Assuming organics are as hygroscopic as ammonium sulfate consistently overpredicted CCN concentrations. On average, the water-soluble organic carbon (WSOC) fraction is 60 ± 14% of the organic aerosol. We show that CCN closure can be significantly improved by incorporating knowledge of the WSOC fraction with a prescribed organic hygroscopicity parameter ($\kappa$ = 0.16 or effective $\kappa$ ∼ 0.3). This implies that the hygroscopicity of organic mass is primarily a function of the WSOC fraction. The overall aerosol hygroscopicity parameter varies between 0.08 and 0.88. Furthermore, droplet activation kinetics are variable and 60% of particles are smaller than the size characteristic of rapid droplet growth.

Description: An expression is derived from first principles for the refractivity of air at L band frequencies, which includes GPS, as well as other GNSS satellite radionavigation signals. Under conditions of pressure, temperature, and moisture content found in the Earth's atmosphere, the expression has an average relative error of approximately 0.01%. This level of accuracy is required to guarantee that the expression does not introduce bias, when it is used within the context of Numerical Weather Prediction (NWP) applications. The thermodynamic dependences of the air's refractivity N are revisited, and the possible sources of uncertainty are analyzed. A first principles microphysical model is constructed, which relates the refractivity at L band frequencies with several measurable properties of matter. The experimental values that are critical for this purpose are already available in the literature and are of high accuracy. Based on this model, a simple expression suitable for atmospheric and weather applications is proposed: N ≡ (n − 1) · 106 = N0 · (1 + $\frac{10^{-6}}{6}$ N0) where N0 = (222.682 + 0.069 · τ) · ρd + (6701.605 + 6385.886 · τ) · ρw with ρd and ρw as the densities of dry air and water vapor in the air (kg/m3), τ = 273.15/T − 1, and T as the absolute temperature in K. The dependence of the coefficients in the expression with respect to the input physical parameters is analyzed. Given the error of the experimental parameters, it is concluded that the proposed expression improves the accuracy to meet the needs of NWP applications.

Description: The influences of the springtime northern Indian biomass burning are shown for the first time over the central Himalayas by using three years (2007–2009) of surface and space based observations along with a radiative transfer model. Near-surface ozone, black carbon (BC), spectral aerosol optical depths (AODs) and the meteorological parameters are measured at a high altitude site Nainital (29.37°N, 79.45°E, 1958 m amsl) located in the central Himalayas. The satellite observations include the MODIS derived fire counts and AOD (0.55 μm), and OMI derived tropospheric column NO2, ultraviolet aerosol index and single scattering albedo. MODIS fire counts and BC observations are used to identify the fire-impacted periods (372 h during 2007–2009) and hence the induced enhancements in surface BC, AOD (0.5 μm) and ozone are estimated to be 1802 ng m−3 (∼145%), 0.3 (∼150%) and 19 ppbv (∼34%) respectively. Large enhancements (53–100%) are also seen in the satellite derived parameters over a 2° × 2° region around Nainital. The present analysis highlights the northern Indian biomass burning induced cooling at the surface (−27 W m−2) and top of the atmosphere (−8 W m−2) in the lesser polluted high altitude regions of the central Himalayas. This cooling leads to an additional atmospheric warming of 19 W m−2 and increases the lower atmospheric heating rate by 0.8 K day−1. These biomass burning induced changes over the central Himalayan atmosphere during spring may also lead to enhanced short-wave absorption above clouds and might have an impact on the monsoonal rainfall.

Description: Photophoretic forces act on a particle which has an inhomogeneous surface, especially with respect to the accommodation coefficient. Gas molecules incident on it leave the surface after more or less accommodation, resulting in a force and torque on the particle. Due to Brownian rotation of the particle the net force is zero. If the particle is also exposed to an orientating torque, the photophoretic force points in a preferred direction resulting in net photoporetic force. The orienting torque can be caused by gravity or by the earth magnetic field. Photophoretic forces are important if the mean free path of the gas molecules is larger than the size of the particles. This is the case for all particles found in the stratosphere and mesosphere. The direction of the photophoretic force can be opposite to gravity and it can exceed the weight of the particle. In that case the particle is lifted upwards, until it reaches conditions where the two forces are equal. The stratification of the stratospheric and mesospheric aerosol is given fact. The temperature profile of the atmosphere, the incoming solar flux density, and the emitted terrestrial infrared radiation, are used to investigate systematically the role of photophoresis in the formation of aerosol layers at specific altitudes for various types of particles around the globe. Photophoresis can be responsible for aerosol layer formation at altitudes of 20, 30, 50, 70 and 85 km. The layers form in specific geographic regions and at certain seasons.

Description: In this work absolute values of gravity wave (GW) momentum flux are derived from global temperature measurements by the satellite instruments High Resolution Dynamics Limb Sounder (HIRDLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). Momentum fluxes in the stratosphere are derived for both instruments and for SABER in the whole mesosphere. The large-scale atmospheric background state is removed by a two-dimensional Fourier decomposition in longitude and time, covering even planetary-scale waves with periods as short as 1–2 days. Therefore, it is possible to provide global distributions of GW momentum flux from observations for the first time in the mesosphere. Seasonal as well as longer-term variations of the global momentum flux distribution are discussed. GWs likely contribute significantly to the equatorward tilt of the polar night jet and to the poleward tilt of the summertime mesospheric jet. Our results suggest that GWs can undergo large latitudinal shifts while propagating upward. In particular, GWs generated by deep convection in the subtropical monsoon regions probably contribute significantly to the mesospheric summertime wind reversal at mid- and high latitudes. Variations in the GW longitudinal distribution caused by those convectively generated GWs are still observed in the mesosphere and could be important for the generation of the quasi two-day wave. Indications for quasi-biennial oscillation (QBO) induced variations of GW momentum flux are found in the subtropics. Also variations at time scales of about one 11-year solar cycle are observed and might indicate a negative correlation between solar flux and GW momentum flux.

Description: Overshooting deep convection plays an important role in regulating the water vapor content of the tropical tropopause layer and is also an important mechanism for transporting water vapor into the lower stratosphere (LS). The aim of this study is to examine the effect of aerosols as cloud condensation nuclei (CCN) on the water vapor content of the LS via single isolated overshooting thunderstorms. The development of a severe Hector thunderstorm in northern Australia observed during the Stratospheric-Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere/Aerosol and Chemical Transport in Tropical Convection (SCOUT-O3/ACTIVE) campaign is simulated using a three-dimensional nonhydrostatic convective cloud model with a double-moment bulk microphysics scheme. The results show that the ice hydrometeors account for over 50% of the total condensate mass, indicating that ice processes play an important role in regulating the structure of thunderstorms in the tropics. A large number of ice particles occurring in the LS are not formed in situ but are transported upward by convective overshooting with subsequent mixing. Sensitivity tests show that the increase in cloud droplet numbers induced by increasing CCN concentrations would increase the number concentrations of the ice crystals transported to the LS, which had the effect of reducing the sizes and fall speeds of the ice crystal, thereby causing a moistening of the LS by sublimation of the injected ice particles. This result suggests that aerosols in the boundary layer can affect stratospheric water vapor via overshooting deep convection.

Description: By using numerical experiments and observational data, this study examined the uplifting and thermal effects of the Tibetan Plateau (TP) on downstream airflow in early summer. Our principal finding is that the uplifting effect of the TP in an Atmospheric General Climate Model (AGCM), including air made warmer than its surroundings climatologically by the huge topography, results mainly in a local response in the atmosphere, i.e., a large ridge north of the TP in the troposphere in June. There was no Rossby wave response to the uplifting effect. However, simulations and statistical analyses strongly suggested that the anomalous TP atmospheric heating associated with global climate warming tends to excite a Rossby wave originating from the TP via Lake Baikal and continuing to move through the Okhotsk Sea to downstream areas. The appearance of the Rossby wave coincides with the positive phase of the eastern part of a normal stationary wave originating in the Caspian Sea traveling via the Okhotsk Sea to the sea area east of Japan that often occurs in June. Thus the TP atmospheric heating acts as an additional wave source in relaying and enhancing the eastern part of the normal wave propagation. Its path usually lies beyond 40°N latitude, which is where the westerly jet stream takes over the role of waveguide.

Description: Instantaneous estimates of the power released by fire (fire radiative power, FRP) are available with satellite active fire detection products. The temporal integral of FRP provides an estimate of the fire radiative energy (FRE) that is related linearly to the amount of biomass burned needed by the atmospheric emissions modeling community. The FRE, however, is sensitive to satellite temporal and spatial FRP undersampling due to infrequent satellite overpasses, cloud and smoke obscuration, and failure to detect cool and/or small fires. Satellite FRPs derived over individual burned areas and fires have been observed to exhibit power law distributions. This property is exploited to develop a new way to derive FRE, as the product of the fire duration and the expected FRP value derived from the FRP power law probability distribution function. The method is demonstrated and validated by the use of FRP data measured with a dual-band radiometer over prescribed fires in the United States and by the use of FRP data retrieved from moderate resolution imaging spectroradiometer (MODIS) active-fire detections over Brazilian deforestation and Australian savanna fires. The biomass burned derived using the conventional FRP temporal integration and power law FRE estimation methods is compared with biomass burned measurements (prescribed fires) and available fuel load information reported in the literature (Australian and Brazilian fires). The results indicate that the FRE power law derivation method may provide more reliable burned biomass estimates under sparse satellite FRP sampling conditions and correct for satellite active-fire detection omission errors if the FRP power law distribution parameters and the fire duration are known.

Description: Using the SSW (Stratospheric Sudden Warming) event in 2009 as a representative case, the temporal evolution of the responses of OH and O2 infrared atmospheric (0–0) nightglow emissions to SSW events is analyzed using the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)/SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) data. The results show that during the mesospheric cooling that occurs during the stratospheric warming stage of SSW events, the brightness of OH and O2 nightglow emissions and the thicknesses of OH and O2 emission layers decrease noticeably and the peak heights of the emissions ascend. During the recovery stage in the mesosphere, the brightness of both nightglow emissions and the thicknesses of the emission layers increase dramatically and the peak heights of the emissions descend. These emission variations are mainly caused by perturbations in temperature and the transport of O in the MLT (Mesosphere Lower Thermosphere) region. For the SSW event that started in January 2009, the onset times of the cooling stage and recovery stage in the mesosphere are ∼2 days ahead of the onset times of the warming stage and recovery stage of the SSW event, respectively. For this event, the influence of the SSW on the OH and O2 nightglow emissions increases with latitude between 50°N and 80°N.

Description: The ice cloud estimates in current global models exhibit significant inconsistency, resulting in a significant amount of uncertainties in climate forecasting. Vertically resolved ice water content (IWC) is recently available from new satellite products, such as CloudSat, providing important observational constraints for evaluating the global models. To account for the varied nature of the model parameterization schemes, it is valuable to develop methods to distinguish the cloud versus precipitating ice components from the remotely sensed estimates in order to carry out meaningful model-data comparisons. The present study develops a new technique that partitions CloudSat total IWC into small and large ice hydrometeors, using the ice particle size distribution (PSD) parameters provided by the retrieval algorithm. The global statistics of CloudSat-retrieved PSD are analyzed for the filtered subsets on the basis of convection and precipitation flags to identify appropriate particle size separation. Results are compared with previous partitioning estimates and suggest that the small particles contribute to ∼25–45% of the global mean total IWC in the upper to middle troposphere. Sensitivity measures with respect to the PSD parameters and the retrieval algorithm are presented. The current estimates are applied to evaluate the IWC estimates from the European Centre for Medium-Range Weather Forecasts model and the finite-volume multiscale modeling framework model, pointing to specific areas of potential model improvements. These results are discussed in terms of applications to model diagnostics, providing implications for reducing the uncertainty in the model representation of cloud feedback and precipitation.

Description: This study evaluates the simulated cloud properties, especially the simulated raindrop size distribution, by the Chinese Academy of Meteorological Sciences bulk microphysics scheme (CAMS BMS) and two other two-moment microphysics schemes (Morrison and WDM6) in the Weather Research and Forecasting model (WRF v3.1). Measurements from a mesoscale convective system that occurred on 14 June 2008 during the Southwest Monsoon Experiment (SoWMEX) and Terrain-influenced Monsoon Rainfall Experiment (TiMREX) are used. The model reflectivity (ZH), differential reflectivity (ZDR), and microwave brightness temperature (TB) are compared with the corresponding observations by the S band dual-polarization Doppler radar (S-Pol) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). Results show that the simulated ZDR, which is sensitive to the drop size distribution (DSD) of raindrops, from the original CAMS BMS and Morrison schemes are larger than those from the S-Pol observations. On the other hand, the simulated ZDR values from the WDM6 scheme are smaller than the radar observations. To improve the model results, modifications are made by controlling the intercept parameter of raindrop DSD and by increasing the raindrop breakup rates in the original CAMS BMS scheme. The modifications reduce the raindrop size and consequently increase the rain evaporation rate. The improved simulations of ZH and ZDR indicate that the modified CAMS BMS scheme adequately simulates the amount and size of liquid hydrometeors. Moreover, the simulated brightness temperatures at liquid water absorption frequency and the simulated surface precipitation rates are also significantly improved by using the modified CAMS BMS scheme.

Description: This article presents an analysis of the spatiotemporal changes (1960–2100) in temperature and precipitation extremes of a typical arid zone (i.e., the Tarim River Basin) in Central Asia. The latest observations in the past five decades (1960–2009) and Coupled General Circulation Model (CGCM) multimodel ensemble projections (2010–2100) using the Bayesian Model Average (BMA) approach are employed for analysis in this study. Results indicate: (1) Most warm (cold) extreme temperature indices have shown significantly positive (negative) trends in the Tarim River Basin in past five decades, while only slight changes in precipitation extremes can be observed. From the spatial perspective, more significantly warm (cold) extremes are found in the desert zones than in upstream mountain zones (i.e., the Tian Shan Mountain and Kunlun Mountain systems which surround the basin). Whereas, there are no identical spatial patterns for the change in extreme precipitation; (2) Ensemble of five CGCM models in Phase 3 of the Coupled Model Intercomparison Project (CMIP3) based on the BMA method suggests that the increasing consecutive dry days (CDD), together with the decreasing frost day (FD) and increasing warm nights frequency (TN90) may lead to more frequent droughts in Tarim in future. Meanwhile, slight increase of annual count of days with precipitation of more than 10 mm (R10), maximum 5-day precipitation total (R5D), simple daily intensity index (SDII), and annual total precipitation with precipitation 〉95th percentile (R95) in projections indicate a probability of flood occurrence in summer together with frequent occurrence of droughts. The results can provide beneficial reference to water resource and eco-environment management strategies in arid zones for associated policymakers and stakeholders.

Description: This paper compares four selected characteristics (mean annual rainfall; mean number of wet days in the year; mean number of days with 2mm or more of rain; and mean 95% quantile of daily rainfall) computed from the Brazilian rain gauge network in the Amazon region, and from the satellite-derived data sets TRMM 3B42 and CMORPH. Comparisons were made at 488 sites over the years 2003–2005, the period for which all three sources provide data, and rainfall characteristics were calculated only from those days in the year with data from all three sources. TRMM and rain gauges mean annual rainfalls were fairly similar, but CMORPH estimates were greater than either, although the statistical significance of the differences were greatly reduced when spatial correlation was allowed for. However, differences between the mean numbers of days with rain (and those with 2mm or more) calculated from the three sources, were very marked, and the differences usually persisted when the errors in the differences included the effects of spatial correlation. Mean 95% quantile of daily rainfall calculated from CMORPH and from rain gauges were shown to be very similar. Over all 488 sites, however, mean annual rainfall calculated from CMORPH was considerably greater than that calculated from rain gauge records, and this result was explained by an analysis of daily rainfalls that exceeded their 95% quantile. The paper emphasizes that, unless spatial correlation is allowed for, uncertainty in rainfall characteristics will be under-estimated, and the apparent statistical significance of differences between values obtained from alternative data sets will be over-estimated.

Description: Three 10 year ensemble decadal forecast experiments have been performed with the European Centre for Medium-Range Weather Forecasts coupled forecast system using an initialization strategy common in seasonal forecasting with realistic initial conditions. One experiment initializes the ocean in a standard way using an ocean-only simulation forced with an atmospheric reanalysis and with strong relaxation to observed sea surface temperatures. The other two experiments initialize the ocean from a similar ocean-only run that, in addition, assimilates subsurface observations. This is the first time that these experiments were performed. The system drifts from the realistic initial conditions toward the model climate, the drift being of the same order as, if not larger than, the interannual signal. There are small drift differences in the three experiments that reflect mainly the influence of dynamical ocean processes in controlling the adjustment between the initialized state and the model climate in the extratropics. In spite of the drift, the predictions show that the system is able to skillfully predict some of the interannual variability of the global and regional air and ocean temperature. No significant forecast quality benefit of the assimilation of ocean observations is found over the extratropics, although a negative impact of the assimilation of incorrect expendable bathythermograph profiles has been found for the global mean upper ocean heat content and the Atlantic multidecadal oscillation. The results illustrate the importance of reducing the important model drift and the ocean analysis uncertainty.

Description: The Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) satellite was successfully launched in 2006 and has provided an unprecedented opportunity to study cloud and aerosol layers using range-resolved laser remote sensing. Dedicated validation flights were conducted using the airborne Cloud Physics Lidar (CPL) to validate the CALIPSO Level 1 and 2 data products. This paper presents results from coincident CALIPSO and CPL measurements of ice cloud spatial properties. Flight segment case studies are shown as well as statistics for all coincident measurements during the CALIPSO-CloudSat Validation Experiment (CC-VEX). CALIPSO layer detection algorithms for cirrus clouds are reliable in comparison with CPL, with best agreement occurring during nighttime coincident segments when the signal-to-noise ratio (SNR) of both instruments is greatest. However, the two instruments disagree on ice cloud spatial properties in two distinct cases. CALIPSO experiences less sensitivity to optically thin cirrus due to lower SNR when compared to CPL data at identical spatial scales. The incorporation of extended spatial averaging in the CALIPSO layer detection algorithm succeeds in detecting the optically thin cirrus, but the averaging process occasionally results in spatial smearing, both horizontally and vertically, of broken cirrus clouds. The second disparity occurs because, in contrast to CPL, multiple scattering contributes significantly to CALIPSO lidar measurements of cirrus clouds. As a result, the CALIPSO signal penetrates deeper into opaque cirrus clouds, and in these cases CALIPSO will report lower apparent cloud base altitudes than CPL.

Description: In this study, we determined the characteristics of mesospheric wave structures over South Pole Station (90°S) derived from sodium airglow imaging observations. During the winter months of 2003 to 2005 (105 nights), we extracted a total of 768 wave events and separated them into two types (band-type gravity waves and ripples) according to their horizontal wavelengths. The distributions of the observed wave parameters, except for the horizontal propagation directions, were similar to those obtained by imaging observations at other latitudes. The observed gravity waves showed a preference for propagation toward 30°–60°E and 210°–240°E, whereas the ripples showed a preference for motion toward 90°–120°E and 300°–330°E. The gravity waves had a weak tendency of being observed in 0100–0700 UT, although the ripples did not show such a time dependence. We also investigated the characteristics of atmospheric instabilities from the alignment of the phase fronts of the observed ripples.

Description: We present analysis of thunderstorm data collected with a liquid nitrogen–cooled germanium spectrometer with energies between 13 keV–2.6 MeV that was deployed at Langmuir Lab on South Baldy Peak in New Mexico for June through August 2005. The motivation was to search for gamma ray emissions from radioactive chlorine-39 and chlorine-38, as suggested by Greenfield et al. (2003). Based on the observations, we place an upper limit on the rate of chlorine production through such a process (6.8 × 10−17 chlorine atoms per argon atom). This rate is sufficiently low to suggest that the anomalous gamma ray count increases observed by Greenfield et al. (2003) were not caused by radioactive chlorine.

Description: One year of instantaneous top-of-atmosphere (TOA) and surface shortwave and longwave irradiances are computed using cloud and aerosol properties derived from instruments on the A-Train Constellation: the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, the CloudSat Cloud Profiling Radar (CPR), and the Aqua Moderate Resolution Imaging Spectrometer (MODIS). When modeled irradiances are compared with those computed with cloud properties derived from MODIS radiances by a Clouds and the Earth's Radiant Energy System (CERES) cloud algorithm, the global and annual mean of modeled instantaneous TOA irradiances decreases by 12.5 W m−2 (5.0%) for reflected shortwave and 2.5 W m−2 (1.1%) for longwave irradiances. As a result, the global annual mean of instantaneous TOA irradiances agrees better with CERES-derived irradiances to within 0.5W m−2 (out of 237.8 W m−2) for reflected shortwave and 2.6W m−2 (out of 240.1 W m−2) for longwave irradiances. In addition, the global annual mean of instantaneous surface downward longwave irradiances increases by 3.6 W m−2 (1.0%) when CALIOP- and CPR-derived cloud properties are used. The global annual mean of instantaneous surface downward shortwave irradiances also increases by 8.6 W m−2 (1.6%), indicating that the net surface irradiance increases when CALIOP- and CPR-derived cloud properties are used. Increasing the surface downward longwave irradiance is caused by larger cloud fractions (the global annual mean by 0.11, 0.04 excluding clouds with optical thickness less than 0.3) and lower cloud base heights (the global annual mean by 1.6 km). The increase of the surface downward longwave irradiance in the Arctic exceeds 10 W m−2 (∼4%) in winter because CALIOP and CPR detect more clouds in comparison with the cloud detection by the CERES cloud algorithm during polar night. The global annual mean surface downward longwave irradiance of 345.4 W m−2 is estimated by combining the modeled instantaneous surface longwave irradiance computed with CALIOP and CPR cloud profiles with the global annual mean longwave irradiance from the CERES product (AVG), which includes the diurnal variation of the irradiance. The estimated bias error is −1.5 W m−2 and the uncertainty is 6.9 W m−2. The uncertainty is predominately caused by the near-surface temperature and column water vapor amount uncertainties.

Description: A pre-industrial multicentury simulation with Bergen Climate Model Version 2 (BCM in brief) is used to investigate the linkage between the Atlantic Multidecadal Oscillation (AMO) and the Indian Summer Monsoon (ISM). The results suggest that the model reproduces the general characters of the observed linkage between AMO and ISM, and that a positive AMO favors more rainfall over India from July to October. The ISM is intensified and the seasonal withdrawal of ISM delayed with one month, in agreement with previous observational and model's results. Further diagnoses indicate that this impact is achieved through an atmospheric teleconnection pattern. A propagating Rossby wave train from the North Atlantic across South Asia leads to enhanced South Asia high and consequently a strengthening of the ISM.

Description: To assess the properties of aerosol particles generated over the surf zone, two experiments were held at the pier of Scripps Institution of Oceanography (SIO), La Jolla CA, and at the pier of the U.S. Army Corps of Engineers Field Research Facility (FRF) in Duck NC. On both sites concentrations of surf-generated sea spray particles, wave parameters and meteorological conditions were measured. The surf-aerosol concentrations in the diameter range 0.2–10 microns were obtained from the difference in aerosol size distributions measured upwind and downwind of the surf zone. It was found that the flux of surf-generated particles at diameters at formation can be expressed in terms of wave energy dissipation, which itself is related to the properties of the incoming wavefield and the bathymetry of the beach. Although the flux can also be modeled in terms of wind speed, this relation is considered to be not universal and limited to low- to medium wind speeds. In Duck NC, two transport experiments were performed under offshore flow conditions. In this case, the surf-aerosol concentrations were obtained from the differences in three aerosol size distributions, measured just before and just behind the surf zone and up to 16 km downwind (out to sea). No significant decrease in concentration was observed at the farthest range, which suggests that an appreciable amount of surf-generated aerosols is advected over tens of kilometers.

Description: After our previous study about methane (CH4) emissions from littoral marshes of the Three Gorges Reservoir (TGR), Chinese dams have raised a world-wide concern. Through measurements from the surface of the TGR, a CH4 emission rate was recorded as 0.26 ± 0.38 mg CH4 m−2 h−1 (Mean ± SD), relatively low compared with those from other hydropower reservoirs. We also recorded CH4 emission rate from the surface of downstream water, which was also relatively low (0.24 ± 0.37 mg CH4 m−2 h−1). Such result may indicate that TGR is not a great CH4 emitter (not “CH4 menace”). One possible reason for such a low emission rate is that measures to maintain water quality and protect environment and ecosystem decrease the input of organic materials (for methanogenesis), which in turn limits the CH4 production in the sediment of the TGR. We also found that CH4 emission from the flooding drawdown area (0.29 ± 0.37 mg CH4 m−2 h−1) was higher than other permanently flooded sites (0.23 ± 0.38 mg CH4 m−2 h−1). Because of annual vegetation re-growth, the drawdown zone is the especially important carbon source for methanogenesis in flooding season. Interestingly, we also observed that mean CH4 emission was significantly higher in winter than in spring and summer. This was partly due to seasonal dynamics of hydrology. In order to estimate the net CH4 emissions caused by the reservoir and reservoir operation, the best approach would be Life Cycle Analysis.

Description: Estimation of evapotranspiration (ET) over large heterogeneous areas using numerous satellite-based algorithms is increasing; however, further analysis of uncertainties is limited. The objective of this study was to evaluate impacts of varying input variables, size of the modeling domain, and spatial resolution of satellite sensors on sensible heat flux (H) estimates from the Surface Energy Balance Algorithm for Land (SEBAL). First, sensitivity analysis of SEBAL is conducted by varying its input variables using Moderate Resolution Imaging Spectroradiometer (MODIS) data for 29 cloud-free days in 2007 covering the Baiyangdian watershed in North China. Domain dependence of the H estimates is quantified by estimating H for subwatersheds of different sizes and the entire watershed using MODIS data for 4 cloud-free days in May 2007. Landsat Thematic Mapper (TM) and MODIS based H estimates are compared to evaluate the effect of spatial resolution of satellite sensors. Results of sensitivity analysis indicate that the H estimates from SEBAL are most sensitive to temperatures of hot and cold pixels and available energy of the hot pixel. Results of domain dependence show that the mean absolute percentage difference (MAPD) and root mean square deviation (RMSD) in the H estimates between different domain sizes up to 53.9% and 75.7 W m−2, respectively. Although areally averaged H estimates from MODIS and Landsat TM sensors are similar, the MODIS-based H estimates show an RMSD of 52.3 W m−2 and a bias of 26.5 W m−2 relative to Landsat TM-based counterparts. Unlike other models, the standard deviation of H estimates from SEBAL using high spatial resolution images can be smaller than that using low spatial resolution images. Furthermore, H estimates from the input upscaling scheme (aggregating input variables) are generally consistent with those from the output upscaling scheme (aggregating the output) for the same sensor, given similar differences between hot and cold pixels for low and high spatial resolution. The resulting H flux and ET estimates from SEBAL can therefore vary with differing extreme pixels selected by the operator, domain size, and spatial resolution of satellite sensors. This study provides insights into various factors that should be considered when applying SEBAL to estimate ET and helps correctly interpret the SEBAL outputs.

Description: Accurate estimates of basin-wide water and energy cycles are essential for improving the integrated water resources management (IWRM), especially for relatively dry conditions. This study aims to evaluate and apply a fine-resolution global data set (Global Land Data Assimilation System with Noah Land Surface Model, GLDAS/Noah; 3-h, 0.25-degree) in a semiarid mesoscale basin (∼15000 km2). Four supporting objectives are proposed: (1) validating a Water and Energy Budget-based Distributed Hydrological Model (WEB-DHM) for GLDAS/Noah evaluation and application; (2) evaluating GLDAS forcing data (precipitation; near-surface air temperature, Tair; downward shortwave radiation, Rsw,d; downward longwave radiation, Rlw,d); (3) investigating GLDAS/Noah outputs (land surface temperature, LST; evapotranspiration; fluxes); (4) evaluating the applicability of GLDAS forcing in modeling basin-wide water cycles. Japanese 25-year reanalysis and in situ observations (precipitation; Tair; Rsw,d; discharge) are used for GLDAS/Noah evaluation. Main results include: (1) WEB-DHM can reproduce daily discharge, 8-day LST and monthly surface soil moisture (point scale) fairly well; (2) the GLDAS is of high quality for daily and monthly precipitation, Tair, monthly Rlw,d, while it overestimates monthly Rsw,d; (3) the GLDAS/Noah agrees well with the verified WEB-DHM and JRA-25 in terms of LST, upward shortwave and longwave radiation. While the net radiation, evapotranspiration, latent and sensible heat fluxes modeled by GLDAS/Noah are larger than WEB-DHM and JRA-25 simulations in wet seasons; (4) the basin-integrated discharges and evapotranspiration can be reproduced reasonably well by WEB-DHM fed with GLDAS forcing except linear corrections of Rsw,d. These findings would benefit the IWRM in ungauged or poorly gauged river basins around the world.

Description: This paper assesses the impacts of horizontal resolution, snow physics, and atmospheric forcing in snow cover simulations by the European Centre for Medium-Range Weather Forecasts (ECWMF) land surface model Hydrology Tiled ECMWF Scheme for Surface Exchanges (HTESSEL). Off-line simulations are carried out forced by the ECMWF deterministic short-term weather forecasts (WFC) with a resolution of 25 km from March 2006 to June 2010. The horizontal-resolution impact on snow cover is addressed by performing simulations at 25, 80, and 200 km forced by WFC. The impact of atmospheric forcing on snow cover is assessed by forcing the model additionally with the ECMWF Era-Interim (ERAI) reanalysis, at 80 km resolution. Snow physics effects are analyzed by performing an extra simulation forced by WFC using a different snow scheme. The simulations are validated against four independent observational data sets: (1) snow water equivalent (SWE) over Switzerland; (2) snow cover duration in Europe (SNOWCLIM); (3) interactive multisensor snow and ice-mapping system (IMS) snow cover; and (4) Moderate Resolution Imaging Spectroradiometer (MODIS) surface albedo. ERAI forced simulations show a systematic underestimation of SWE and snow cover fraction, which is due to an underprediction of snowfall by ERAI. The snow physics experiment highlights the sensitivity of the model to the partitioning between rainfall and snowfall when rainfall interception in the snowpack is neglected. The horizontal resolution has a crucial role in characterizing the snow cover over complex terrain (e.g., orographic areas, coastal and lakes regions). However, improved snow physical parameterizations and meteorological forcing are shown to be the key elements to achieve more accurate simulations of the snowpack and of the snow-atmosphere interactions, also over flat terrain.

Description: Overlap statistics of cumuliform boundary-layer clouds are studied using large-eddy simulations at high resolutions. The cloud overlap is found to be highly inefficient, due to the typical irregularity of cumuliform clouds over a wide range of scales. The detection of such inefficient overlap is enabled in this study by i) applying fine enough discretizations and ii) by limiting the analysis to exclusively cumuliform boundary-layer cloud fields. It is argued that these two factors explain the differences with some previous studies on cloud overlap. In contrast, good agreement exists with previously reported observations of cloud overlap as derived from lidar measurements of liquid water clouds at small cloud covers. Various candidate functional forms are fitted to the results, suggesting that an inverse linear function is most successful in reproducing the observed behavior. The sensitivity of cloud overlap to various aspects is assessed, reporting a minimal or non-systematic dependence on discretization and vertical wind-shear, as opposed to a strong case-dependence, the latter probably reflecting differences in the cloud size distribution. Finally, calculations with an offline radiation scheme suggest that accounting for the inefficient overlap in cumuliform cloud fields in a general circulation model can change the top-of-atmosphere short-wave cloud radiative forcing by −20 to −40 W m−2, depending on vertical discretization. This corresponds to about 50 to 100% of the typical values in areas of persistent shallow cumulus, respectively.