ALBERT

Description: The diurnal variations from a high-resolution regional climate model (Regional Spectral Model; RSM) are analyzed from 6 independent decade long integrations using lateral boundary forcing data separately from the National Centers for Environmental Prediction Reanalysis 2 (NCEPR2), and European Center for Medium-Range Weather Forecasts (ECMWF) 40-year Reanalysis (ERA40) and the 20 th Century Reanalysis (20CR). With each of these lateral boundary forcing data, the RSM is integrated separately using two convection schemes: the Relaxed Arakawa-Schubert (RAS) and Kain-Fritsch (KF) schemes. The results show that RSM integrations forced with 20CR have the least fidelity in depicting the seasonal cycle and diurnal variability of precipitation and surface temperature over the Southeastern United States (SEUS). The remaining four model simulations show comparable skills. The differences in the diurnal amplitude of rainfall during the summer months of the 20CR forced integration from the corresponding NCEPR2 forced integration, for example, is found to be largely from the transient component of the moisture flux convergence. The root mean square error (RMSE) of the seasonal cycle of precipitation and surface temperature of the other four simulations (not forced by 20CR) were comparable to each other and highest in the summer months. But the RMSE of the diurnal amplitude of precipitation and the timing of its diurnal zenith were largest during winter months and least during summer and fall months in the four model simulations (not forced by 20CR). The diurnal amplitude of surface temperature in comparison showed far less fidelity in all models. The phase of the diurnal maximum of surface temperature however showed significantly better validation with corresponding observations in all of the 6 model simulations

Description: A high-resolution transect of atmospheric soundings across the Kuroshio Current in the East China Sea was conducted onboard a ship in June 2012 with the objective of analyzing the influence of the complex sea surface temperature (SST) distribution on the Baiu frontal zone (BFZ). Expendable bathythermograph castings and continuous surface meteorological observations were also examined. Two distinct mesoscale atmospheric fronts, characterized by changes of wind direction in the lower troposphere and surface air temperature (SAT), were found in the BFZ. One (northern) atmospheric front was observed around the SST front in relation to a warm water tongue extending from the Kuroshio. A high SST region around the northern atmospheric front enhances unstable near surface stratification and intensifies turbulent heat flux. They help modify the marine atmospheric boundary layer in the BFZ. The other (southern) atmospheric front was at the southern end of the BFZ. Intense evaporation over the Kuroshio and moisture transport by southerly winds were important in forming the conditionally unstable air masses in the lower troposphere of the BFZ.

Description: We analyze the variability of mean age of air (AoA) and of the local effects of the stratospheric residual circulation and eddy mixing on AoA within the framework of the isentropic zonal mean continuity equation. AoA for the period 1988–2013 has been simulated with the Lagrangian chemistry transport model CLaMS driven by ERA-Interim winds and diabatic heating rates. Model simulated AoA in the lower stratosphere shows good agreement with both in-situ observations and satellite observations from MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), even regarding interannual variability and changes during the last decade. The interannual variability throughout the lower stratosphere is largely affected by the QBO-induced circulation and mixing anomalies, with year-to-year AoA changes of about 0.5 years. The decadal 2002–2012 change shows decreasing AoA in the lowest stratosphere, below about 450 K. Above, AoA increases in the NH and decreases in the SH. Mixing appears to be crucial for understanding AoA variability, with local AoA changes resulting from a close balance between residual circulation and mixing effects. Locally, mixing increases AoA at low latitudes (40S-40N) and decreases AoA at higher latitudes. Strongest mixing occurs below about 500 K, consistent with the separation between shallow and deep circulation branches. The effect of mixing integrated along the air parcel path, however, significantly increases AoA globally, except in the polar lower stratosphere. Changes of local effects of residual circulation and mixing during the last decade are supportive of a strengthening shallow circulation branch in the lowest stratosphere and a southward shifting circulation pattern above.

Description: The interaction between sea ice and atmosphere depends strongly on the near-surface transfer coefficients for momentum and heat. A parametrization of these coefficients is developed on the basis of an existing parametrization of drag coefficients for neutral stratification that accounts for form drag caused by the edges of ice floes and melt ponds. This scheme is extended to better account for the dependence of surface wind on limiting cases of high and low ice concentration and to include near-surface stability effects over open water and ice on form drag. The stability correction is formulated on the basis of stability functions from Monin-Obukhov similarity theory and also using the Louis concept with stability functions depending on thebulk Richardson numbers. Furthermore, a parametrization is proposed that includes the effect of edge related turbulence also on heat transfer coefficients. The parametrizations are available in different levels of complexity. The lowest level only needs sea ice concentration and surface temperature as input while the more complex level needs additional sea ice characteristics. An important property of our parametrization is that form drag caused by ice edges depends on the stability over both ice and water which is in contrast to the skin drag over ice. Results of the parametrization show that stability has a large impact on form drag and, thereby, determines the value of sea ice concentration for which the transfer coefficients reach their maxima. Depending on the stratification, these maxima can occur anywhere between ice concentrations of 20 and 80%.

Description: The eruption of Mount Pinatubo in 1991 injected a large amount of SO 2 into the stratosphere, which formed sulfate aerosols. Increased scattering and absorption of UV radiation by the enhanced stratospheric SO 2 and aerosols decreased the amount of UV radiation reaching the troposphere, causing changes in tropospheric photochemistry. These changes affected the oxidizing capacity of the atmosphere and the removal rate of CH 4 in the years following the eruption. We use the three-dimensional chemistry transport model TM5 coupled to the aerosol microphysics module M7 to simulate the evolution of SO 2 and sulfate aerosols from the Pinatubo eruption. Their effect on tropospheric photolysis frequencies and concentrations of OH and CH 4 are quantified for the first time. We find that UV attenuation by stratospheric sulfur decreased the photolysis frequencies of both ozone and NO 2 by about 2% globally, decreasing global OH concentrations by a similar amount in the first two years after the eruption. SO 2 absorption mainly affects OH primary production by ozone photolysis, while aerosol scattering also alters OH recycling. The effect of stratospheric sulfur on global OH and CH 4 is dominated by the effect of aerosol extinction, while SO 2 absorption contributes by 12.5% to the overall effect in the first year after the eruption. The reduction in OH concentrations causes an increase in the CH 4 growth rate of 4 and 2 ppb/yr in the first and second year after the eruption, respectively, contributing 11 Tg to the 27 Tg observed CH 4 burden change in late 1991 and early 1992.

Description: Emission source contributions of tropospheric ozone (O 3 ) were comprehensively investigated by using the higher-order decoupled direct method (HDDM) for sensitivity analysis and the ozone source apportionment technology (OSAT) for mass balance analysis in the comprehensive air-quality model with extensions (CAMx). The response of O 3 to emissions reductions at various levels in mainland China, Korea, and Japan were estimated and compared with results calculated by the brute force method (BFM) where one model parameter is varied at a time. Emissions were assessed at three receptor sites in Japan that experienced severe pollution events in May 2009. For emissions from China, HDDM assessed O 3 response with a bias of only up to 3 ppbv (a relative error of 4.5%) even for a 50% reduction, but failed to assess a more extreme reduction. OSAT was reasonably accurate at 100% reduction, with a −4 ppbv (−7%) bias, but was less accurate at moderate ranges of reduction (⊔50-70%). For emissions from Korea and Japan, HDDM captured the nonlinear response at all receptor sites and at all reduction levels to within 1% in all but one case; however, the bias of OSAT increased with the increasing reduction of emissions. One possible reason for this is that OSAT does not account for NO titration. To address this, a term for potential ozone (PO; O 3 and NO 2 together) was introduced. Using of PO instead of O 3 improved the performance of OSAT, especially for emissions reductions from Korea and Japan. The proposed approach with PO refined the OSAT results and did not degrade HDDM performance.

Description: Using observed precipitation and the NCEP-NCAR reanalysis, the changes in the metrics of the summer precipitation in China, the dominance of frequency and intensity of daily extreme precipitation, and the linkage with changes in moisture and air temperature are explored. Results show that over the recent 50 years, total summer rainfall increased over the southeast and the west and decreased over the northeast. The changes in the frequency, identified with the 95% threshold and Poisson regression, and rainfall extremes show similar spatial patterns. The relative importance of the changes in frequency and intensity in the variability and changes in extreme precipitation is estimated. It is shown that, while the interannual variability of the rainfall amount is dominated by the frequency change in almost all stations, the long-term change of rainfall amount can be dominated by both frequency and intensity, depending on the station. The change in the rainfall total is linked to changes in atmospheric moisture and temperature. Results show that the variability and change of the rainfall total can be dominated by changes in both moisture and air temperature, and the relative importance depends on the region.

Description: We examine variations in water vapor in air entering the stratosphere through the tropical tropopause layer (TTL) over the past three decades in satellite data and in a trajectory model. Most of the variance can be explained by three processes that affect the TTL: the quasi-biennial oscillation, the strength of the Brewer-Dobson circulation, and the temperature of the tropical troposphere. When these factors act in phase, significant variations in water entering the stratosphere are possible. We also find that volcanic eruptions, which inject aerosol into the TTL, affect the amount of water entering the stratosphere. While there is clear decadal variability in the data and models, we find little evidence for a long-term trend in water entering the stratosphere through the TTL over the past 3 decades.

Description: Eight months (June 2011-January 2012) of aerosol property data were obtained at the remote site of Alborán Island (35.95 ° N, 3.03 ° W) in the western Mediterranean basin. The aim of this work is to assess the aerosol properties according to air-mass origin and transport over this remote station with a special focus on air mass transport from North Africa. For air masses coming from North Africa, different aerosol properties showed strong contributions from mineral dust lifted from desert areas. Nevertheless, during these desert dust intrusions, some atmospheric aerosol properties are clearly different from pure mineral dust particles. Thus, Angström exponent α(440–870) presents larger values than those reported for pure desert dust measured close to dust source regions. These results combine with α(440,670)-α(670,870) ≥ 0.1 and low single scattering albedo (ω(λ)) values, especially at the largest wavelengths. Most of the desert dust intrusions over Alborán can be described as a mixture of dust and anthropogenic particles. The analyses support that our results apply to North Africa desert dust air masses transported from different source areas. Therefore, our results indicate a significant contribution of fine absorbing particles during desert dust intrusions over Alborán arriving from different source regions. The aerosol optical depth (AOD) data retrieved from sun-photometer measurements have been used to check MODIS retrievals, and they show reasonable agreement, especially for North African air masses.

Description: The impact of aircraft type on contrail evolution is assessed using a large-eddy simulation (LES) model with Lagrangian ice microphysics. Six different aircraft ranging from the small regional airliner Bombardier CRJ to the largest aircraft Airbus A380 are taken into account. Differences in wake vortex properties and fuel flow lead to considerable variations in the early contrail geometric depth and ice crystal number. Larger aircraft produce contrails with more ice crystals (assuming that the number of initially generated ice crystals per kg fuel is constant). These initial differences are reduced in the first minutes, as the ice crystal loss during the vortex phase is stronger for larger aircraft. In supersaturated air, contrails of large aircraft are much deeper after 5 minutes than those of small aircraft. A parametrization for the final vertical displacement of the wake vortex system is provided, depending only on the initial vortex circulation and stratification. Cloud resolving simulations are used to examine whether the aircraft-induced initial differences have a long-lasting mark. These simulations suggest that the synoptic scenario controls the contrail-cirrus evolution qualitatively. However, quantitative differences between the contrail-cirrus properties of the various aircraft remain over the total simulation period of six hours. The total extinctions of A380-produced contrails are about 1.5 to 2.5 times higher than those from contrails of a Bombardier CRJ.

Description: Quantifying the effects of urban land-use/land-cover with regard to surface radiation and heat-flux regulation is important to ecological planning and heat stress mitigation. To retrieve the spatial pattern of heat fluxes in the Beijing metropolitan area, China, a remote sensing–based energy balance model was calibrated with synchronously measured energy fluxes including net radiation, latent heat flux (LE), and sensible heat flux (H). Our model calibration approach avoided the uncertainties due to subjective judgments in previous empirical parameterization methods. The land surface temperature (LST), H, and Bowen ratio (β) of Beijing were found to increase along the outskirt-suburban-urban gradient, with strong spatial variation. LST and H were negatively correlated with vegetation fraction cover (VFC). For example, the modern high-rise residential areas with relatively higher VFC had lower H and β than the traditional low-rise residential areas. Our findings indicate thermal dissipation through vegetation transpiration might play an important role in urban heat regulation. Notably, the thermal-dissipating strength of vegetation (calculated as LE/VFC) declined exponentially with increased VFC. For the purpose of heat stress regulation, we recommend upgrading the traditional low-rise residential areas to modern high-rise residential areas and focusing urban greenery projects in areas whose VFC 〈 0.1, where the heat-regulating service by urban vegetation could be twice as effective as in other places.

Description: ABSTRACT Urban areas have different climatology with respect to their rural surroundings. Though urbanization is a worldwide phenomenon, it is especially prevalent in India, where urban areas have experienced an unprecedented rate of growth over the last 30 years. Here we take up an observational study to understand the influence of urbanization on the characteristics of precipitation (specifically extremes) in India. We identify 42 urban regions, and compare their extreme rainfall characteristics with those of surrounding rural areas. We observe that, on an overall scale, the urban signatures on extreme rainfall are not prominently and consistently visible, but they are spatially non uniform. Zonal analysis reveals significant impacts of urbanization on extreme rainfall in central and western regions of India. An additional examination, to understand the influences of urbanization on heavy rainfall climatology, is carried with station level data using a statistical method, quantile regression. This is performed for the most populated city of India, Mumbai, in pair with a nearby non–urban area, Alibaug; both having similar geographic location. The derived extreme rainfall regression quantiles reveal the sensitivity of extreme rainfall events to the increased urbanization. Overall the study identifies the climatological zones in India, where, increased urbanization affects regional rainfall pattern and extremes, with a detailed case study of Mumbai. This also calls attention to the need of further experimental investigation, for the identification of the key climatological processes, in different regions of India, affected by increased urbanization.

Description: The Victoria mode (VM) represents the second dominant mode (EOF2) of North Pacific variability, independent of the Pacific Decadal Oscillation (PDO), and is defined as the EOF2 of SST anomalies (SSTAs) in the North Pacific poleward of 20°N. The present study indicates that the VM is closely linked to the development of El Niño–Southern Oscillation (ENSO). The VM may effectively act as an ocean bridge (or conduit) through which the extratropical atmospheric variability in the North Pacific influences ENSO. The VM can trigger the onset of ENSO via the following two dominant processes: 1) surface air–sea coupling associated with the VM in the subtropical/tropical Pacific and 2) evolution of subsurface ocean temperature anomalies along the equator associated with the VM. These two processes may force sufficient surface warming to occur in the central–eastern equatorial Pacific from spring to summer, which in turn initiates an ENSO event. The VM influence on ENSO relies on a basin-scale air–sea interaction dynamic, as opposed to more local-scale dynamics typically associated with the seasonal footprinting mechanism (SFM) or Pacific meridional mode (PMM). The majority of VM events are followed by ENSO events. These ENSO events triggered by VM include El Niño Modoki (EM) as well as conventional El Niño. There is no evidence that the VM tends to be more conducive to the initialization of EM than conventional El Niño.

Description: It has been well documented that there is an anticyclonic anomaly over the western North Pacific (WNPAC, hereafter) during El Niño decaying summer. This El Niño − WNPAC relationship is greatly useful for the seasonal prediction of summer climate in the WNP and East Asia. In this study, we investigate the modification of the El Niño–WNPAC relationship induced by a weakened Atlantic thermohaline circulation (THC) in a water-hosing experiment. The results suggest that the WNPAC during the El Niño decaying summer, as well as the associated precipitation anomaly over the WNP, is intensified under the weakened THC. On the one hand, this intensification is in response to the increased amplitude and frequency of El Niño events in the water-hosing experiment. On the other hand, this intensification is also because of greater climatological humidity over the western to central North Pacific under the weakened THC. We suggest that the increase of climatological humidity over the western to central North Pacific during summer under the weakened THC is favourable for enhanced interannual variability of precipitation, and therefore favourable for the intensification of the WNPAC during El Niño decaying summer. This study suggests a possible modulation of the ENSO–WNP summer monsoon relationship by the low frequency fluctuation of Atlantic sea surface temperature (SST). The results offer an explanation for the observed modification of the multidecadal fluctuation of El Niño–WNPAC relationship by the Atlantic multidecadal oscillation (AMO).

Description: Line-by-line radiative transfer computations show that the logarithmic dependence of radiative forcing on gas concentration not only applies to broadband irradiation fluxes such as in the well-known case of the CO 2 forcing, but also applies to the spectral radiance change due to both CO 2 and other gases, such as H 2 O. That the logarithmic relationship holds for monochromatic radiance requires an explanation beyond the conventional ideas based on the spectroscopic features of the gas absorption lines. We show that the phenomenon can be explained by an Emission Layer Displacement Model , which describes the radiance response to gas perturbation under normal atmospheric conditions such as temperature linearly varying with height and gas concentration exponentially decaying with height.

Description: Seasonal mean rainfall projections for Hawai‘i are given based on statistical downscaling of the latest CMIP5 global model results for two future representative concentration pathways (RCP4.5 and RCP8.5). The spatial information content of our statistical downscaling (SD) method is improved over previous efforts through the inclusion of spatially extensive, high quality monthly rainfall data set and the use of improved large-scale climate predictor information. Predictor variables include moisture transport in the middle atmosphere (700 hPa), vertical temperature gradients, and geopotential height fields of the 1000 and 500 hPa layers. The results allow for the first time to derive a spatially interpolated map with future rainfall change estimates for the main Hawaiian Islands. The statistical downscaling was applied to project wet (November-April) and dry (May-October) season rainfall anomalies for the mid and late 21 st century. Overall, the statistical downscaling gives more reliable results for the wet season than the dry season. The wet season results indicate a pronounced dipole structure between windward-facing mountain slopes and the leeward side of most of the islands. The climatically wet regions on the windward slopes of the mountain regions are expected to become wetter or remain stable in their seasonal precipitation amounts. On the climatically dry leeward sides Kaua‘i, O‘ahu Maui and Hawai‘i Island, future precipitation exhibits the strongest drying trends. The projected future rainfall anomaly pattern is associated with a circulation anomaly that resembles a shift in the position or strength of the subtropical high and the average location of extratropical troughs. These new results suggest that a negative trend dominates the area-averaged changes in the statistical downscaling over the Hawaiian Islands. However, the islands are expected to experience a greater contrast between the wet and dry regions in future.

Description: [1]  Acquiring accurate measurements of water vapor at the low mixing ratios (〈 10 ppm) encountered in the upper troposphere and lower stratosphere has proven to be a significant analytical challenge evidenced by persistent disagreements between high-precision hygrometers. These disagreements have caused uncertainties in the description of the physical processes controlling dehydration of air in the tropical tropopause layer and entry of water into the stratosphere, and have hindered validation of satellite water vapor retrievals. A 2011 airborne intercomparison of a large group of in situ hygrometers onboard the NASA WB-57F high-altitude research aircraft and balloons has provided an excellent opportunity to evaluate progress in the scientific community towards improved measurement agreement. In this work we intercompare the measurements from the Mid-latitude Airborne Cirrus Properties Experiment (MACPEX) and discuss the quality of agreement. Differences between values reported by the instruments were reduced in comparison to some prior campaigns, but were non-negligible and on the order of 20% (0.8 ppm). Our analysis suggests that unrecognized errors in the quantification of instrumental background for some or all of the hygrometers are a likely cause. Until these errors are understood, differences at this level will continue to somewhat limit our understanding of cirrus microphysical processes and dehydration in the tropical tropopause layer.

Description: [1]  We analyse the stratospheric Kelvin and Rossby-gravity wave packets with periods of a few days in nine high-top (i.e. with stratosphere) models of the fifth Coupled Model Intercomparison Project (CMIP5). These models simulate realistic aspects of these waves, and represent them better than the tropospheric convectively coupled waves analyzed in previous studies. [2]  There is nevertheless a large spread among the models, and those with a Quasi-Biennial Oscillation (QBO) produce larger amplitude waves than the models without a QBO. For the Rossby-gravity waves this is explained by the fact that models without a QBO never have positive zonal mean zonal winds in the lower stratosphere, a situation that is favorable to the propagation of Rossby-gravity waves. For the Kelvin waves, larger amplitudes in the presence of a QBO is counter intuitive because Kelvin waves are expected to have larger amplitude when the zonal mean zonal wind is negative, and this is always satisfied in models without a QBO. We attribute the larger amplitude to the fact that models tuned to have a QBO require finer vertical resolution in the stratosphere. [3]  We also find that models with large precipitation variability tend to produce larger amplitude waves. However, the effect is not as pronounced as was found in previous studies. In fact, even models with weak precipitation variability still have quite realistic stratospheric waves, indicating either that (i) other sources can be significant or that (ii) the dynamical filtering mitigates the differences in the sources between models.

Description: [1]  A high-resolution global atmospheric dataset (DA126) is used to understand the East Asian summer precipitation variability. It is found that a fine resolution of the DA126 precipitation data is able to reveal the detailed structures of the rainfall variability over East Asia and southern China in comparison with global analysis precipitation datasets such as the CMAP. The first two empirical orthogonal functions (EOFs) of the DA126 precipitation data over East Asia accurately reflect a decadal shift in rainfall over southern China in the mid-1990s. Furthermore, the first EOF-related precipitation of the DA126 is related to the tropical Pacific sea surface temperature (SST) variability (i.e., ENSO) and the second EOF-related precipitation is associated with the Indian Ocean SST variability. Consequently, the tropical Pacific and the Indian Ocean SSTs have different associations with the East Asian monsoon precipitation variability. However, it is difficult to find such a relationship in the first two EOFs of the CMAP dataset over East Asia. Using the DA126 precipitation dataset, our further analysis indicates that warming of both the tropical Pacific and the Indian Ocean causes an increase in the rainfall anomaly over southern China after the mid-1990s, which results in a decadal shift in the rainfall anomaly after the mid-1990s. In addition, the first EOF-related precipitation is associated with both the Pacific-Japan-like (PJ-like) pattern and the Eurasian-like pattern. In contrast, the second EOF-related precipitation is only associated with the PJ-like wave trains from the western Pacific to East Asia.

Description: [1]  We have diagnosed the lifetimes of long-lived source gases emitted at the surface and removed in the stratosphere using six three-dimensional chemistry-climate models (CCMs) and a two-dimensional model. The models all used the same standard photochemical data. We investigate the effect of different definitions of lifetimes, including running the models with both mixing ratio (MBC) and flux (FBC) boundary conditions. Within the same model, the lifetimes diagnosed by different methods agree very well. Using FBCs versus MBCs leads to a different tracer burden as the implied lifetime contained in the MBC value does not necessarily match a model's own calculated lifetime. In general, there are much larger differences in the lifetimes calculated by different models, the main causes of which are variations in the modelled rates of ascent and horizontal mixing in the tropical mid-lower stratosphere. The model runs have been used to compute instantaneous and steady-state lifetimes. For chlorofluorocarbons (CFCs) their atmospheric distribution was far from steady state in their growth phase through to the 1980s and the diagnosed instantaneous lifetime is accordingly much longer. Following the cessation of emissions, the resulting decay of CFCs is much closer to steady-state. For 2100 conditions the model circulation speeds generally increase, but a thicker ozone layer due to recovery and climate change reduces photolysis rates. These effects compensate so the net impact on modelled lifetimes is small. For future assessments of stratospheric ozone use of FBCs would allow a consistent balance between rate of CFC removal and model circulation rate.

Description: [1]  In January 2011, the state of the polar vortex in the midlatitudes changed significantly due to a minor Sudden Stratospheric Warming event. As a result, a bi-directional duct for infrasound propagation developed in the middle atmosphere that persistedfor two weeks. The ducts were due to two zonal wind jets, one between 30-50 km and the other around 70 km altitude. In this paper, using microbarom source modeling, a previously unidentified source region in the eastern Mediterranean is identified,besides the more well known microbarom source regions in the Atlantic Ocean. Infrasound data is then presented in which the above mentioned bi-directional duct is observed in microbarom signals recorded at the IMS station I48TN in Tunisia from the Mediterranean region to the east and from the Atlantic Ocean to the west. While the frequency bands of the two sources overlap, the Mediterranean signal is coherent up to about 0.6 Hz. This observation is consistent with the microbarom source modeling; the discrepancy in the frequency band is related to differences in the ocean wave spectra for the two basins considered. This work demonstrates the sensitivity of infrasound to stratospheric dynamics and illustrates that the classic paradigm of a unidirectional stratospheric duct for infrasound propagation can be broken during a Sudden Stratospheric Warming event.

Description: [1]  Clear experimental evidence of the Twomey effect for shallow trade wind cumuli near Barbados is presented. Effective droplet radius ( r eff ) and cloud optical thickness ( τ ), retrieved from helicopter–borne spectral cloud–reflected radiance measurements, and spectral cloud reflectivity ( γ λ ) are correlated with collocated in situ observations of the number concentration of aerosol particles from the sub–cloud layer ( N ). N denotes the concentration of particles larger than 80nm in diameter and represents particles in the activation mode. In situ cloud microphysical and aerosol parameters were sampled by the Airborne Cloud Turbulence Observation System (ACTOS). Spectral cloud–reflected radiance data were collected by the Spectral Modular Airborne Radiation measurements sysTem (SMART-HELIOS). With increasing N a shift in the probability density functions of τ and γ λ towards larger values is observed, while the mean values and observed ranges of retrieved r eff decrease. The relative susceptibilities ( RS ) of r eff , τ and γ λ to N are derived for bins of constant liquid water path ( LWP ). The resulting values of RS are in the range of 0.35 for r eff and τ , and 0.27 for γ λ . These results are close to the maximum susceptibility possible from theory. Overall, the shallow cumuli sampled near Barbados show characteristics of homogeneous, plane–parallel clouds. Comparisons of RS derived from in situ measured r eff and from a microphysical parcel model are in close agreement.

Description: [1]  The long-term measurements at the Barrow and Atqasuk sites have been processed to develop the climatology of aerosol and cloud properties at interannual, seasonal, and diurnal temporal scales. At the Barrow site, the surface temperature exhibits an increasing trend in both thawed and frozen seasons over the period studied here, about one decade. Corresponding to the warming, the snow melting day arrives earlier and the non-snow-cover duration increases. Aerosol optical depth (AOD) increased during the periods of 2001-2003 and 2005-2009, and decreased during 2003-2005. The liquid water path (LWP), cloud optical depth (COD), and cloud fraction exhibit apparently decreasing trends from 2002 to 2007 and increased significantly after 2008. In the frozen season, the Arctic haze and ice clouds are dominant, while in the thawed season, the oceanic biogenic aerosols and liquid water clouds or mix-phase clouds are dominant. The cloud droplet effective radius during the thawed season is larger than that during the frozen season. The diurnal variations of aerosol and cloud related atmospheric properties are not obvious at these two sites. During the sunshine periods, the aerosol has a cooling effect on the surface through direct aerosol radiative forcing. In the frozen season clouds have a positive impact on the net surface radiation, and the WVP, LWP, and COD have good positive correlations with the surface temperature, suggesting that the cloud–radiation feedback is positive. In the thawed season, clouds have a negative impact on the net surface radiation. The impact of surface temperature on cloud amount is also negative, although the statistics are not as robust in the frozen season.

Description: [1]  Differences between stratospheric water vapor measurements by NOAA frost point hygrometers (FPHs) and the Aura Microwave Limb Sounder (MLS) are evaluated for the period August 2004 through December 2012 at Boulder, Colorado (40.0°N, 105.2°W), Hilo, Hawaii (19.7°N, 155.1°W), and Lauder, New Zealand (45.0°S, 169.7°E). Two groups of MLS profiles coincident with the FPH soundings at each site are identified using unique sets of temporal and spatial criteria. Before evaluating the differences between coincident FPH and MLS profiles each FPH profile is convolved with the MLS averaging kernels for 8 pressure levels from 100 to 26 hPa (~16 to 25 km) to reduce its vertical resolution to that of the MLS water vapor retrievals. The mean FPH–MLS differences at every pressure level (100 to 26 hPa) are well within the combined measurement uncertainties of the two instruments. However, the mean differences at 100 and 83 hPa are statistically significant and negative, ranging from –0.46 ± 0.22 ppmv (–10.3 ± 4.8%) to –0.10 ± 0.05 ppmv (–2.2 ± 1.2%). Mean differences at the 6 pressure levels from 68 to 26 hPa are on average 0.8% (0.04 ppmv) and only a few are statistically significant. The FPH–MLS differences at each site are examined for temporal trends using weighted linear regression analyses. The vast majority of trends determined here are not statistically significant and most are smaller than the minimum trends detectable in this analysis. Except at 100 and 83 hPa the average agreement between MLS retrievals and FPH measurements of stratospheric water vapor is better than 1%.

Description: [1]  Recent observations reveal a seasonally occurring layer of aerosol located from 0 ∘ to 100 ∘ E, 20 ∘ to 45 ∘ N and extending vertically from about 13 km to 18 km; this has been termed the Asian Tropopause Aerosol Layer (ATAL) and its existence is closely associated with the Asian summer monsoon circulation. Observational studies argue that the ATAL is a recent phenomenon, as the layer was not observed in the satellite record prior to 1998. This suggests that the ATAL may be of anthropogenic origin associated with increased emissions, most notably sulfur dioxide (SO 2 ), from increased industrial activity in China and India starting during the same time. Here we test the hypothesis that SO 2 emitted from Asia led to the formation of the ATAL using an aerosol microphysical model coupled to a global chemistry climate model. This is the first modeling study to specifically examine the ATAL and its possible origin. From our results, we conclude that the ATAL is most likely due to anthropogenic emissions, but its source cannot solely be attributed to emissions from Asia. Specifically, the results indicate that Chinese and Indian emissions contribute ∼ 30% of the sulfate aerosol extinction in the ATAL during volcanically quiescent periods. We also show that even small volcanic eruptions preclude our ability to make any conclusions about the existence of the ATAL before 1998 with observations alone.

Description: [1]  New observations with a 3D Lightning Mapping Array and high speed video are presented and discussed. The first set of observations show that under certain thunderstorm conditions wind turbine blades can produce electric discharges at regular intervals of ~3 seconds in relation to its rotation, over periods of time that range from a few minutes up to hours. This periodic effect has not been observed in static towers indicating that the effect of rotation is playing a critical role. The repeated discharges can occur tens of kilometers away from electrically active thunderstorm areas, and may or may not precede a fully developed upward lightning discharge from the turbine. Similar to rockets used for triggering lightning the fast movement of the blade tip plays an important role on the initiation of the discharge. The movement of the rotor blades allows the tip to ‘runaway’ from the generated corona charge. The second observation is an uncommon upward/downward flash triggered by a wind turbine. In that flash, a negative upward leader was initiated from a wind turbine without preceding lightning activity. The flash produced a negative cloud-to-ground stroke several kilometers from the initiation point. The third observation corresponds to a high speed video record showing simultaneous upward positive leaders from a group of wind turbines triggered by a preceding intra-cloud flash. The fact that multiple leaders develop simultaneously indicates a poor shielding effect among them. All these observations provide some special features on the initiation of lightning by non-static and complex tall structures.

Description: [1]  Tides forced by large-scale weather systems in the tropical troposphere introduce significant longitudinal and local time variability in the upper atmosphere. This paper presents variability of tidal harmonics of the latent heating from 2002–2011 associated with the El Niño-Southern Oscillation (ENSO) and the tidal wind response in the mesosphere/lower thermosphere (MLT) region. Emphasis is on the strong ENSO cycle 2009–2011. Latent tidal heating rates are computed from TRMM satellite precipitation data, with added radiative heating from MERRA reanalysis, as functions of time, latitude, and altitude. The heating rates for the two most affected nonmigrating tides (DE3 and DE2) are examined and compared with MLT tidal wind variability from TIDI/TIMED. Principal component analysis (PCA) is used to identify the tidal modes most affected by ENSO. Our results indicate that the tidal response to ENSO is largest during winter for both of the tides, with the largest response occurring in the DE3 tidal winds during the La Niña phase, with an increase of roughly 70% for the winter months of 2010/2011, and negligible response during the El Niño phase. The ENSO effect in the tidal forcing closely resembles the first symmetric and antisymmetric Hough modes of DE3 and DE2, thus being an efficient mechanism to transmit the ENSO signal into the MLT tides.

Description: [1]  An important key for the understanding of the dynamic response to large tropical volcanic eruptions is the warming of the tropical lower stratosphere and the concomitant intensification of the polar vortices. Although this mechanism is reproduced by most GCMs today, most models still fail in producing an appropriate winter warming pattern in the Northern Hemisphere. In this study ensemble sensitivity experiments were carried out with a coupled atmosphere–ocean model to assess the influence of different ozone climatologies on the atmospheric dynamics and in particular on the northern hemispheric winter warming. The ensemble experiments were perturbed by a single Tambora-like eruption. Larger meridional gradients in the lower stratospheric ozone favor the coupling of zonal wind anomalies between the stratosphere and the troposphere after the eruption. The associated sea level pressure, temperature, and precipitation patterns are more pronounced and the northern hemispheric winter warming is highly significant. Conversely, weaker meridional ozone gradients lead to a weaker response of the winter warming and the associated patterns. The differences in the number of stratosphere–troposphere coupling events between the ensembles experiments indicate a non-linear response behavior of the dynamics with respect to the ozone and the volcanic forcing.

Description: [1]  The skill of seasonal precipitation forecasts is assessed worldwide —grid point by grid point— for the forty-year period 1961-2000, considering the ENSEMBLES multi-model hindcast and applying a tercile-based probabilistic approach in terms of the ROC Skill Score (ROCSS). Although predictability varies with region, season and lead-time, results indicate that 1) significant skill is mainly located in the tropics —20 to 40% of the total land areas,— 2) overall, SON (MAM) is the most (least) skillful season, and 3) the skill weakens (with respect to the one-month lead case) at four-month lead —especially in JJA,— although the ROCSS spatial patterns are broadly preserved —particularly in northern South America and the Malay archipelago.— [2]  The contribution of ENSO events to this forty-year skill is also analyzed, based on the idea that the seasonal predictability may be mainly driven by El Niño and La Niña precipitation teleconnections and, consequently, limited by the ability of the different seasonal forecasting models to accurately reproduce them. Results show that the ROCSS spatial patterns for 1) the full period 1961-2000 and 2) El Niño and La Niña events are highly correlated –over 0.85.— Moreover, the observed teleconnection patterns are properly simulated (predicted) —with spatialcorrelations around 0.8— by most of the models at both one and four months lead-time.

Description: [1]  This study analyzes 15 years of Tropical Rainfall Measuring Mission (TRMM) satellite data, together with surface observations of thunderstorms and visibility, to study trends and relationships between aerosols and thunderstorms in Southeast China. TRMM data used are from the lightning imaging sensor (LIS) and the precipitation radar (PR). Surface data are human-observed thunderstorm occurrence and visibility for the period of 1990-2012 at 70 plain stations and 4 mountain stations. Thunderstorm and lightning activity, as well as PR echo-top heights, have all increased significantly over the region during the period under study, while regional mean visibility has decreased greatly at the plain stations. The daily rainfall amount during thunderstorm days has increased significantly, but rainfall without thunderstorms has no trend during this period. In comparison, the four mountain weather stations at elevations greater than 1100 m showed little trend in the number of thunderstorm days during the period of 1990-2012. The ratio of the number of thunderstorm days between plain and mountain stations has increased significantly. The distinct trends seen between plain and mountain stations may originate from large differences in aerosol concentration between the plain and mountain regions. The accumulation of pollution aerosols in the plain region likely invigorates thunderstorms, whereas a lesser, or no, impact on intense convection is found over high-altitude regions.

Description: [1]  During volcanic eruptions, empirical relationships are used to estimate mass eruption rate from plume height. Although simple, such relationships can be inaccurate and can underestimate rates for eruptions in windy conditions. 1-D plume models can incorporate atmospheric conditions and are hypothesized to give potentially more accurate estimates. Here, I present a 1-D model for plumes in cross wind and use it to simulate 25 historical eruptions where plume height H obs was well observed, and where mass eruption rate M obs could be calculated from mapped deposit mass and observed duration. The simulations considered wind, temperature, and phase changes of water. Atmospheric conditions were obtained from the NCEP/NCAR Reanalysis 2.5 degree model. Simulations calculate the minimum, maximum, and average values ( M min , M max , and M avg ) that fit the plume height. Eruption rates were also estimated from the empirical formula M empir  = 140H obs 4.14 ( M empir is in kg s -1 , H obs is in km). For these eruptions, the standard error of the residual in log space is about 0.53 for M avg and 0.50 for M empir . Thus for this dataset, the model is slightly less accurate at predicting M obs than the empirical curve. The inability of this model to improve eruption-rate estimates may lie in the limited accuracy of even well-observed plume heights, inaccurate model formulation, and (or) the fact that most eruptions in the dataset were not highly influenced by wind or moisture. For the low, wind-blown plume of April 14-18, 2010 at Eyjafjallajökull, where an accurate plume-height time series is available, modeled rates do agree better with M obs than M empir .

Description: [1]  Stratospheric water vapor has an important effect on Earth's climate. Considering the significance of overshooting deep convection in modulating the water vapor content (WVC) of the lower stratosphere (LS), we use a three-dimensional convective cloud model to simulate the effects of various silver iodide (AgI) seeding scenarios on tropical overshooting deep convection occurred on 30 November 2005 in Darwin, Australia. The primary motivation for this study is to investigate whether the WVC in the LS can be artificially modified by deliberate cloud seeding. It is found that AgI seeding done at the early stages of clouds produces significant effects on cloud microphysical and dynamical properties, and that further affects the WVC in the LS, while seeding at the mature stages of clouds has only a slight impact. The response of stratospheric water vapor to changes in the amount of seeding agent is nonlinear. The seeding with a small (large) amount of AgI increases (decreases) the WVC in the LS, due to enhanced (reduced) production and vertical transport of cloud ice from the troposphere and subsequent sublimation in the stratosphere. The results show that stratospheric water vapor can be artificially altered by deliberate cloud seeding with proper amount of seeding agent. This study also shows an important role of graupel in regulating cloud microphysics and dynamics, and in modifying the WVC in the LS.

Description: [1]  To confirm whether surface winds strengthen above warm waters around oceanic fronts using in-situ data, a field measurement was conducted using both expendable bathythermographs and Global Positioning System sondes released concurrently across the Kuroshio front in the East China Sea in December 2010. In contrast to previous studies mainly based on satellite observations, the finding of the present field survey is the local weakening of surface winds at the northern flank of the Kuroshio front. From the above field observation in conjunction with a regional numerical model experiment, it is suggested that northwesterly winds crossing the Kuroshio front from the cooler side first weaken at the northern flank of the front because of the onset of upward transfer of the “non-slip” condition at the sea surface. Thereafter, as the atmospheric mixed layer with warm and humid air mass develops gradually downwind over the Kuroshio region, the surface winds are gradually accelerated by the momentum mixing with strong winds aloft. The surface winds remain strong over the cool East China Sea shelf, and it is thus considered that the surface winds only weaken at the northern flank of the Kuroshio front. However, numerical modeling indicates that this local weakening of the surface winds occurs as a transient state with a short duration, and such a structure has thus rarely been detected in the long-term averaged wind fields observed by satellites.

Description: [1]  The 222 Radon-tracer method is a powerful tool to estimate local and regional surface emissions of (e.g.) greenhouse gases. In this paper we demonstrate that in practice, the method as it is commonly used, produces inaccurate results in case of non-homogeneously spread emission sources and we propose a different approach to account for this. We have applied the new methodology to ambient observations of CO 2 and 222 Radon to estimate CO 2 surface emissions for the city of Bern, Switzerland. Furthermore, by utilizing combined measurements of CO 2 and δ (O 2 /N 2 ) we obtain valuable information about the spatial and temporal variability of the main emission sources. Mean net CO 2 emissions based on two years of observations are estimated at: (11.2 ± 2.9) kt km -2 a -1 . Oxidative ratios indicate a significant influence from the regional biosphere in summer/spring and fossil fuel combustion processes in winter/autumn. Our data indicate that the emissions from fossil fuels are, to a large degree, related to the combustion of natural gas which is used for heating purposes.

Description: [1]  The daily maximum and minimum temperatures observed at the 1897 meteorological stations of China over the past 60 years (1951-2010) are analyzed in this study to examine the interdecadal variation of frequency for record-breaking event (RBE) of temperature in the context of global warming. The results indicate that the frequency of record-breaking high temperature in the first decade of the 21th century is the highest in the three decades from the 1980s to the 2000s, implying a distinct warming trend. Meanwhile, frequencies of record-breaking low temperature in the 1990s and the beginning of the 21th century are also significant. In particular, the RBEs of low temperature occurred over most of China in the 1990s but concentrated in northern China during the 2000s. To understand why the record-low temperatures in northern China are repeatedly broken in the 2000s, the related East Asian Winter Monsoon (EAWM) variability is investigated. The EOF analysis of surface air temperature reveals that the northern mode of the EAWM variability, which is highly associated with the Arctic Oscillation (AO) activities at both interdecadal and interannual timescales, has been intensifying since late 1990s. Corresponding to the intensification of the northern mode of the EAWM variability and the negative phase of AO in the 2000s, the Siberian High and East Asian trough intensify while the polar-front jet stream strengthens and the subtropical westerly jet stream abnormally shifts northward. As a result, anomalously strong cold air masses, originated from Siberia, intrude into East Asia, but are blocked by the enhanced northward subtropical westerly jet and cannot reach low latitudes area. Therefore the extremely strong cold air masses are amassed in mid-high latitudes of East Asia, resulting in RBEs of low temperature in this area.

Description: [1]  The Ice Particle Size Distribution (PSD) is fundamental to the quantitative description of a cloud. It is also crucial in the development of remote sensing retrieval techniques using radar and/or lidar measurements. The PSD allows one to link characteristics of individual particles (area, mass, scattering properties) to characteristics of an ensemble of particles in a sampling volume (e.g. visible extinction ( σ ), Ice Water Content (IWC), radar reflectivity (Z)). The aim of this study is to describe a normalisation technique to represent the PSD. We update an earlier study by including recent in-situ measurements covering a large variety of ice clouds spanning temperatures ranging between -80 °C and 0 °C. This new data set also includes direct measurements of IWC. We demonstrate that is possible to scale the PSD in size space by the volume weighted diameter D m and in the concentration space by the intercept parameter and obtain the intrinsic shape of the PSD. Therefore, by combining , D m and a modified gamma function representing the normalised PSD shape, we are able to approximate key cloud variables (such as IWC) as well as cloud properties which can be remotely observed (such as Z) with an absolute mean relative error smaller than 20%. The underlying idea is to be able to retrieve the PSD using two independent measurements. We also propose parameterizations for ice cloud key parameters derived from the normalised PSD. We also investigate the effects of uncertainty present in the ice crystal mass-size relationships on the parameterizations and the normalised PSD approach.

Description: [1]  The MERRA Aerosol Reanalysis (MERRAero) has been recently developed at NASA's Global Modeling Assimilation Office (GMAO). This reanalysis is based on a version of the GEOS-5 model radiatively coupled with GOCART aerosols, and it includes assimilation of bias-corrected Aerosol Optical Thickness (AOT) from the MODIS sensor on both Terra and Aqua satellites. In October over the period 2002-2009, MERRAero showed that AOT was lower over the east of the Ganges basin than over the north-west of the Ganges basin: this was despite the fact that the east of the Ganges basin should have produced higher anthropogenic aerosol emissions because of higher population density, increased industrial output and transportation. This is evidence that higher aerosol emissions do not always correspond to higher AOT over the areas where the effects of meteorological factors on AOT dominate those of aerosol emissions. MODIS AOT assimilation was essential for correcting modeled AOT mainly over the north-west of the Ganges basin, where AOT increments were maximal. Over the east of the Ganges basin and north-west BoB, AOT increments were low and MODIS AOT assimilation did not contribute significantly to modeled AOT. Our analysis showed that increasing AOT trends over north-west BoB (exceeding those over the east of the Ganges basin) were reproduced by GEOS-5, not because of MODIS AOT assimilation, but mainly because of the model capability of reproducing meteorological factors contributing to AOT trends. Moreover, vertically integrated aerosol mass flux was sensitive to wind convergence causing aerosol accumulation over north-west BoB.

Description: [1]  Numerous studies have emphasized that climate simulations are subject to various biases and uncertainties. The objective of this study is to cross-validate 34 Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations of precipitation against the Global Precipitation Climatology Project (GPCP) data, quantifying model pattern discrepancies and biases for both entire data distributions and their upper tails. The results of the Volumetric Hit Index ( VHI ) analysis of the total monthly precipitation amounts show that most CMIP5 simulations are in good agreement with GPCP patterns in many areas, but that their replication of observed precipitation over arid regions and certain sub-continental regions (e.g., northern Eurasia, eastern Russia, central Australia) is problematical. Overall, the VHI of the multi-model ensemble mean and median also are superior to that of the individual CMIP5 models. However, at high quantiles of reference data (e.g., the 75th and 90th percentiles), all climate models display low skill in simulating precipitation, except over North America, the Amazon, and central Africa. Analyses of total bias ( B ) in CMIP5 simulations reveal that most models overestimate precipitation over regions of complex topography (e.g. western North and South America and southern Africa and Asia), while underestimating it over arid regions. Also, while most climate model simulations show low biases over Europe, inter-model variations in bias over Australia and Amazonia are considerable. The Quantile Bias ( QB ) analyses indicate that CMIP5 simulations are even more biased at high quantiles of precipitation. It is found that a simple mean-field bias removal improves the overall B and VHI values, but does not make a significant improvement in these model performance metrics at high quantiles of precipitation.

Description: Conventional global climate models (GCMs) often consider radiation interactions only with small-particle/suspended cloud mass, ignoring large-particle/falling and convective core cloud mass. We characterize the radiation and atmospheric circulation impacts of frozen precipitating hydrometeors (i.e., snow), using the National Center for Atmospheric Research (NCAR)-coupled GCM, by conducting sensitivity experiments that turn off the radiation interaction with snow. The changes associated with the exclusion of precipitating hydrometeors exhibit a number differences consistent with biases in CMIP3 and CMIP5, including more outgoing longwave (LW) flux at the top of atmosphere (TOA) and downward shortwave (SW) flux at the surface in the heavily precipitating regions. Neglecting the radiation interaction of snow increases the net radiative cooling near the cloud top with the resulting increased instability triggering more convection in the heavily precipitating regions of the tropics. In addition, the increased differential vertical heating leads to a weakening of the low-level mean flow and an apparent low-level eastward advection from the warm pool resulting in moisture convergence south of the ITCZ and north of the SPCZ. This westerly bias, with effective warm and moist air transport, might be a contributing factor in the model's northeastward overextension of the SPCZ and the concomitant changes in sea surface temperatures, upward motion, and precipitation. Broader dynamical impacts include a stronger local meridional overturning circulation over the mid- and east Pacific, and commensurate changes in low- and upper-level winds, large-scale ascending motion, with a notable similarity to the systematic bias in this region in CMIP5 upper-level zonal winds.

Description: The Visible Infrared Imaging Radiometer Suite (VIIRS) is the next-generation polar-orbiting operational environmental sensor with a capability for global aerosol observations. The VIIRS aerosol Environmental Data Record (EDR) is expected to continue the decade-long successful multi-spectral aerosol retrieval from the NASA's Earth Observing System (EOS) MODerate resolution Imaging Spectroradiometer (MODIS) for scientific research and applications. Since the launch of the Suomi National Polar-orbiting Partnership (S-NPP), the VIIRS aerosol Calibration/Validation team has been continuously monitoring, evaluating and improving the performance of VIIRS aerosol retrievals. In this study, the VIIRS aerosol optical thickness (AOT) at 550 nm EDR at current Provisional maturity level is evaluated by comparing it with MODIS retrievals and measurements from the AErosol RObotic NETwork (AERONET) and the Maritime Aerosol Network (MAN). The VIIRS global mean AOT at 550 nm differs from that of MODIS by approximately -0.01 over ocean and 0.03 over land (0.00 and -0.01 for the collocated retrievals), but shows larger regional biases. Global validation with AERONET and with MAN measurements shows biases of 0.01 over ocean and -0.01 over land, with about 64% and 71% of retrievals falling within the expected uncertainty range established by MODIS over ocean (±(0.03 + 0.05AOT)) and over land (±(0.05 + 0.15AOT)), respectively. The VIIRS retrievals over land exhibit slight overestimation over vegetated surfaces and underestimation over soil-dominated surfaces. These results show that the VIIRS AOT at 550 nm product provides a solid global dataset for quantitative scientific investigations and environmental monitoring.

Description: This study is based on the analysis of Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) data measured over Hyderabad, India during the years 2006-2008. Tropospheric profiles of O 3 show clear seasonality with high and low values during the pre-monsoon and monsoon seasons, respectively. Analysis of back trajectory and fire count data indicates major roles for long-range transport and biomass burning in the seasonal variation of O 3 . Typically, lower levels of O 3 in the monsoon season were due to the flow of marine air and negligible regional biomass burning, while higher levels in other seasons were due to transport of continental air. In the upper troposphere, relatively low levels of O 3 during the monsoon and post-monsoon seasons were associated with deep convection. In the free troposphere, levels of O 3 also show year-to-year variability as the values in the pre-monsoon of 2006 were higher by about 30 ppbv compared to 2008. The year-to-year variations were mainly due to transition from El Niño (2006) to La Niña (2008). The higher and lower levels of O 3 were associated with strong and weak wind shears, respectively. Typically, vertical variations of O 3 were anti-correlated with the lapse rate profile. The lower O 3 levels were observed in the stable layers, but higher values in the mid-troposphere were caused by long-range transport. In the PBL region, the mixing ratio of O 3 shows strong dependencies on meteorological parameters. The Chemistry Climate Model (CCM2) reasonably reproduced the observed profiles of O 3 except for the pre-monsoon season.

Description: Radiative forcing by aerosols and tropospheric ozone could play a significant role in recent Arctic warming. These species are in general poorly accounted for in climate models. We use the GEOS-Chem global chemical transport model to construct a 3-D representation of Arctic aerosols and ozone that is consistent with observations and can be used in climate simulations. We focus on 2008, when extensive observations were made from different platforms as part of the International Polar Year. Comparison to aircraft (ARCTAS), surface, and ship cruise (ICEALOT, ASCOS) observations suggests that GEOS-Chem provides in general a successful year-round simulation of Arctic black carbon (BC), organic carbon (OC), sulfate, and dust aerosol. BC has major fuel combustion and boreal fire sources, OC is mainly from fires, sulfate has a mix of anthropogenic and natural sources, and dust is mostly from the Sahara. The model is successful in simulating aerosol optical depth (AOD) observations from AERONET stations in the Arctic; the sharp drop from spring to summer appears driven in part by the smaller size of sulfate aerosol in summer. The anthropogenic contribution to Arctic AOD is a factor of 4 larger in spring than summer and is mainly sulfate. Simulation of absorbing aerosol optical depth (AAOD) indicates that non-BC aerosol (OC and dust) contributed 24% of Arctic AAOD at 550 nm and 37% of absorbing mass deposited to the snow pack in 2008. Open fires contributed half of AAOD at 550 nm and half of deposition to the snowpack.

Description: [1]  In this study, we investigated spatial and temporal changes in precipitation over the COordinated Regional climate Downscaling EXperiment (CORDEX) East Asia domain, for present (1986–2005) and future (2031–2050) periods using the Regional Climate Model version 4 (RegCM4). Future meteorology produced by the Hadley Center Global Environmental Model version 2 coupled with the Atmosphere–Ocean (HadGEM2-AO) following global climate change scenarios (RCP 4.5 and 8.5) was used as meteorological boundary conditions for the RegCM4. Six sub-regions (South Korea, North China, South China, Japan, Mongolia, and India) in the CORDEX East Asia domain were considered for analysis. The RegCM4 simulated spatial distributions of precipitation over East Asia with a correlation coefficient of 0.7 against CRU data. The simulation skills of its temporal variability varied based on geographical regions and seasons, showing relatively poorer performance (under-estimation in rainfall amount) in summer than in winter, in general. The future climate simulations by the RegCM4 presented that the East Asian continental regions will be warmer and more humid, leading to increased precipitation amounts, especially in the summer. The summer precipitation amount was projected to increase by about 5%, on average, over the East Asian domain, 5 − 15% in most sub-regions, and even higher (44% and 24%) in the South Korean region for the RCP 4.5 and 8.5 scenarios, respectively. It was also expected that heavy rainfall (〉 50 mm/day) events may occur more frequently in the future possibly owing to meteorological changes that are favorable to convective heavy precipitation.

Description: Four years of CloudSat data have been analyzed over a region of the East Atlantic Ocean in order to examine the influence of aerosols on deep convection. The satellite data were combined with information about aerosols taken from the Global and Regional Earth-System Monitoring Using Satellite and in-situ Data (GEMS) model. Only those profiles fitting the definition of deep convective clouds were analyzed. Overall, the cloud center of gravity, cloud top, and rain top were all found to increase with increased aerosol loading. These effects were largely independent of the environment, and the differences between the cleanest and most polluted clouds sampled were found to be statistically significant. When examining an even smaller subset of DCCs likely to be part of the convective core, similar trends were seen. These observations suggest that convective invigoration occurs with increased aerosol loading, leading to deeper, stronger storms in polluted environments.

Description: The ratio of liquid water to ice in a cloud, largely controlled by the presence of ice nuclei and cloud temperature, alters cloud radiative effects. This study quantitatively examines how the liquid fraction of clouds influences various climate feedbacks using the NCAR Community Atmosphere Model (CAM). Climate feedback parameters were calculated using equilibrated temperature changes in response to increases in the atmospheric concentration of carbon dioxide in CAM Version 3.0 with a slab ocean model. Two sets of model experiments are designed such that cloud liquid fraction linearly decreases with a decrease in temperature down to −20°C (Experiment “C20”) and −40°C (Experiment “C40”). Thus, at the same sub-zero temperature, C20 yields fewer liquid droplets (and more ice crystals) than C40. Comparison of the results of experiments C20 and C40 reveals that experiment C20 is characterized by stronger cloud and temperature feedbacks in the tropics (30°N–30°S) (by 0.25 and –0.28 W m −2  K −1 , respectively), but weaker cloud, temperature and albedo feedbacks (by –0.20, 0.11, and –0.07 W m −2  K −1 ) in the extratropics. Compensation of these climate feedback changes leads to a net climate feedback change of ~7.28% of that of C40 in the model. These results suggest that adjustment of the cloud phase function affects all types of feedbacks (with the smallest effect on water vapor feedback). Although the net change in total climate feedback is small due to the cancellation of positive and negative individual feedback changes, some of the individual changes are relatively large. This illustrates the importance of the influence of cloud phase partitioning for all major climate feedbacks, and by extension, for future climate change predictions.

Description: Diurnal and seasonal controls on water vapor fluxes were investigated in a subtropical mangrove forest in Everglades National Park, Florida. Energy partitioning between sensible and latent heat fluxes was highly variable during the 2004-2005 study period. During the dry season, the mangrove forest behaved akin to a semiarid ecosystem as most of the available energy was partitioned into sensible heat, which gave Bowen ratio values exceeding 1.0 and minimum latent heat fluxes of 5 MJ day -1 . In contrast, during the wet season the mangrove forest acted as a well-watered, broadleaved deciduous forest, with Bowen ratio values of 0.25 and latent heat fluxes reaching 18 MJ day -1 . During the dry season, high salinity levels (〉 30 parts per thousand, ppt) caused evapotranspiration to decline and correspondingly resulted in reduced canopy conductance. From multiple linear regression, daily average canopy conductance to water vapor declined with increasing salinity, vapor pressure deficit, and daily sums of solar irradiance but increased with air temperature and friction velocity. Using these relationships, appropriately modified Penman-Monteith and Priestley-Taylor models reliably reproduced seasonal trends in daily evapotranspiration. Such numerical models, using site-specific parameters, are crucial for constructing seasonal water budgets, constraining hydrological models, and driving regional climate models over mangrove forests.

Description: The relationship between low-level cloud optical depth and atmospheric and surface air temperature is examined in the control climate of thirteen climate models to determine if cloud optical depth-temperature relationships found in observations are replicated in climate models, and if climate model behavior found in control climate simulations provides information about the optical depth feedback in climate warming simulations forced by increasing carbon dioxide. A positive relationship between cloud optical depth and cloud temperature exists in all models for low clouds with relatively cold temperatures at middle and high latitudes, whereas a negative relationship exists for warmer low clouds in the tropics and subtropics. This relationship is qualitatively similar to that in an earlier analysis of satellite observations, although modeled regression slopes tend to be too positive and their inter-model spread is large. In the models, the cold cloud response comes from increases in cloud water content with increasing temperature, while the warm cloud response comes from decreases in physical thickness with increasing cloud temperature. The inter-model and inter-regional spread of low-cloud optical depth feedback in climate warming simulations is well predicted by the corresponding spread in the relationships between optical depth and temperature for the current climate, suggesting that this aspect of cloud feedback may be constrained by observations. Because models have a positive bias relative to observations in the optical depth-temperature relationship, shortwave cloud feedbacks for climate changes may be more positive than climate models currently simulate.

Description: Seasonal characteristics of aerosol optical properties (AOP) in SKYNET Hefei site are studied using a sky-radiometer from March 2007 to May 2013. The aerosol optical depth (AOD), Angstrom exponent (AE), volume size distributions, single scattering albedo (SSA), refractive index and asymmetry factor (ASY) of aerosols are simultaneously retrieved using the SKYRAD.pack version 4.2 software. During the study period, the AOD varied seasonally, with the maximum value of 1.02 ± 0.42 at 500 nm occurring in the summer and the highest AOD (1.13 ± 0.42) occurred in June due to stagnant climate conditions and accumulation of polluted aerosols before the East Asian summer monsoon. The variation in AE showed a different pattern, with the minimum (0.97 ± 0.28) and maximum values (1.30 ± 0.22) occurring during the spring and fall seasons, respectively. The relatively low value of AE in spring is related to the emission of Asian dust events. The aerosol volume size distributions can be expressed by the tri-modal patterns for each season, consisting of a fine mode with R 〈 0.6um, a coarse mode with R 〉 2.5um and a middle mode located between them. The real part of the refractive index increased with wavelength (380-870 nm) while the imaginary part of the refractive index decreased for all seasons except for the summer. The seasonal mean values of SSA were 0.97 ± 0.02(summer), 0.95 ± 0.03(spring), 0.93 ± 0.04(autumn), and 0.91 ± 0.04(winter) at 380 nm indicating more absorbing aerosol in the autumn and winter months. Furthermore, aerosol properties were greatly modified by condensation growth as evidenced by the positive dependencies of AOD, SSA and ASY on relative humidity.

Description: For quantification and prediction of global warming due to anthropogenic carbon dioxide (CO 2 ) emissions, high resolution determination of carbon cycle reservoir size and rates of exchange is vital. Isotope ratio measurements of 13 C and 18 O in CO 2 have proven their utility in resolving the carbon cycle. Here we report a long term record of triple oxygen isotopic composition of tropospheric CO 2 . A presence of a steady state stratospheric component (Δ 17 O = ln[(δ 17 O + 1) – 0.522 ln(δ 18 O + 1)] = 0.08 ± 0.04 ‰) is observed in tropospheric CO 2 after modeling via a long-term spline fit of the record, adding an additional technique for quantification of CO 2 fluxes. The δ 18 O of CO 2 (measured as O 2 for oxygen triple-isotopic composition) from the present study is compared to the measurements of the Scripps Institute of Oceanography (SIO) CO 2 network (measured as CO 2 for δ 13 C and δ 18 O) and found to be comparable. We note that the triple oxygen isotopic signal (Δ 17 O) significantly decreased to 0.02 ± 0.02 ‰ from the steady state value of 0.08 ± 0.04 ‰ during the 1997-1999 time period. The possible causes for this depletion are evaluated and discussed. An enhanced role of global primary productivity, facilitating water-CO 2 isotope exchange is possible, and future measurements and modeling strategies may develop this further.

Description: The middle and lower reaches of the Yangtze River basin (MLRY) suffered a once-in-a-fifty-year drought during the spring of 2011. The abnormality of the atmospheric circulation in the spring is characterized by the deeper-than-normal trough over East Asia and the northwest Pacific while stronger-than-normal high pressure system over the west, which results in a stronger-than-normal meridional circulation. Meanwhile, the western Pacific subtropical high is weaker than normal and retreats to the east, so the spring monsoonal moist airs from lower latitudes are relatively weak. The anomalous northerly wind in higher latitudes suppresses the northward motion of the moist airs, and brings dry airs to the MLRY. The northerly wind also suppresses the northward motion of warm airs, and brings cold airs to the MLRY. So finally the air over the MLRY is drier, but colder, than normal, which is different from many other droughts. The coldness of the air plays a negative role to the drought. Results from comparing the C q and C T , the measures of the changes in moisture and temperature between this spring and the normal spring defined based on the tight precipitation-relative humidity relation, indicate that the dryness of the air is much stronger than the coldness, and the air over the MLRY thus maintains a lower-than-normal relative humidity. The animation of daily C q and C T shows that dry and cold airs move from the northwest to the MLRY in a manner of low frequency oscillation.

Description: We present the first comprehensive intercomparison of currently available satellite ozone climatologies in the upper troposphere / lower stratosphere (UTLS) (300-70 hPa) as part of the Stratosphere-troposphere Processes and their Role in Climate (SPARC) Data Initiative. The Tropospheric Emission Spectrometer (TES) instrument is the only nadir-viewing instrument in this initiative, as well as the only instrument with a focus on tropospheric composition. We apply the TES observational operator to ozone climatologies from the more highly vertically resolved limb-viewing instruments. This minimizes the impact of differences in vertical resolution among the instruments and allows identification of systematic differences in the large-scale structure and variability of UTLS ozone. We find that the climatologies from most of the limb-viewing instruments show positive differences (ranging from 5 to 75%) with respect to TES in the tropical UTLS, and comparison to a “zonal mean” ozonesonde climatology indicates that these differences likely represent a positive bias for p ≤ 100 hPa. In the extratropics, there is good agreement among the climatologies regarding the timing and magnitude of the ozone seasonal cycle (differences in the peak-to-peak amplitude of 〈15%) when the TES observational operator is applied, as well as very consistent midlatitude interannual variability. The discrepancies in ozone temporal variability are larger in the tropics, with differences between the datasets of up to 55% in the seasonal cycle amplitude. However, the differences among the climatologies are everywhere much smaller than the range produced by current chemistry-climate models, indicating that the multiple-instrument ensemble is useful for quantitatively evaluating these models.

Description: It has long been recognized that differences in climate model simulated cloud feedbacks are a primary source of uncertainties for the model predicted surface temperature change induced by increasing greenhouse gases such as CO 2 . Large-scale circulation broadly determines when and where clouds form and how they evolve. However, the linkage between large-scale circulation change and cloud radiative effect (CRE) change under global warming has not been thoroughly studied. By analyzing 15 climate models, we show that the change of the Hadley Circulation exhibits meridionally varying weakening and strengthening structures, physically consistent with the cloud changes in distinct cloud regimes. The regions that experience a weakening (strengthening) of the zonal-mean circulation account for 54% (46%) of the multi-model-mean top-of-atmosphere (TOA) CRE change integrated over 45°S-40°N. The simulated Hadley Circulation structure changes per degree of surface warming differ greatly between the models and the inter-model spread in the Hadley Circulation change is well correlated with the inter-model spread in the TOA CRE change. This correlation underscores the close interactions between large-scale circulation and clouds and suggests that the uncertainties of cloud feedbacks and climate sensitivity reside in the intimate coupling between large-scale circulation and clouds. New model performance metrics proposed in this work, which emphasize how models reproduce satellite observed spatial variations of zonal-mean cloud fraction and relative humidity associated with the Hadley Circulation, indicate that the models closer to the satellite observations tend to have equilibrium climate sensitivity higher than the multi-model-mean.

Description: Single scattering albedo (SSA) retrievals obtained with CIMEL sun-sky radiometers from the AERONET aerosol monitoring network were used to make comparisons with simultaneous in-situ sampling from aircraft profiles carried out by the NASA Langley Aerosol Group Experiment (LARGE) team in the summer of 2011 during the coincident DRAGON-MD (Distributed Regional Aerosol Gridded Observational Network-Maryland) and DISCOVER-AQ (Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality) experiments. The single scattering albedos (interpolated to 550 nm) derived from AERONET measurements for aerosol optical depth (AOD) at 440 nm ≥ 0.4 (mean SSA: 0.979) were on average 0.011 lower than the values derived from the LARGE profile measurements (mean SSA: 0.99). The maximum difference observed was 0.023 with all the observed differences within the combined uncertainty for the stated SSA accuracy (0.03 for AERONET; 0.02 for LARGE). Single scattering albedo averages were also analyzed for lower aerosol loading conditions (AOD ≥ 0.2) and a dependence on aerosol optical depth was noted with significantly lower single scattering albedos observed for lower AOD in both AERONET and LARGE datasets. Various explanations for the SSA trend were explored based on other retrieval products including volume median radius and imaginary refractive index as well as column water vapor measurements. Additionally, these SSA trends with AOD were evaluated for one of the DRAGON-MD study sites, Goddard Space Flight Center, and two other Mid-Atlantic AERONET sites over the long-term record dating to 1999.

Description: By combining observations and numerical simulations, we investigated the responses of the surface energy budget and the convective boundary layer (CBL) dynamics to the presence of aerosols. A detailed dataset containing (thermo)dynamic observations at CESAR (Cabauw Experimental Site for Atmospheric Research) and aerosol information from the European Integrated Project on Aerosol, Cloud, Climate, and Air Quality Interactions (IMPACT/EUCAARI) was employed to design numerical experiments reproducing two typical clear-sky days, each characterized by contrasting thermodynamic initial profiles: (i) residual layer above a strong surface inversion, and (ii) well-mixed CBL connected to the free troposphere by a capping inversion, without the residual layer in between. A large-eddy simulation (LES) model and a mixed-layer (MXL) model, coupled to a broadband radiative transfer code and a land-surface model, were used to study the impacts of aerosols on shortwave radiation. Both the LES model and the MXL model results reproduced satisfactorily the observations for both days. A sensitivity analysis on a wide range of aerosol properties was conducted. Our results showed that higher loads of aerosols decreased irradiance imposing an energy restriction at the surface, delaying the morning onset of the CBL and advancing its afternoon collapse. Moderately to strongly absorbing aerosols increased the heating rate contributing positively to increase the afternoon CBL height, potential temperature and to decrease Bowen ratio. In contrast, scattering aerosols were associated with smaller heating rates and cooler and shallower CBLs. Our findings advocate the need for accounting for the aerosol influence in analyzing surface and CBL dynamics.

Description: This study utilizes hydrometeor sonde and radiometer sonde in-situ observations to simultaneously evaluate ice cloud microphysics and radiative fluxes. In addition, the impact of non-sphericity and heterogeneous ice nucleation schemes on radiative fluxes are examined using a double-moment bulk cloud microphysics scheme on a mid-latitude frontal system. The distribution of simulated outgoing longwave radiation (OLR) is systematically reduced by assuming the presence of columnar ice crystals instead of planar ice crystals because of the difference in the effective radii (the projected area) between the two shapes. However, the choice of the heterogeneous ice nucleation schemes drastically changes the distribution of OLR by modifying the number concentration of the cloud ice ( N i ) (more than ten-fold). The observed shortwave fluxes are useful for evaluating the simulated number concentration of cloud ice when non-spherical single scattering properties are used instead of spherical single scattering properties. The dependence of the asymmetry factor on the effective radius is the key to quantitatively estimating the ice cloud radiative forcing and determining the aerosol indirect effect on ice clouds. Based on the comparison of shortwave fluxes, the cloud microphysics scheme was found to underestimate the N i near the cloud base (a robust bias). A possible method of modifying the bias is discussed.

Description: A new study of gravity waves produced by thunderstorms was performed using continuous recordings at the IS17 (Ivory Coast) infrasound station of the International Monitoring System (IMS) developed for the verification of the Comprehensive nuclear-Test-Ban Treaty (CTBT). A typical case study is presented for a large thunderstorm on April 10 th and 11 th , 2006 lasting near 14 hours. Comparison with cloud temperature measured by the Meteosat6 satellite shows that wave activity is large when the cloud temperature is low inside convection cells located over the station. Statistics based on 10 year data show that the wave activity is intense throughout the year with peak periods in May and October and less intense activity in January in good agreement with the local keraunic level. The seasonal variations of the wave azimuth highlight clear trends from northward direction from February to August, to southward direction from August to December. Lightning flashes, observed from space show a similar motion confirming that thunderstorms are the main sources of the gravity wave activity. The gravity wave azimuth follows the seasonal motion of the tropical rainbelt partly related to the InterTropical Convergence Zone of the winds. The contribution of other possible sources, such as wind over relief is weak because surface winds are weak in this region and only oceans are present South of the station. We conclude that the large observed wave activity is mainly produced by convection associated to thunderstorms.

Description: Daily maximum and minimum temperatures over global land are fundamental climate variables, and their difference represents the diurnal temperature range (DTR). While the differences between the monthly averaged DTR (MDTR) and the range of monthly averaged hourly temperature diurnal cycle (RMDT) are easy to understand qualitatively, their differences have not been quantified over global land areas. Based on our newly-developed in-situ data (CRU)-reanalysis (MERRA) merged hourly temperature data from 1979 to 2009, RMDT in January is found to be much smaller than that in July over high northern latitudes, as it is much more affected by the diurnal radiative forcing than by the horizontal advection of temperature. In contrast, MDTR in January is comparable to that in July over high northern latitudes, but it is much larger than January RMDT, as it primarily reflects the movement of lower-frequency synoptic weather systems. The area-averaged RMDT trends north of 40˚N are near zero in November, December, and January, while the trends of MDTR are negative. These results suggest the need to use both the traditional MDTR and RMDT suggested here in future observational and modeling studies. Furthermore, MDTR and its trend are more sensitive to the starting hour of a 24-hour day used in the calculations than those for RMDT, and this factor also needs to be considered in model evaluations using observational data.

Description: Small accumulation precipitation events are critical for the high latitude hydrological cycle. They contribute to more than 50% of total accumulation in the area, and occur at a greater frequency than high accumulation events. Despite their importance, the processes controlling them have not been investigated in sufficient detail. This study characterizes an unexpected high latitude snowfall event at Iqaluit, Nunavut and surrounding area during the Storm Studies in the Arctic (STAR) field project. High-resolution data collected, from both ground based and airborne Doppler radar, along with upper air and surface observations, provided the basis for analysis of the conditions that led to the event and offer some insight as to why it was not well forecast by the Canadian operational model. Several factors worked in concert to produce this event. Low-level convection and upslope processes were important in cloud and precipitation generation over the orography upstream. When combined with additional lift from the passing of a weak trough, cloud and precipitation production were enhanced, allowing these features to penetrate over the terrain, and resulted in precipitation at Iqaluit. Analysis of the Global Environmental Multi-scale Limited Area Model (2.5 km resolution) suggests that upstream convection and upslope processes were affected by model errors. As a consequence, precipitation onset was delayed and total accumulation was 50% lower than observations. Results indicate that the complexity of precipitation events in the region represents a significant challenge for predicting and modeling and to understanding their role in the regions hydrological cycle.

Description: No abstract is available for this article.

Description: [1]  Surface ozone is a secondary air pollutant formed from reactions between nitrogen oxides (NO x  = NO + NO 2 ) and volatile organic compounds in the presence of sunlight. In this work we examine effects of the climate pattern known as the El-Niño Southern Oscillation (ENSO) and NO x variability on surface ozone from 1990 to 2007 over the South African Highveld, a heavily populated region in South Africa with numerous industrial facilities. Over summer and autumn (December-May) on the Highveld, El-Niño, as signified by positive sea surface temperature (SST) anomalies over the central Pacific Ocean, is typically associated with drier and warmer than normal conditions favoring ozone formation. Conversely, La-Niña, or negative SST anomalies over the central Pacific Ocean, is typically associated with cloudier and above normal rainfall conditions, hindering ozone production. We use a generalized regression model to identify any linear dependence that the Highveld ozone, measured at five air quality monitoring stations, may have on ENSO and NO x . Our results indicate that four out of the five stations exhibit a statistically significant sensitivity to ENSO at some point over the December-May period where El Niño amplifies ozone formation and La Niña reduces ozone formation. Three out of the five stations reveal statistically significant sensitivity to NO x variability, primarily in winter and spring. Accounting for ENSO and NO x effects throughout the study period of 18 years, two stations exhibit statistically significant negative ozone trends in spring and September, one station displays statistically significant positive trend in August, and two stations show no statistically significant change in surface ozone.

Description: [1]  Co-located SEVIRI retrieved and GERB fluxes at the top of atmosphere are used to provide, for the first time, an observationally based estimate of the cloud-free net direct radiative effect (DRE) of mineral dust aerosol from geostationary satellite observations, providing new insights into the influence of time of day on the magnitude and sign of the shortwave, longwave and overall net effect during sunlit hours. Focusing on the GERBILS campaign over north Africa during June 2007, the presence of mineral dust aerosol reduces the outgoing longwave radiation at all times of day with the peak reduction clearly following the diurnal cycle of surface temperature. The instantaneous shortwave DRE shows strong dependencies on pristine sky albedo and solar zenith angle such that the same dust loading can induce a postive or negative value dependent on time of day. However, the area mean net DRE over the GERBILS period is dominated by the longwave component at all sampled times of day, with mineral dust inducing a reduction in outgoing net flux of the order of 10 Wm -2 . Hence, in the mean sense Saharan dust is found to warm the Earth-Atmosphere system over northern Africa and the Middle East.

Description: Nitrate is one of the major PM 2.5 components in polluted environments and its concentration could be greatly elevated during pollution episodes. High temporal resolution measurements of aerosol components provide valuable information in probing sources and formation process. In this study, nitrate and other inorganic soluble ions on PM 2.5 were measured half-hourly with a semi-continuous system (Particle-Into-Liquid-System coupled with two ion chromatographs) at a residential location in Hong Kong in December 2009. High temporal variation was observed in the hourly concentrations of nitrate in PM 2.5 , varying from 0.8 to 40.5 µg m -3 . A pollution episode with hourly visibility down to 3.7 ± 1.0 km and NO 2 as high as 80.7 ± 14.4 ppb was encountered. During the episode hours, PM 2.5 NO 3 - concentration reached 27.8 ± 8.0 µg m -3 , an increase by 5.2 times in comparison with the normal hours. Despite the high level, nitrate was fully balanced by NH 4 + , indicating abundant presence of NH 3 . Varied size distributions among coarse and fine particles were observed for nitrate, as recorded in nine sets of size-segregated samples collected during the observation period. During the episode, 84% of nitrate was found in the fine mode, contributing significantly to the elevation of PM 2.5 . The size-resolved ionic composition suggests that the fine mode nitrate is more likely to become significant when fine particles are less acidic and when less sea-salt particles are available to compete for gaseous HNO 3 . An Observation-Based Model for Secondary Inorganic Aerosols was applied to investigate the relative importance of homogeneous and heterogeneous reactions to production of NO 3 - potential (sum of HNO 3 (g) and aerosol nitrate) during the episode. The modeling analysis shows that both homogeneous and heterogeneous formation pathways for HNO 3 were much more active during the episode. Gas-phase production of HNO 3 through reaction of NO 2  + OH followed by neutralization by NH 3 was the dominant pathway during the initial rapid build-up of nitrate around noon time but the heterogeneous N 2 O 5 hydrolysis pathway made a sizable contribution in the subsequent few hours due to sustained high concentrations of NO 2 (62-104 ppb) combined with reduced photolysis loss of N 2 O 5 . This case study illustrates the important role of NH 3 and NO 2 in contributing to elevated PM 2.5 in a polluted high NO x environment through the formation of nitrate.

Description: Updraft velocities strongly control the activation of aerosol particles or that component that act as Cloud Condensation Nuclei (CCN). For km-scale models, vertical motions are partially resolved but the sub-grid scale (SGS) contribution needs to be parametrised or constrained to properly represent the activation of CCNs. This study presents a method to estimate the missing SGS (or unresolved) contribution to vertical velocity variability in models with horizontal grid sizes up to ~2 km. A framework based on Large Eddy Simulations (LES) and high-resolution aircraft observations of stratocumulus and shallow cumulus clouds has been developed and applied to output from the United Kingdom Met-Office Unified Model (UM) operating at km-scale resolutions in numerical weather prediction configuration. For a stratocumulus deck simulation, we show that the UM 1 km model underestimates significantly the variability of updraft velocity with an averaged cloud-base standard deviation of between 0.04 and 0.05 ms − 1 compared to LES and aircraft estimates of 0.38 and 0.54 ms − 1 respectively. Once the SGS variability is considered, the UM corrected averages are between 0.34 and 0.44 ms − 1 . Off-linecalculations of CCN activated fraction using an activation scheme have been performed to illustrate the implication of including the SGS vertical velocity. It suggests increased CCN activated fraction from 0.52 to 0.89 (respectively 0.10 to 0.54) for a clean (respectively polluted) aerosol environment for simulations with a 1 km horizontal grid-size. Our results highlights the importance of representing the SGS vertical velocity in km-scale simulations of aerosol-cloud interactions.

Description: Aerosols play a significant yet complex and central role in the Earth´s radiation budget, and knowledge of long-term changes in the atmospheric turbidity induced by aerosols is therefore fundamental for a better understanding of climate change. However, there is little available information on changes in aerosol concentration in the atmosphere, especially prior to the 1980s. The present paper reviews publications reporting the suitability of sunshine duration records with regard to detecting changes in atmospheric aerosols. Some of the studies reviewed propose methods for estimating aerosol-related magnitudes, such as turbidity, from sunshine deficit at approximately sunrise and sunset, when the impact of aerosols on the solar beam is more easily observed. In addition, there is abundant evidence that one cause of the decadal changes observed in sunshine duration records involves variations in atmospheric aerosol loading. Possible directions for future research are also suggested: in particular, detailed studies of the burn (not only its length but also its width) registered by means of Campbell-Stokes sunshine recorders may provide a way of creating time series of atmospheric aerosol loading metrics dating back to over 120 years from the present.

Description: While the calibration accuracy of Advanced Microwave Sounding Unit-A (AMSU-A) was well characterized during its prelaunch period, its on-orbit performance remains important after the launch of each satellite. In this study, Global Positioning System (GPS) radio occultation (RO) data are used to carry out a postlaunch calibration to obtain an estimate of the accuracy of brightness temperatures measured by an AMSU-A instrument. At each scan angle, the mean difference and a linear regression relationship between AMSU-A observations and GPS RO brightness temperature simulations can firstly be derived using data in clear-sky conditions over ocean. The AMSU-A upper-air sounding channels are then calibrated to GPS RO brightness temperature simulations using either the mean difference or the linear regression relationship. The effect of the above two calibration methods on the biases of AMSU-A data with respect to NCEP GFS 6-h forecast fields is finally examined. It is found that the magnitude and sign of biases vary with channels. After GPS RO calibration, biases are negative and of the same magnitude for all AMSU-A upper-air sounding channels. The negative biases of AMSU-A brightness temperature are associated with a small warm bias in the physical temperature of NCEP GFS 6-h forecasts compared with GPS RO temperature retrieval above the low troposphere.

Description: This study focuses on the decadal variability of tropical cyclone (TC) activity over the South China Sea (SCS) since the 1970s and its possible cause behind. It is found that TC activity over the SCS experiences a significant decadal change around the mid-1990s. Compared to the period from the 1970s to the early 1990s, the number of TCs formed in the SCS remarkably increases from the mid-1990s through the 2000s. In particular, this change of TC genesis is closely related to a decadal shift in atmospheric intraseasonal variability (ISV) that occurred in 1994. The ISV on the 30-60 days time scale over the SCS has been increasing since the mid-1990s, and the increased TC frequency after 1994 is attributed primarily to the active convection induced by the enhancement of the SCS ISV. In addition, the TC activities before the mid-1990s are mostly confined within the SCS basin. However, more TCs form over the SCS and move northeastward since the mid-1990s, and finally enter the East China Sea and the Philippine Sea. Anomalies of westerly over the northern SCS after 1994 are responsible for the northeastward moving of TCs.

Description: The dynamical and microphysical processes that influence water vapor concentrations in the tropical tropopause layer (TTL) are investigated in simulations of ice clouds along backward trajectories of air parcels sampled during three flights of the Airborne Tropical Tropopause Experiment (ATTREX) over the central to eastern tropical Pacific in boreal fall 2011. ERA-Interim reanalysis temperatures interpolated onto the flight tracks have a negligible (–0.09 K) cold bias compared to aircraft measurements of tropical cold point temperature thus permitting case-study simulations of TTL dehydration. When the effects of subgrid-scale waves, cloud microphysical processes and convection are considered, the simulated water vapor mixing ratios on the final day of 40-d backward trajectories exhibit a mean profile that is within 20-30% of the mean of the aircraft measurements collected during vertical profiling maneuvers between the 350 and 410 K potential temperature levels. Averaged over the three flights, temperature variability driven by subgrid-scale waves dehydrated the 360-390 K layer by approximately –0.5 ppmv, whereas including homogeneous freezing of aqueous aerosols and subsequent sublimation and rehydration of ice crystals increased water vapor below the 380 K level by about +1 ppmv. The predominant impact of convection was to moisten the TTL, resulting in an average enhancement below the 370 K level by +1 to 5 ppmv. Accurate (to within 0.5-1 ppmv) predictions of TTL water vapor using trajectory models require proper representations of waves, in situ ice cloud formation and convective influence, which together determine the saturation history of air parcels.

Description: A fuel-based inventory for vehicle emissions is presented for carbon dioxide (CO 2 ), and mapped at various spatial resolutions (10 km, 4 km, 1 km, and 500 m) using fuel sales and traffic count data. The mapping is done separately for gasoline-powered vehicles and heavy-duty diesel trucks. Emissions estimates from this study are compared with the Emissions Database for Global Atmospheric Research (EDGAR) and VULCAN. All three inventories agree at the national level within 5%. EDGAR uses road density as a surrogate to apportion vehicle emissions, which leads to 20-80% overestimates of on-road CO 2 emissions in the largest U.S. cities. High-resolution emission maps are presented for Los Angeles, New York City, San Francisco-San Jose, Houston, and Dallas-Fort Worth. Sharp emission gradients that exist near major highways are not apparent when emissions are mapped at 10 km resolution. High CO 2 emission fluxes over highways become apparent at grid resolutions of 1 km and finer. Temporal variations in vehicle emissions are characterized using extensive day- and time-specific traffic count data, and are described over diurnal, day of week, and seasonal time scales. Clear differences are observed when comparing light- and heavy-duty vehicle traffic patterns and comparing urban and rural areas. Decadal emission trends were analyzed from 2000 to 2007 when traffic volumes were increasing, and a more recent period (2007-2010) when traffic volumes declined due to recession. We found large non-uniform changes in on-road CO 2 emissions over a period of ~5 years, highlighting the importance of timely updates to motor vehicle emission inventories.

Description: The contribution of the evapotranspiration from a certain region to the precipitation over the same area is referred to as water recycling. In this paper, we explore the spatio-temporal links between the recycling mechanism and the Iberian rainfall regime. We use a 9-km resolution WRF simulation of 18 years (1990-2007) to compute local and regional recycling ratios over Iberia, at the monthly scale, through both an analytical and a numerical recycling model. In contrast to coastal areas, the interior of Iberia experiences a relative maximum of precipitation in spring, suggesting a prominent role of land-atmosphere interactions on the inland precipitation regime during this period of the year. Local recycling ratios are the highest in spring and early summer, coinciding with those areas where this spring peak of rainfall represents the absolute maximum in the annual cycle. This confirms that recycling processes are crucial to explain the Iberian spring precipitation, particularly over the eastern and north-eastern sectors. Average monthly recycling values range from 0.04 in December to 0.14 in June according to the numerical model and from 0.03 in December to 0.07 in May according to the analytical procedure. Our analysis shows that the highest values of recycling are limited by the coexistence of two necessary mechanisms: 1) the availability of sufficient soil moisture and 2) the occurrence of appropriate synoptic configurations favoring the development of convective regimes. The analyzed surplus of rainfall in spring has a critical impact on agriculture over large semiarid regions of the interior of Iberia.

Description: The radiation schemes in the Weather Research and Forecasting (WRF) model have previously not accounted for the presence of subgrid-scale cumulus clouds, thereby resulting in unattenuated shortwave radiation which can lead to overly energetic convection and overpredicted surface precipitation. This deficiency can become problematic when applying WRF as a regional climate model (RCM). Therefore, modifications were made to the WRF model to allow the Kain-Fritsch (KF) convective parameterization to provide subgrid-scale cloud fraction and condensate feedbacks to the Rapid Radiative Transfer Model – Global (RRTMG) shortwave and longwave radiation schemes. The effects of these changes are analyzed via three-year simulations using the standard and modified versions of WRF, comparing the modeled results with the North American Regional Reanalysis (NARR) and Climate Forecast System Reanalysis data, as well as with available data from the Surface Radiation Network and Clouds and Earth's Radiant Energy System. During the summer period, including subgrid cloudiness estimated by KF in the RRTMG reduces the surface shortwave radiation, leading to less buoyant energy, which is reflected in a smaller diabatic convective available potential energy, thereby alleviating the overly-energetic convection. Overall, these changes have reduced the overprediction of monthly regionally-averaged precipitation during summer for this RCM application; e.g., by as much as 49 mm for the southeastern U.S., to within 0.7% of the NARR value of 221 mm. These code modifications have been incorporated as an option available in the latest version of WRF (v3.6).

Description: Recent decades have seen increased melting of the Greenland ice sheet. On 11 July 2012, nearly the entire surface of the ice sheet melted; such rare events last occurred in 1889 and, prior to that, during the Medieval Climate Anomaly. Studies of the 2012 event associated the presence of a thin, warm elevated liquid cloud layer with surface temperatures rising above the melting point at Summit Station, some 3212 m above sea level. Here we explore other potential factors in July 2012 associated with this unusual melting. These include 1) warm air originating from a record North American heat wave, 2) transitions in the Arctic Oscillation, 3) transport of water vapor via an Atmospheric River over the Atlantic to Greenland, and 4) the presence of warm ocean waters south of Greenland. For the 1889 episode, the Twentieth Century Reanalysis and historical records showed similar factors at work. However, markers of biomass burning were evident in ice cores from 1889 which may reflect another possible factor in these rare events. We suggest that extreme Greenland summer melt episodes, such as those recorded recently and in the late Holocene, could have involved a similar combination of slow climate processes, including prolonged North American droughts/heat waves and North Atlantic warm oceanic temperature anomalies, together with fast processes, such as excursions of the Arctic Oscillation, and transport of warm, humid air in Atmospheric Rivers to Greenland. It is the fast processes that underlie the rarity of such events and influence their predictability.

Description: Using two different operational Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) cloud optical depth (COD) retrievals (0.86 versus 1.6 µm), we evaluate the impact of above-cloud smoke aerosol particles on near-IR (0.86 µm) COD retrievals. Aerosol Index (AI) from the collocated Ozone Monitoring Instrument (OMI) are used to identify above-cloud aerosol particle loading over the southern Atlantic Ocean, including both smoke and dust from the African sub-continent. Collocated Cloud-Aerosol Lidar and Infrared Pathfinder (CALIPSO) data constrain cloud phase and provide contextual above-cloud Aerosol optical depth (AOD). The frequency of occurrence of above-cloud aerosol events is depicted on a global scale for the spring and summer seasons from OMI and CALIOP. Seasonal frequencies for smoke-over-cloud off the southwestern Africa coastline reach 20-50% in boreal summer. We find a corresponding low COD bias of 10-20% for standard MODIS COD retrievals when averaged OMI AI are larger than 1. No such bias is found over the Saharan dust outflow region off northern Africa, since both MODIS 0.86 and 1.6 µm channels are vulnerable to radiance attenuation due to dust particles. A similar result is found for a smaller domain, in the Gulf of Tonkin region, from smoke advection over marine stratocumulus clouds and outflow into the northern South China Sea in spring. This study shows the necessity of accounting for the above-cloud aerosol events for future studies using standard MODIS cloud products in biomass burning outflow regions, through the use of collocated OMI AI and supplementary MODIS 1.6 µm COD products.

Description: Two recent papers have quantified long-term ozone (O3) changes observed at northern mid-latitude sites that are believed to represent baseline (here understood as representative of continental to hemispheric scales) conditions. Three chemistry climate models (NCAR CAM-chem, GFDL-CM3 and GISS-E2-R) have calculated retrospective tropospheric O3 concentrations as part of the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) and Coupled Model Intercomparison Project Phase 5 (CMIP5) model intercomparisons. We present an approach for quantitative comparisons of model results with measurements for seasonally averaged O3 concentrations. There is considerable qualitative agreement between the measurements and the models, but there are also substantial and consistent quantitative disagreements. Most notably models 1) overestimate absolute O3 mixing ratios, on average by ~5 to 17 ppbv in the year 2000, 2) capture only ~50% of O3 changes observed over the past five to six decades, and little of observed seasonal differences, and 3) capture ~25 to 45% of the rate of change of the long-term changes. These disagreements are significant enough to indicate that only limited confidence can be placed on estimates of present-day radiative forcing of tropospheric O3 derived from modeled historic concentration changes and on predicted future O3 concentrations. Evidently our understanding of tropospheric O3, or the incorporation of chemistry and transport processes into current chemical climate models, is incomplete. Modeled O3 trends approximately parallel estimated trends in anthropogenic emissions of NOX, an important O3 precursor, while measured O3 changes increase more rapidly than these emission estimates.

Description: We use the adjoint of a global 3-D chemical transport model (GEOS-Chem) to optimize ammonia (NH 3 ) emissions in the U.S., European Union, and China by inversion of 2005–2008 network data for wet deposition fluxes. Optimized emissions are derived on a 2 °  × 2.5 ° grid for individual months and years. Error characterization in the optimization includes and corrects for model errors in precipitation. Annual optimized emissions are 2.8Tg NH 3  − N a − 1 for the contiguous U.S., 3.1Tg NH 3  − N a − 1 for the European Union, and 8.4Tg NH 3  − N a − 1 for China. Comparisons to previous inventories for the U.S. and European Union show consistency (~± 15 % ) in annual totals but some large spatial and seasonal differences. We develop a new global bottom-up inventory of NH 3 emissions (MASAGE_NH3) to interpret results of the adjoint optimization in terms of the underlying emission processes. MASAGE_NH3 includes detailed information on the contributions of individual crop and livestock types to NH 3 emissions on a 0.5 °  × 0.5 ° grid. Its seasonality is driven by meteorology and crop operations. MASAGE_NH3 captures the seasonality and magnitude of the optimized emissions in most of the regions considered here. We thus find that U.S. emissions peak in the spring in the Midwest due to corn fertilization and in the summer elsewhere due to manure. The seasonality of European emissions is more homogeneous with a well-defined maximum in spring associated with manure and mineral fertilizer application. There is some evidence for the effect of European regulations of NH 3 emissions, notably a large fall decrease in northern Europe and high emissions in Spain (where emissions are not regulated). Emissions in China peak in summer because of the summertime application of fertilizer for double cropping.

Description: High-speed video records of two bipolar cloud-to-ground flashes were analyzed in detail. They both began with a single positive return stroke that was followed by more than one subsequent weak negative stroke. Due to the elevated cloud-base height of its parent thunderstorm, the preparatory processes of each subsequent negative stroke were documented optically below cloud base. In the first event (Case 1) it was observed that all four subsequent negative strokes were initiated by recoil leaders that retraced one horizontal channel segment previously ionized by the positive leader. Those recoil leaders connected to the original vertical channel segment and propagated towards ground, producing four subsequent strokes that had the same ground contact point as the original positive discharge. The second event (Case 2), in contrast, presented fifteen subsequent strokes that were initiated by recoil leaders that did not reach the original channel of the positive stroke. They diverged vertically towards ground, making contact approximately 11 kilometers away from the original positive strike point. These results constitute the first optical evidence that both single- and multiple-channel bipolar flashes occur as a consequence of recoil leader activity in the branches of the initial positive return stroke. For both events their total channel length increased continuously at a rate of the order of 10 4  m s -1 , comparable to speeds reported for typical positive leaders.

Description: [1]  Interactively coupled climate chemistry models (CCMs) extend the number of feedback mechanisms in climate change simulations by including chemical feedbacks. In this study the radiative feedback from ozone changes on climate response and climate sensitivity is quantified for a series of simulations driven by CO 2 increases on top of a present-day reference concentration level. Other possibly relevant feedbacks via atmospheric chemistry, e.g. via CH 4 and N 2 O, are not fully quantified in the CCM setup as their concentrations are essentially fixed at the surface. In case of a CO 2 doubling simulation the ozone feedback reduces the climate sensitivity parameter by 3.4%, from 0.70 K/(Wm -2 ) without interactive chemistry to 0.68 K/(Wm -2 ). In case of a 4*CO 2 simulation the reduction of the climate sensitivity parameter increases to 8.4%. An analysis of feedbacks reveals that the negative feedbacks of stratospheric ozone and the associated negative feedback change in stratospheric water vapor are mainly responsible for this damping. The feedback from tropospheric ozone changes is positive but much smaller. The nonlinearity in the climate sensitivity damping with increased CO 2 concentrations is shown to be due to nonlinear feedbacks of ozone and stratospheric water vapor.

Description: [1]  We calculated the photolysis rate coefficients of five ozone isotopologues/isotopomers as functions of altitude up to 80 km using recent ab-initio absorption cross sections [ Ndengué et al 2010] and an averaged actinic flux. Three of the five ozone isotopologues/isotopomers are symmetric, with a single isotopic dissociation channel, and two are asymmetric with two isotopic dissociation channels. The specific contributions of the Chappuis, Huggins and Hartley bands to the photolysis rates and enrichments have been determined as a function of altitude. The Chappuis and Hartley bands have a dominant contribution to the photolysis rates, respectively at low and high altitudes, but these two bands are characterized by small fractionations. In contrast, the Huggins band has a minor contribution to the overall photolysis rate at any altitude but it generates most of the strong fractionation which peaks around 35 km. The photolysis fractionations are " mass-dependent" in contrast with those due to the ozone formation process which are “non-mass-dependent”. The altitude dependences of our photolysis fractionations are in qualitative agreement with those of Liang et al .[2006] but differ quantitatively, most notably because the contribution of the Huggins band has been re-evaluated. The branching ratio between the two electronic dissociation channels (either O( 3 P) or O( 1 D) products) has been used to calculated the isotopic enrichments of the reactive O( 1 D) induced by ozone photolysis. The isotopic photolysis rates can be included in a global kinetic model that includes ozone formation processes and collision with O 2 and other oxygen containing species.

Description: [1]  The radiative forcing (RF) related to atmospheric ozone variations is generally derived from radiative transfer codes using as input either observation-based datasets or atmospheric information provided by chemistry-climate models. This paper proposes a method to calculate the ozone RF at the Earth's surface in the ultraviolet (UV) spectral range for all-sky conditions based on the estimation of the ozone efficiency (OE) from experimental data that is subsequently applied to changes on the total ozone column (TOC) since late 1970s. The OE is defined as the rate at which the solar UV irradiance is “forced” per TOC-unit, being estimated for all-sky conditions from UV-B (280-320 nm) and TOC data recorded with a Bentham spectroradiometer at Granada (Spain). The results showed a clear seasonal pattern in the OE values with largest monthly averages (in absolute terms) in July (-4.2 ± 0.3 mW/m 2 per Dobson Unit) and the smallest in January (-0.7 ± 0.3 mW/m 2 per Dobson Unit) which was mainly related to the strong influence of the solar elevation in the efficiency values. The continuous and consistent TOC dataset (1979-2008) provided by the Multi Sensor Reanalysis (MSR) over the study site showed that spring months (March, April and May) present the largest annual TOC changes relative to 1979 while summer months (June, July and August) exhibit small variations. Thus, spring has the largest contribution (~ 53%) to annual ozone RF followed by summer (~ 17%), winter (~ 16%) and autumn (~ 14%). The evolution of the ozone RF relative to 1979 in the UV-B range at Granada showed large inter-annual fluctuations with positive values for most of years (between 5 and 40 mW/m 2 ). Finally, the long-term evolution of the ozone RF exhibited a positive trend until the mid- 1990s and, subsequently, a weak negative trend until the end of the analyzed period in accordance with the TOC trend over the study site.

Description: [1]  An unusually large, explosive convective cloud system was observed over the equatorial Indian Ocean on 28 November 2011 during the DYNAMO [Dynamics of the Madden-Julian Oscillation (MJO)] field campaign. The rapid development and expansion of extremely cold cloud tops (minimum −96 °C) gave the impression of an “explosion”. The significance of this large mesoscale convective system (MCS) is its size and rapid development during the initiation of a strong MJO event. Observations from the DYNAMO sounding array show that the large MCS developed within a well-defined synoptic-scale cyclonic circulation associated with an equatorial low-pressure system with characteristics of a mixed Rossby-gravity wave that dominated the flow in the DYNAMO array. Prior to the development of the MCS, the equatorial flow was characterized by strong vertical wind shear with low-level westerlies and upper-level easterlies. A region of decreased wind shear and enhanced upper-level divergence emerged over the DYNAMO array concurrently with the passage of the westward-moving mixed Rossby-gravity wave-related low-pressure system and convective activity during the MJO initiation. The in situ sounding observations suggest that widespread deep convection upstream of the large MCS may have contributed to the reduction of the upper-level easterlies through vertical momentum transport and convective outflow. Both the reduction in vertical wind shear and enhanced low-level convergence induced by the equatorial low-pressure system created a favorable environment for the rapid development of the large MCS. This study examines the development of the MCS and the associated synoptic-scale equatorial low-pressure system within the large-scale MJO circulation using high temporal resolution (3-hourly) in situ sounding observations from DYNAMO, which provide new insights into the interaction between convection and environmental flow during MJO initiation over the equatorial Indian Ocean.

Description: [1]  Natural carbon sources and sinks over China's terrestrial land ecosystems have an uncertain magnitude, but possibly an important role in the global carbon budget. In this paper we present an estimate of net ecosystem exchange of CO 2 (NEE) over China for the years 2001-2010 using the CarbonTracker Data Assimilation System for CO 2 (CTDAS). Additional Chinese and Asian CO 2 observations are used in CTDAS to improve our estimate. We found that the combined terrestrial ecosystems in China absorbed about –0.33 Pg Cy r -1 during 2001-2010, compensating approximately 20% of the total CO 2 emissions from fossil fuel burning and cement manufacturing from China (+1.70 Pg C yr -1 ). The uncertainty on Chinese terrestrial carbon exchange estimates as derived from a set of sensitivity experiments suggests a range of –0.29 to –0.64 Pg C yr -1 . This total Chinese terrestrial CO 2 sink is attributed to the three major biomes (forests, crop lands and grass/shrub lands) with estimated CO 2 fluxes of –0.12 Pg C yr -1 (range from –0.09 to –0.19 Pg C yr -1 ), –0.12 Pg C yr -1 (range from –0.09 to –0.26 Pg C yr -1 ) and –0.09 Pg C yr -1 (range from –0.09 to –0.17 Pg C yr -1 ), respectively. The peak-to-peak amplitude of inter-annual variability (IAV) of the Chinese terrestrial ecosystem carbon flux is 0.21 Pg C yr -1 (~64% of mean annual average), with the smallest CO 2 sink (–0.19 Pg C yr -1 ) in 2003 and the largest CO 2 sink (–0.40 Pg C yr -1 ) in 2007 corresponding with favorable temperature in spring/winter. We stress that our estimate of terrestrial ecosystem CO 2 uptake based on inverse modeling strongly depends on a limited number of atmospheric CO 2 observations used. More observations in China specifically, and in Asia in general are needed to improve the accuracy of terrestrial carbon budgeting for this region.

Description: [1]  In this study, an explicit representation of vertical momentum transport by convective cloud systems, including mesoscale convective systems (MCSs), is proposed and tested in a multi-scale modeling framework (MMF). The embedded cloud-resolving model (CRM) provides vertical momentum transport in one horizontal direction. The vertical momentum transport in the other direction is assumed to be proportional to the vertical mass flux diagnosed from the CRM in addition to the effects of entrainment and detrainment. In order to represent both upgradient and downgradient vertical momentum transports, the orientation of the embedded CRM must change with time instead of being stationary typically in MMFs. The orientation is determined by the stratification of the lower troposphere and environmental wind shear. Introducing the variation of the orientations of the embedded CRM is responsible for reducing the stationary anomalous precipitation and many improvements. Improvements are strengthened when the CRM simulated vertical momentum transport is allowed to modify the large-scale circulation simulated by the host general circulation model. These include an improved spatial distribution, amplitude and intraseasonal variability of the surface precipitation in the tropics, more realistic zonal-mean diabatic heating and drying patterns, more reasonable zonal-mean large-scale circulations and the East Asian summer monsoon circulation, and an improved, annual-mean implied meridional ocean transport in the Southern Hemisphere. Further tests of this convective momentum transport parameterization scheme will be performed with a higher-resolution MMF to further understand its roles in the intraseasonal oscillation and tropical waves, monsoon circulation, and zonal-mean large-scale circulations.

Description: [1]  This study investigates the general macrophysical and microphysical properties of single-layer warm clouds over the Northern Hemisphere (NH) using standard CloudSat products during 2008. The yearly averaged occurrence frequency of single-layer warm clouds is 20.9% over oceans and 9.5% over land, respectively. The cloud top heights (CTH) over land in different latitude zones exhibit a broader spectral width and more frequent occurrence of strong cloud systems in tropics than that over higher latitudes. The maximum values of liquid water content (LWC) over land are approximately 10-30% smaller but the occurring altitudes are about 0.5 km higher than those over oceans with the same liquid water path (LWP). The reflectivity profiles clearly demonstrate the growth processes of cloud particles and how these processes vary with increasing LWP. The evolutions of reflectivity over land show a frequency shift toward weaker dBZ values relative to that over oceans with the same LWP, suggesting that high aerosol concentrations may induce suppressed drizzle and delayed precipitation in warm clouds. Additionally, there exists a significant difference between warm clouds formation at the initial stage over ocean and land areas. When LWP is 0.1-0.2 kg m -2 and optical depth exceeds 25, the dBZ values over oceans increase rapidly downward with increasing optical depth (decreasing height). The faster growth may be caused by the evaporation-condensation mechanism. Over land, however, large droplets are unlikely to appear in the early stage possibly because continental aerosols prevent the evaporation-condensation mechanism to occur, resulting in partially delayed drizzle formation.

Description: ABSTRACT [1]  Thirty Coupled Model Intercomparison Project phase 5 (CMIP5) preindustrial simulations are examined to contrast the impacts of the two types of El Niño on US winter temperatures. The CMIP5 models are found to be more capable of simulating the observed Eastern Pacific (EP) El Niño impacts (a warm-northeast, cold-southwest pattern over the US) but less capable of simulating the observed Central Pacific (CP) El Niño impacts (a warm-northwest, cold-southeast pattern). During EP El Niño events, large sea surface temperature (SST) anomalies are located in the eastern equatorial Pacific that enable the events to influence the Walker circulation giving rise to a basin-wide pattern of outgoing longwave radiation (OLR) anomalies. The modeled atmospheric responses to the EP El Niño are thus less sensitive to the detailed structure of the simulated SST anomalies and can be well simulated by most of the CMIP5 models. In contrast, the SST anomalies associated with the CP El Niño are located in the central equatorial Pacific and affect the strength of the Walker Circulation less effectively than the EP El Niño. OLR anomalies are local, rather than basin-wide, and located to the west of the SST anomalies. The modeled atmospheric responses to the CP El Niño, therefore, depend more on how realistically the CP El Niño SST anomalies are simulated in the models. As a result, the CP El Niño's impact on US winter temperature, which is controlled by the atmospheric wave train response to the OLR forcing, is less well simulated by the CMIP5 models. This conclusion is supported by an examination of the Pacific North American and Tropical/Northern Hemisphere patterns produced by the CMIP5 models in response to the two types of El Niño.

Description: [1]  The microphysical characteristics of precipitating convection occurring in various stages of the Madden-Julian Oscillation (MJO) over the Indian Ocean are determined from data obtained from the National Center for Atmospheric Research dual-polarimetric Doppler S-band radar, S-PolKa, deployed as part of the Dynamics of the MJO (DYNAMO) field experiment. Active MJO events with increased rainfall occurred in October, November, and December of 2011. During each of these active MJO phases, in addition to enhanced rainfall, convection became deeper and ice-phase microphysics played a greater role. S-PolKa consistently showed non-oriented small ice particles dominating the radar echoes at altitudes of 9-10 km, dry aggregates concentrated between 7 and 9 km, and wet aggregates and graupel near the melting level (~5 km). Graupel occurred mainly in actively convective towers, while the wet aggregates occurred almost exclusively in the stratiform regions of mesoscale convective systems (MCSs). During each of the three multi-week MJO active phases, the maximum rainfall occurred in short bursts lasting a few days. Each multi-day rainy period began with deepening convective elements and a concurrent increase in occurrence of dry aggregates, which maximized just prior to organization into MCSs. The peak rainfall occurrence coincided with the maximum coverage of the radar domain by MCSs, reflecting large stratiform regions that exhibited the most frequent occurrence of wet aggregates. During the December active MJO phase, however, the MCSs were shallower and had a slightly lower tendency for wet aggregates in the stratiform regions, and, therefore, generally weaker brightbands.

Description: [1]  Airborne volcanic ash can pose a hazard to aviation, agriculture and to both human and animal health. It is therefore important that ash clouds are monitored both day and night, even when they travel far from their source. Infrared satellite data provide perhaps the only means of doing this and, since the hugely expensive ash crisis that followed the 2010 Eyjafjalljökull eruption, much research has been carried out into techniques for discriminating ash in such data, and for deriving key properties. Such techniques are generally specific to data from particular sensors, and most approaches result in a binary classification of pixels into ‘ash’ and ‘ash free’ classes with no indication of the classification certainty for individual pixels. Furthermore, almost all operational methods rely on expert-set thresholds to determine what constitutes ‘ash’, and can therefore be criticized for being subjective and dependent on expertise that may not remain with an institution. Very few existing methods exploit available contemporaneous atmospheric data to inform the detection, despite the sensitivity of most techniques to atmospheric parameters. The Bayesian method proposed here does exploit such data, and gives a probabilistic, physically based classification. We provide an example of the method's implementation for a scene containing both land and sea observations, and a large area of desert dust (often misidentified as ash by other methods). The technique has already been successfully applied to other detection problems in remote sensing, and this work shows that it will be a useful and effective tool for ash detection.

Description: [1]  Total 550 nm aerosol optical depth (AOD), surface fine particulate matter (PM 2.5 ), and meteorological observations were assimilated with continuously cycling three-dimensional variational (3DVAR), ensemble square root Kalman filter (EnSRF), and “hybrid” variational-ensemble data assimilation (DA) systems. The hybrid system's background error covariances (BECs) were a blend of those in 3DVAR and produced by the cycling EnSRF system, and the 3DVAR, EnSRF, and hybrid systems differed almost exclusively by their BECs. New analyses were produced every 6-hrs between 0000 UTC 1 June and 1800 UTC 14 July 2010 over a domain encompassing the contiguous United States (CONUS) and adjacent areas. Additionally, a control experiment that only assimilated meteorological observations was performed. Each 1800 UTC analysis initialized a 48-hr Weather Research and Forecasting-Chemistry (WRF-Chem) model forecast. These forecasts were evaluated with a focus on air quality prediction. The ensemble aerosol spread was generally insufficient, particularly over the western CONUS. However, despite the sub-optimal ensemble spread, the hybrid system performed quite well and usually produced the best aerosol forecasts. Additionally, both the 3DVAR- and EnSRF- initialized forecasts typically outperformed the control. These results are encouraging and suggest the resiliency of the hybrid method. Improved aerosol ensembles should translate into even better future hybrid forecasts.

Description: [1]  Potential evapotranspiration is an important component of hydrological modeling. In this study, the objective is to project potential evapotranspiration in the future period 2011-2040 and understand their changes in Zhejiang Province, East China. The sensitivity of potential evapotranspiration to five climatic variables (solar radiation, daily minimum and maximum air temperature, relative humidity and wind speed) is analyzed based on observation data from 1955-2008 using a global sensitivity analysis method, Sobol's method. The changes in potential evapotranspiration during the future period are generated using one regional climate model, PRECIS with two global climate models - ECHAM5 and HadCM3, and their causes are analyzed based on sensitivity analysis results. Global sensitivity analysis results reveal substantial spatial-temporal variations in the sensitivity of potential evapotranspiration to climatic variables and unignorable interactions among climatic variables. Rather similar spatial change patterns of annual mean PET are generated for both GCMs; however, seasonal or monthly changes are very different due to different spatial-temporal changes in climatic variables. Different contributory sources to potential evapotranspiration changes are identified at different months and stations; the PET changes in 2011-2040 are mainly due to three climatic variables including solar radiation, relative humidity and daily minimum temperature.

Description: ABSTRACT [1]  Continuous measurements of ozone (O 3 ), carbon monoxide (CO), beryllium-7 ( 7 Be) and lead-210 ( 210 Pb) in aerosols along with relative humidity (RH) were carried out at the Mt. Hehuan site (24.05°N, 121.10°E, 3380 m.a.s.l.) from September 2011 to September 2012 in order to investigate the seasonality of stratospheric influence (SI) and its effect on surface ozone concentration in the subtropical free troposphere over central Taiwan. During the 13-month period, the measured O 3 concentration fluctuated around a mean of 41 ppb and showed a broad springtime maximum and summertime minimum that can be attributed to regional circulation over subtropical Asia. Beryllium-7, CO, RH and the normalized fraction of 7 Be, f( 7 Be, 210 Pb), were used to identify SI days based on several criteria. The results showed that the seasonal patterns of SI days identified by different criteria were very similar, with the absolute values deviating within ~29% from those based on our reference methodology. Of the total analyzed days, fourteen SI days (approximately 4.6%) were found, indicating that SI phenomenon in the subtropical region is much less frequent than at northern mid-latitudes. About two-thirds of the SI days occurred in the winter, whereas none were found in the summer. The seasonality of SI occurrence could be related to the position of the subtropical jet stream (SJS). The proportion of surface O 3 derived from the stratosphere was estimated to be only 1.3% on a yearly basis, increasing to ~3.2% in the winter and 27% during the SI days, demonstrating the importance of downward transport of stratospheric air in affecting the level of surface ozone.

Description: [1]  Over the past couple of decades, there has been much discussion about the role of tilted heating in the evolution of the Madden Julian Oscillation (MJO). It is widely believed that the inability of many GCMs to produce a robust MJO is directly related to deficiencies in the model-produced heating distributions in the tropics. Given the MJO's importance in tropical climate, we need to better understand what heating distributions are required in climate models to produce strong MJOs. This study directly addresses the role of tilted heating in the Community Atmospheric Model version 4 (CAM4). Various idealized and observed heating distributions are added to the tropical atmosphere in CAM4. These include idealized heating that tilts westward with height in both the lower and upper atmosphere, as well as various tilted and untilted heating in the form of either idealized blobs or observations derived from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and the International Satellite Cloud Climatology Project (ISCCP). In all cases, CAM4 is integrated for 15 years and the MJO is analyzed. We find that low-level heating ahead of the MJO convective center is critical for the initial strengthening and later maintenance of the MJO. However, tilted heating is not necessary to simulate a realistic MJO. In addition, excess upper-level heating, whether tilted or not, appears to degrade the MJO signal, although Rossby wave modes are evident only in runs with additional upper-level heating. Kelvin waves are evident during strong MJOs for all simulations, but are only visible over the central Pacific. Results suggest that accurate shallow convective parameterizations may be more important than deep convective ones in the evolution of the MJO.

Description: [1]  The relationship between the layer-mean radar reflectivity, Ze , and the columnar effective particle radius, Re , in evolving shallow warm clouds was investigated by numerical experiments using a hybrid cloud microphysical model and a forward simulator of satellite measurements. Changes in the cloud/rain droplet size distributions were traced in a kinematically driven warm cloud for various values of number concentration of cloud condensation nuclei (CCN) and maximum updraft velocity. In contrast to previous interpretations of the observed data, we found four paths for the relationships between Ze and Re during the lifetime of a warm cloud. In the first path, both Ze and Re increase with an approximate sixth-power dependency, indicating a stage of condensational growth of droplets without raindrops in the cloud. In the second path, only Ze increases rapidly, while Re remains almost constant ( Re 2nd ), indicating a stage in which few raindrops emerge in the cloud before appreciable precipitation occurs at the surface. This second path was newly identified in this study. In the third path, Re increases rapidly while Ze does not change greatly, indicating a stage of coalescence of droplets. Precipitation reaches the surface at the end of the third path. In the fourth path, both Ze and Re decrease, indicating a greater contribution of raindrop evaporation and weakening or termination of precipitation. The maximum values of Ze and Re and the constant value of Re 2nd for the second stage depend on the CCN number concentration and the updraft velocity.

Description: [1]  We present combined Raman and elastic backscatter lidar observations that were carried out in Zhongshan, PRD (Pearl River Delta), China, during two periods in 2009: one haze pollution period and one moderate pollution period. During the haze period , high AOD (0.86 and 1.20 at 355 nm) and medium Ångström exponents (1.23 and 1.35 at 355 nm/532 nm) were observed. In the moderate pollution period, the corresponding parameters were comparatively lower with values of 0.83 and 0.74 at 355 nm for AOD and 1.108 and 0.98 at 355 nm/532 nm for Ångström exponent.The mean lidar ratios in the two periods were 64 ± 10sr and 56 ±9 sr, respectively, at 355 nm. The lidar ratio during haze phase was a bit higher compared to that in previous observations in this area. The calculated values of the lidar ratio and Ångström exponent were also used to help identify aerosol types. The Ångström exponent was calculated for the extinction from the wavelength pair 355 nm/532 nm, with high values of around 1.35 for the haze event. The particle size distribution (PSD) and single scattering albedo (SSA) derived from sunphotometer measurements indicate the presence of rather small particles. The three-day back trajectories from a HYSPLIT model in the haze period indicate that the air masses in the lower layer were advected from the southeast coast of China, where incomplete combustion of carbonaceous fuels and straw burning are frequently found in Shanghai during the heating period in winter. In the moderate pollution period, the air mass passed through western China, indicating a combination of some pollution from South Asia in case of strong convection, local aerosol aging, and smoke from adjacent fire burning spots in the PRD region.

Description: No abstract is available for this article.

Description: [1]  Natural seepage of methane from the lithosphere to the atmosphere occurs in regions with large natural gas deposits. According to some authors, it accounts for roughly 5% of the global methane budget. I explore a new approach to estimate methane fluxes based on the maturation of kerogen, which is the hydrocarbon polymer present in petroleum source rocks, and whose decomposition leads to the formation of oil and natural gas. The temporal change in the atomic H/C ratio of kerogen lets us estimate the total carbon mass released by it in the form of oil and natural gas. Then the time interval of active kerogen decomposition lets us estimate the average annual formation rate of oil and natural gas in any given petroleum system, which I demonstrate here using the Uinta Basin of eastern Utah as an example. Obviously, this is an upper bound to the average annual rate at which natural gas seeps into the atmosphere. After adjusting for bio-oxidation of natural gas, I conclude that the average annual seepage rate in the Uinta Basin is not greater than (3100 ± 900) tonne y –1 . This is (0.5 ± 0.15)% of the total flux of methane into the atmosphere over the Basin, as measured during aircraft flights. I speculate about the difference between the regional 0.5% and the global 5% estimates.

Description: [1]  Mixed-phase clouds (clouds that consist of both cloud droplets and ice crystals) are frequently present in the Earth's atmosphere and influence the Earth's energy budget through their radiative partitioning of cloud water is compared among water phase. In this study, the phase partitioning of cloud water is compared among six global climate models (GCMs) and with CALIOP retrievals. It is found that the GCMs predict vastly different distributions of cloud phase for a given temperature, and none of them are capable of reproducing the spatial distribution or magnitude of the observed phase partitioning. While some GCMs produced liquid water paths comparable to satellite observations, they all failed to preserve sufficient liquid water at mixed-phase cloud temperatures. Our results suggest that validating GCMs using only the vertically integrated water contents could lead to amplified differences in cloud radiative feedbacks. The sensitivity of the simulated cloud phase in GCMs to the choice of heterogeneous ice nucleation parameterization is also investigated. The response to a change in ice nucleation is quite different for each GCM, and the implementation of the same ice nucleation parameterization in all models does not reduce the spread in simulated phase among GCMs. The results suggest that processes subsequent to ice nucleation are at least as important in determining phase, and should be the focus of future studies aimed at understanding and reducing differences among the models.

Description: [1]  Size-resolved measurements of the ratios of cloud condensation nuclei (CCN) to condensation nuclei for particles with different hygroscopic growth factors ( g ) and distributions of g at 85% relative humidity were performed for urban aerosols over Nagoya, Japan. The CCN efficiency spectra of less hygroscopic particles ( g of 1.0 and 1.1) were very different from those of more hygroscopic particles ( g of 1.25 and 1.4). While the differences between the CCN activation diameters predicted from g ( d act,g85 ) and those measured ( d act,CCN ) were within 12% for more hygroscopic particles, the differences were larger (16%–41%) for less hygroscopic particles. Possible causes of this included surface tension reduction, the dependence of κ on the concentration of the solution, the existence of sparingly soluble materials, and asphericity of particles. The number concentrations of CCN ( N CCN ) and cloud droplets ( N cd ), and the effective radius of cloud droplets ( R eff ) were estimated from the distributions of g using a cloud parcel model. The influences of the differences between d act,g85 and d act,CCN and the existence of CCN-inactive particles on the model assessment were small. With high updraft velocity, incorporating both less and more hygroscopic particles into the model led to substantial increases in N CCN and N cd , and a decrease in R eff as compared to the hypothetical cases that only more hygroscopic particles were present. The results indicated that less hygroscopic particles significantly contribute to cloud droplet formation and assessments of g distributions are useful in this regard.

Description: [1]  Atmospheric particles often include a complex mixture of nitrate and secondary organic materials accumulated within the same individual particles. Nitrate as an important inorganic component can be chemically formed in the atmosphere. For instance, formation of sodium nitrate (NaNO 3 ) and calcium nitrate (Ca(NO 3 ) 2 ) occurs when nitrogen oxides and nitric acid (HNO 3 ) react with sea salt and calcite, respectively. Organic acids contribute a significant fraction of photochemically formed secondary organics that can condense on the preexisting nitrate-containing particles. Here, we present a systematic microanalysis study on chemical composition of laboratory-generated particles composed of water-soluble organic acids and nitrates (i.e., NaNO 3 and Ca(NO 3 ) 2 ) using computer-controlled scanning electron microscopy with energy-dispersive analysis of X-rays (CCSEM/EDX) and Fourier transform infrared micro-spectroscopy (micro-FTIR). The results show that water-soluble organic acids can react with nitrates and release gaseous HNO 3 during the dehydration process. These reactions are attributed to acid displacement of nitrate with weak organic acids driven by the evaporation of HNO 3 into gas phase because of its relatively high volatility. The reactions result in significant nitrate depletion and formation of organic salts in mixed organic acids/nitrate particles that, in turn, may affect their physical and chemical properties relevant to atmospheric environment and climate. Airborne nitrate concentrations are estimated by thermodynamic calculations corresponding to various nitrate depletions in selected organic acids of atmospheric relevance. The results indicate a potential mechanism of HNO 3 recycling that may further affect concentrations of gas and condensed phase species in the atmosphere and the heterogeneous reaction chemistry between them.

Description: [1]  The VHF Broadband Digital Interferometer (DITF) developed by Osaka University has been improved to allow continuous sampling over the entire duration of a lightning flash and to utilize a generalized cross-correlation technique for determining the lightning source directions. Time series waveforms of 20-80 MHz signals received at three orthogonally located antennas are continuously digitized over multi-second intervals, as opposed to sequences of short-duration triggers. Because of the coherent nature of the measurements, radiation sources are located down into the ambient receiver and environmental noise levels, providing a quantum leap in the ability to study lightning discharge processes. When post-processed using cross-correlation, the measurements provide angular uncertainties less than 1 degree and time resolution better than 1  μ s. Special techniques have been developed to distinguish between actual lightning sources and noise events, with the result being that on the order of 50,000-80,000 radiation sources are located for a typical lightning flash. In this study, 2-dimensional interferometer observations of a classic bi-level intracloud flash are presented and combined with 3-dimensional Lightning Mapping Array observations to produce a quasi-3D map of lightning activity with the time resolution of the interferometer. As an example of the scientific utility of the observations, results are presented for the 3D progression speed of negative leaders associated with intracloud K-leaders.