ALBERT

Description: Continued advancement in the realm of tropical cyclone (TC) forecasting requires a more accurate depiction of these storms at model initialization. This study examines the impact of precipitation assimilation on the representation of TCs in the North American Regional Reanalysis (NARR) before and after the 2004 introduction of precipitation assimilation over ocean in the vicinity of TCs. The probability distribution function of rainfall rates indicates that light (heavy) precipitation was overforecast (underforecast) in the early time period. Since the precipitation assimilation is applied through an adjustment to the latent heating distribution, the data assimilation system in the later time period initializes a low-level moisture and heating profile that is more conducive to the initiation of deep convection and the generation of precipitation. Consequently, the deep convection and enhanced latent heat release lead to a more robust warm core temperature perturbation and a better developed secondary circulation, which supplies the TC with larger quantities of moisture from the large-scale environment. Furthermore, the evolution of TC size, which was objectively estimated though the radius of outermost closed isobar (ROCI), is significantly more skillful ( p 〈0.05) in post-2003 storms. Based on this study, precipitation assimilation leads to a better analysis of temperature, winds, and moisture in the vicinity of TCs, resulting in improved representations of the water budget and storm lifecycle. Therefore, we conclude that efforts towards the development of precipitation assimilation techniques from radar and satellite datasets will be valuable toward the construction of improved TC forecasting tools with more authentic TC representation.

Description: Six years of aerosol size distribution measurements between 20 and 600 nm diameters and total aerosol concentration above 10 nm from March 2008 to February 2014 at the high-alpine site Jungfraujoch are presented. The size distribution was found to be typically bimodal with mode diameters and widths relatively stable throughout the year and the observation period. New particle formation was observed on 14.5% of all days without a seasonal preference. Particles typically grew only into the Aitken mode and did not reach cloud condensation nuclei (CCN) sizes on the time scale of several days. Growth of pre-existing particles in the Aitken mode, on average, contributed very few CCN. We concluded that the dominant fraction of CCN at Jungfraujoch originated in the boundary layer. A number of approaches were used to distinguish free tropospheric (FT) conditions and episodes with planetary boundary layer (PBL) influence. In the absence of PBL injections, the concentration of particles larger than 90 nm ( N 90 , roughly corresponding to the CCN concentration) reached a value ~40 cm −3 while PBL influence caused N 90 concentrations of several hundred or even 1000 cm −3 . Comparing three criteria for free tropospheric conditions, we found FT prevalence for 39% of the time with over 60% during winter and below 20% during summer. It is noteworthy that a simple criterion based on standard trace gas measurements appeared to outperform alternative approaches.

Description: Tropical convection has been observed to contain three cloud modes, the middle of which is cumulus congestus clouds. Congestus clouds act to moisten the tropical atmosphere, may be mixed-phase, and on occasion surpass the freezing level inversion from where they may develop into deeper convection. This study investigates the impacts of enhanced aerosol concentrations on the growth of congestus clouds produced in idealized cloud-resolving model simulations run under a state of radiative convective equilibrium (RCE). High resolution, long duration simulations were completed using the Regional Atmospheric Modeling System (RAMS). Aerosol concentrations between 2 and 4 km AGL were varied from clean to polluted conditions in order to represent the advection of Saharan dust over the Atlantic Ocean. The congestus populations within each aerosol simulation are statistically analyzed using ten days of model output after the simulation reaches RCE. Results indicate that congestus in more polluted conditions produce greater amounts of cloud water and ice mass, enhanced updraft strengths and an increase in the number of congestus cloud tops that extend above the freezing level. Enhanced vapor depositional growth on the populations of more numerous, smaller cloud droplets in the polluted conditions, and the subsequent increase in latent heat release in the warm phase regions of the cloud, are found to be important factors in convective invigoration of these cloud systems. Aerosol feedbacks associated with cold pools and condensate loading also influence the updraft strength, and act in opposition to the warm phase invigoration processes.

Description: With water vapor and clouds expected to effect significant feedbacks on climate, moisture transport through convective processes has important implications for future temperature change. The precipitation efficiency—the ratio of the rates at which precipitation and condensation form ( e  =  P / C )—is useful for characterizing how much boundary layer moisture recycles through precipitation versus mixes into the free troposphere through cloud detrainment. Yet, it is a difficult metric to constrain with traditional observational techniques. This analysis characterizes the precipitation efficiency of convection near the Big Island of Hawaii, USA, using a novel tracer: isotope ratios in water vapor. The synoptic circulation patterns associated with high and low precipitation efficiency are identified, and the importance of large-scale dynamics and local convective processes in regulating vertical distributions of atmospheric constituents important for climate is evaluated. The results suggest high e days are correlated with plume-like transport originating from the relatively clean tropics, while low e days are associated with westerly transport, generated by a branching of the jet stream. Differences in transport pathway clearly modify background concentrations of water vapor and other trace gases measured at Mauna Loa Observatory; however, local convective processes appear to regulate aerosols there. Indeed, differences between observed and simulated diurnal cycles of particle number concentration indicate precipitation scavenges aerosols and possibly facilitates new particle formation when e is high. As measurements of isotope ratios in water vapor expand across the subtropics, the techniques presented here can further our understanding of how synoptic weather, precipitation processes, and climate feedbacks interrelate.

Description: This study evaluates the ability of the Community Atmospheric Model version 5 (CAM5) to reproduce low clouds observed by the Atmospheric Radiation Measurement (ARM) cloud radar at Manus Island of the tropical western Pacific during the Years of Tropical Convection (YOTC). Here, low clouds are defined as clouds with their tops below the freezing level and bases within the boundary layer. Low-cloud statistics in CAM5 simulations and ARM observations are compared in terms of their general occurrence, mean vertical profiles, fraction of precipitating vs. non-precipitating events, diurnal cycle, and monthly time series. Other types of clouds are included to put the comparison in a broader context. The comparison shows that the model overproduces total clouds and their precipitation fraction but underestimates low clouds in general. The model, however, produces excessive low clouds in a thin layer between 954–930 hPa, which coincides with excessive humidity near the top of the mixed layer. This suggests that the erroneously excessive low clouds stem from parameterization of both cloud and turbulence mixing. The model also fails to produce the observed diurnal cycle in low clouds, not exclusively due to the model coarse grid spacing that does not resolve Manus Island. This study demonstrates the utility of ARM long-term cloud observations in the tropical western Pacific in verifying low clouds simulated by global climate models, illustrates issues of using ARM observations in model validation, and provides an example of severe model biases in producing observed low clouds in the tropical western Pacific.

Description: Solar influences on spatial patterns of Eurasian winter climate and possible mechanisms are investigated based on a multiple linear regression method, multi-sources observational and reanalysis data. Robust and significant solar signals are detected in Eurasian surface air temperature (SAT) and strong solar activity evidently warms most area of the continent. The spatial pattern of sea level pressure (SLP) responses to solar activity is similar but not identical to that of the North Atlantic Oscillation (NAO). Compared to the NAO, geographic distribution of solar-induced SLP anomalies shifts eastward, with significantly enhanced influences over northern Eurasia. Relatively weaker solar signals were also found in mid-to-upper troposphere. The spatial pattern of 500 hPa geopotential anomalies resembles a negative Scandinavia Teleconnection pattern and the 200 hPa subtropical jet is weakened, while zonal wind at high latitudes is enhanced due to strong solar activity. The anomalous zonal circulations can be attributed to the “top-down” mechanism. During high solar activity winters, an enhanced stratospheric zonal wind anomaly propagates downward, causing zonal wind anomalies in the troposphere. However, the “bottom-up” mechanisms may provide more reasonable explanations of the distinct solar influences on Eurasian climate. Solar-induced strong warm advection in lower atmosphere tends to increase SAT but decrease SLP, resulting in enhanced solar influences over northern Eurasia. Meanwhile, change in the land-ocean thermal contrast (LOTC) could also amplify the circulation anomaly. Inhomogeneous surface heating caused by anomalous solar activity modifies LOTC, which probably enhances the solar-induced circulation patterns. Such a positive feedback may potentially strengthen the solar influences.

Description: This study reports the chemical composition of particles present along Greenland's North Greenland Eemian Ice Drilling (NEEM) ice core, back to 110,000 years before present. Insoluble and soluble particles larger than 0.45 µm were extracted from the ice core by ice sublimation and their chemical composition was analyzed using scanning electron microscope and energy dispersive X-ray spectroscopy and micro-Raman spectroscopy. We show that the dominant insoluble components are silicates, whereas NaCl, Na 2 SO 4 , CaSO 4 , and CaCO 3 represent major soluble salts. For the first time, particles of CaMg(CO 3 ) 2 and Ca(NO 3 ) 2 •4H 2 O are identified in a Greenland ice core. The chemical speciation of salts varies with past climatic conditions. Whereas the fraction of Na-salts (NaCl + Na 2 SO 4 ) exceeds that of Ca-salts (CaSO 4 + CaCO 3 ) during the Holocene (0.6–11.7 kyr BP), the two fractions are similar during the Bølling-Allerød period (12.9–14.6 kyr BP). During cold climate such as over the Younger Dryas (12.0–12.6 kyr BP) and the Last Glacial Maximum (15.0–26.9 kyr BP), the fraction of Ca-salts exceeds that of Na-salts, showing that the most abundant ion generally controls the salt budget in each period. High-resolution analyses reveal changing particle compositions: those in Holocene ice show seasonal changes, and those in LGM ice show a difference between cloudy bands and clear layers, which again can be largely explained by the availability of ionic components in the atmospheric aerosol body of air masses reaching Greenland.

Description: Characterizing black carbon (BC) concentrations in the seasonal snowpack is of interest because BC deposition on snow can reduce albedo and accelerate melt. In Washington State, USA snowmelt from the seasonal snowpack provides an important source of water resources, but minimal work has been done characterizing BC concentrations in snow in this region. BC concentrations in snow were monitored over two winters (2012 and 2013) at Tronsen Meadow, located near Blewett Pass in the eastern Cascade Mountains in Central Washington, to characterize spatial and temporal variations in BC concentrations, and the processes affecting BC concentrations in the snowpack. BC concentrations were measured using a Single Particle Soot Photometer (SP2). Snowpit BC concentrations at spatial scales ranging from cm to 100 m scales were fairly homogenous during the accumulation season, with greater spatial variability during the melt season due to variable melt patterns. BC concentrations in snow increased in late winter-spring due to an increase in atmospheric BC concentrations and trapping of BC on the snow surface during melt. However, during a period of intense melt in 2013 BC concentrations decreased, likely caused by melt water scavenging. In summer 2012 the Table Mountain forest fire burned adjacent to the study site, and BC concentrations in the snowpack in 2013 were far higher than in previous years, with charred trees post-fire the likely source of the elevated BC.

Description: The interactions between idealized binary tropical cyclones (TCs) on f - and β -planes with different separation distance and thermodynamic soundings obtained from the NCEP/NCAR reanalysis data averaged over the western North Pacific (WNP) are investigated through ensemble three-dimensional numerical simulations with a horizontal resolution of 10 km in a single domain. In the simulations on the f -plane, two TCs show mutual cyclonic rotations with symmetric structures. Two TCs with thermodynamic profiles of larger convective available potential energy (CAPE) and maximum potential intensity (MPI) show greater interaction than those with a smaller CAPE and MPI due to the stronger tangential velocity near the TC center. In the simulations on the β -plane, the two TCs do not merge, because the beta effect prevents the attraction of the two TCs by generating asymmetric motions of the TC with northwestward forcing. The relative strengths of the two TCs change with time and depend on the low-level inflow influenced by the Coriolis parameter. Similar to the results on the f -plane, the two TCs only merge with the thermodynamic soundings of large CAPE and MPI.

Description: The tropical rain belt is a narrow band of clouds near the equator, where the most intense rainfall on the planet occurs. On seasonal timescales, the rain moves across the equator following the sun, resulting in wet and dry seasons in the tropics. The position of the tropical rain belt also varies on longer time scales. Through the latter half of the 20th century, for example, shifts in tropical rainfall have been associated with severe droughts, including the African Sahel and Amazon droughts. Here, I show that climate models project a northward migration of the tropical rain belt through the 21st century, with future anthropogenic aerosol reductions driving the bulk of the shift. Models that include both aerosol indirect effects yield significantly larger northward shifts than models that lack aerosol indirect effects. Moreover, the rate of the shift corresponds to the rate of the decrease of anthropogenic aerosol emissions across different time periods and future emission scenarios. This response is consistent with relative warming of the Northern Hemisphere, a decrease in northward cross-equatorial moist static energy transport, and a northward shift of the Hadley circulation, including the tropical rain belt. The shift is relatively weak in the Atlantic sector, consistent with both a smaller decrease in aerosol emissions and a larger reduction in northward cross equatorial ocean heat flux. Although aerosol effects remain uncertain, I conclude that future reductions in anthropogenic aerosol emissions may be the dominant driver of a 21st century northward shift of the tropical rain belt.

Description: Aerosol deposition over the Southern Ocean and Antarctica has the potential to alter marine productivity and thus ocean carbon uptake while also impacting radiative balance due to scattering and absorption from atmospheric particulates. Quantification of modern emission, transport and deposition of terrestrial dust and other airborne material from Southern Hemisphere sources is challenging due to low emission levels and poor detection from remote sensing platforms. Here, forward trajectory modeling is used to explore atmospheric transport, independent of deposition processes, from 1979 to 2013. Trajectories are initiated from known arid dust source areas in South America (Patagonia), Australia, and southern Africa, with detailed consideration of New Zealand as a potential source. Results suggest that Patagonian and New Zealand dust and other aerosol emissions share strong atmospheric transport during all seasons, allowing even potentially small New Zealand emissions to contribute significantly to Southern Ocean and Antarctic aerosol loading. We find that atmospheric transport controlling distribution of dust and other aerosols shows distinct spatial variability. New Zealand and Patagonia rapidly contribute a high proportion of trajectories to West Antarctica, while in interior East Antarctica source contributions are limited and highly mixed. The sensitivity of existing deep ice core sites to modern atmospheric transport is discussed. Finally, interannual variability of poleward trajectory extension over the Pacific and Atlantic sectors of the Southern Ocean highlights the association of both tropical Pacific sea-surface temperature and high-latitude wind variability (e.g. the Southern Annular Mode) with transport of dust and other aerosols to the Southern Ocean and Antarctica.

Description: We show the results and evaluation with independent measurements from assimilating both MOPITT (Measurements Of Pollution In The Troposphere) and IASI (Infrared Atmospheric Sounding Interferometer) retrieved profiles into the Community Earth System Model (CESM). We used the DART (Data Assimilation Research Testbed) ensemble Kalman Filter technique, with the full atmospheric chemistry CESM component CAM-chem (Community Atmospheric Model with Chemistry). We first discuss the methodology and evaluation of the current data assimilation system with coupled meteorology and chemistry data assimilation. The different capabilities of MOPITT and IASI retrievals are highlighted, with particular attention to instrument vertical sensitivity and coverage and how these impact the analyses. MOPITT and IASI CO retrievals mostly constrain the CO fields close to the main anthropogenic, biogenic and biomass burning CO sources. In the case of IASI CO assimilation, we also observe constraints on CO far from the sources. During the simulation time period (June and July 2008), CO assimilation of both instruments strongly improves the atmospheric CO state as compared to independent observations, with the higher spatial coverage of IASI providing better results on the global scale. However, the enhanced sensitivity of multispectral MOPITT observations to near surface CO over the main source regions provides synergistic effects at regional scales.

Description: Processes that influence the humidity and cirrus cloud abundance in the Tropical Tropopause Layer (TTL) during boreal winter 2006-07 are investigated in simulations of clouds along backward trajectories of parcels ending at the 372 K potential temperature (100 hPa) level in the tropics. Trajectories are calculated using offline calculations of seasonal mean tropical radiative heating rates along with reanalysis temperature and wind data with enhanced wave-driven variability in the TTL. The one-dimensional (vertical) time-dependent cloud microphysical model is initialized with water vapor measurements from the Microwave Limb Sounder and the evolution of clouds along each trajectory is simulated using temperature profiles extracted from reanalysis data and convective cloud top heights estimated from 3-hourly geostationary satellite imagery. Averaged over the tropics, waves dehydrate the 100 hPa level by 0.5 ppmv, while convection and cloud microphysical processes moisten by 0.3 and 0.7 ppmv, respectively. The tropical mean cloud occurrence frequencies in the mid to upper TTL agree well with those based on satellite observations (spatial correlation of 0.8). Waves and convection enhance cloud occurrence at the cold point tropopause by 4% and 2%, respectively. Temporal variability of the heating rates as indicated by the ERA-Interim 6-hourly heating rate fields dehydrates the TTL by 0.4 ppmv and decreases the cloud occurrence by 4% because parcels are more likely to encounter the coldest temperatures and dehydrate near the cold point, limiting cloud formation above. The final dehydration locations of parcels, concentrated near the dateline in the tropical Pacific, are insensitive to various model parameters.

Description: Increasing optical depth poleward of 45° is a robust response to warming in global climate models. Much of this cloud optical depth increase has been hypothesized to be due to transitions from ice-dominated to liquid-dominated mixed-phase cloud. In this study, the importance of liquid-ice partitioning for the optical depth feedback is quantified for 19 CMIP5 models. All models show a monotonic partitioning of ice and liquid as a function of temperature, but the temperature at which ice and liquid are equally mixed (the glaciation temperature) varies by as much as 40K across models. Models that have a higher glaciation temperature are found to have a smaller climatological liquid water path (LWP) and condensed water path, and experience a larger increase in LWP as the climate warms. The ice-liquid partitioning curve of each model may be used to calculate the response of LWP to warming. It is found that the re-partitioning between ice and liquid in a warming climate contributes at least 20% to 80% of the increase in LWP as the climate warms, depending on model. Inter-model differences in the climatological partitioning between ice and liquid are estimated to contribute at least 20% to the inter-model spread in the high-latitude LWP response in the mixed-phase region poleward of 45°S. It is hypothesized that a more thorough evaluation and constraint of GCM mixed-phase cloud parameterizations, and validation of the total condensate and ice-liquid apportionment against observations will yield a substantial reduction in model uncertainty in the high-latitude cloud response to warming.

Description: In this study, we adopt a parametric sensitivity analysis framework that integrates the quasi-Monte Carlo parameter sampling approach and a surrogate model to examine aerosol effects on the East Asian Monsoon climate simulated in the Community Atmosphere Model (CAM5). A total number of 256 CAM5 simulations are conducted to quantify the model responses to the uncertain parameters associated with cloud microphysics parameterizations and aerosol (e.g., sulfate, black carbon (BC), and dust) emission factors and their interactions. Results show that the interaction terms among parameters are important for quantifying the sensitivity of fields of interest, especially precipitation, to the parameters. The relative importance of cloud-microphysics parameters and emission factors (strength) depends on evaluation metrics or the model fields we focused on, and the presence of uncertainty in cloud microphysics imposes an additional challenge in quantifying the impact of aerosols on cloud and climate. Due to their different optical and microphysical properties and spatial distributions, sulfate, BC, and dust aerosols have very different impacts on East Asian Monsoon through aerosol-cloud-radiation interactions. The climatic effects of aerosol do not always have a monotonic response to the change of emission factors. The spatial patterns of both sign and magnitude of aerosol-induced changes in radiative fluxes, cloud, and precipitation could be different, depending on the aerosol types, when parameters are sampled in different ranges of values. We also identify the different cloud microphysical parameters that show the most significant impact on climatic effect induced by sulfate, BC and dust, respectively, in East Asia.

Description: Earth radiation management has been suggested as a way to rapidly counteract global warming in the face of a lack of mitigation efforts, buying time and avoiding potentially catastrophic warming. We compare six different radiation management schemes that use surface, troposphere and stratosphere interventions in a single climate model in which we projected future climate from 2020 to 2099 based on RCP4.5. We analyze the surface air temperature responses to determine how effective the schemes are at returning temperature to its 1986-2005 climatology and analyze precipitation responses to compare side effects. We find crop albedo enhancement is largely ineffective at returning temperature to its 1986-2005 climatology. Desert albedo enhancement causes excessive cooling in the deserts and severe shifts in tropical precipitation. Ocean albedo enhancement, sea-spray geoengineering, cirrus cloud thinning and stratospheric SO 2 injection have the potential to cool more uniformly, but cirrus cloud thinning may not be able to cool by much more than 1 K globally. We find that of the schemes potentially able to return surface air temperature to 1986-2005 climatology under future greenhouse gas warming, none has significantly less severe precipitation side effects than other schemes. Despite different forcing patterns, ocean albedo enhancement, sea-spray geoengineering, cirrus cloud thinning and stratospheric SO 2 injection all result in large scale tropical precipitation responses caused by Hadley cell changes and land precipitation changes largely driven by thermodynamic changes. Widespread regional scale changes in precipitation over land are significantly different from the 1986-2005 climatology and would likely necessitate significant adaptation despite geoengineering.

Description: The wind velocity structure in the upper stratosphere, mesosphere and lower thermosphere (MLT) is studied with the recently developed method of infrasound probing of the atmosphere. The method is based on the effect of infrasound scattering from highly anisotropic wind velocity and temperature inhomogeneities in the middle and upper atmosphere. The scattered infrasound field propagates in the acoustic shadow zones, where it is detected by microbarometers. The vertical profiles of the wind velocity fluctuations in the upper stratosphere (30–52 km) and MLT (90–140 km) are retrieved from the wave forms and travel times of the infrasound signals generated by explosive sources such as volcanoes and surface explosions. The fine-scale wind layered structure in these layers was poorly observed until present time by other remote sensing methods, including radars and satellites. It is found that the MLT atmospheric layer (90–102 km) can contain extremely high vertical gradients of the wind velocity, up to 10 m/s per 100 m. The effect of a fine-scale wind velocity structure on the wave forms of infrasound signals is studied. The vertical wavenumber spectra of the retrieved wind velocity fluctuations are obtained for the upper stratosphere. Despite the difference in the locations of the explosive sources all the obtained spectra show the existence of high vertical wavenumber spectral tail with a −3 power law decay. The obtained spectral characteristics of the wind fluctuations are necessary for improvement of gravity wave drag parametrizations for numerical weather forecast.

Description: We have computed lightning electromagnetic pulses (LEMPs), including the azimuthal magnetic field H ϕ , vertical electric field E z , and horizontal (radial) electric field E h that propagated over 5 to 200 km of flat lossy ground, using the finite-difference time-domain (FDTD) method in the 2D cylindrical coordinate system. This is the first systematic full-wave study of LEMP propagation effects based on a realistic return-stroke model and including the complete return-stroke frequency range. Influences of the return-stroke wavefront speed (ranging from c /2 to c , where c is the speed of light), current risetime (ranging from 0.5 to 5 µs), and ground conductivity (ranging from 0.1 mS/m to ∞) on H ϕ , E z , and E h have been investigated. Also, the FDTD-computed waveforms of E h have been compared with the corresponding ones computed using the Cooray-Rubinstein formula. Peaks of H ϕ , E z , and E h are nearly proportional to the return-stroke wavefront speed. The peak of E h decreases with increasing current risetime, while those of H ϕ and E z are only slightly influenced by it. The peaks of H ϕ and E z are essentially independent of the ground conductivity at a distance of 5 km. Beyond this distance, they appreciably decrease relative to the perfectly conducting ground case, and the decrease is stronger for lower ground conductivity values. The peak of E h increases with decreasing ground conductivity. The computed E h /E z is consistent with measurements of Thomson et al. [1988]. The observed decrease of E z peak and increase of E z risetime due to propagation over 200 km of Florida soil [ Uman et al. 1976; Lin et al. 1979] are reasonably well reproduced by the FDTD simulation with ground conductivity of 1 mS/m.

Description: This study investigates the impact of temperature and moisture profiles from Atmospheric Infrared Sounder (AIRS) on the prediction of the Indian Summer Monsoon, using the variational data assimilation system annexed to the Weather Research and Forecasting (WRF) model. In this study, three numerical experiments are carried out. The first is the control (CTRL) and includes no assimilation; in the second, named Conv, assimilation of conventional Global Telecommunication System data is performed. The third one, named ConvAIRS, is identical to the Conv except that it also includes assimilation of AIRS profiles. The initial fields of tropospheric temperature and water vapour mixing ratio showed significant improvement over the model domain. Assimilation of AIRS profiles has significant impact on predicting the seasonal mean monsoon characteristics such as tropospheric temperature, low level moisture distribution, easterly wind shear and precipitation. The vertical structure of the root mean square error (RMSE) is substantially affected by the assimilation of AIRS profiles, with smaller errors in temperature, humidity and wind magnitude. The consequent improved representation of moisture convergence in the boundary layer (and deep convection as well) causes an increase in precipitation forecast skill. The fact that the monsoonal circulation is better captured, thanks to an improved representation of thermal gradients, which in turn leads to more realistic moisture transport, is particularly noteworthy. Several previous data impact studies with AIRS and other sensors have focused on the short- or medium-range of the forecast. The demonstrated improvement in all the predicted fields associated with the Indian Summer Monsoon, consequent to the assimilation of AIRS profiles, is an innovative finding with large implications to the operational seasonal forecasting capabilities over the Indian subcontinent.

Description: The oceans cover most of the Earth's surface, contain nearly half the total global primary biomass productivity, and are a major source of atmospheric aerosol particles. Here we experimentally investigate links between biological activity in seawater and sea spray aerosol (SSA) flux, a relationship of potential significance for organic aerosol loading and cloud formation over the oceans, and thus for climate globally. Bubbles were generated in laboratory mesocosm experiments either by recirculating impinging water jets or glass frits. Experiments were conducted with Atlantic Ocean seawater collected off the eastern end of Long Island, NY, and with artificial seawater containing cultures of bacteria and phytoplankton Thalassiosira pseudonana , Emiliania huxleyi , and Nannochloris atomus . Changes in SSA size distributions occurred during all phases of bacterial and phytoplankton growth, as characterized by cell concentrations, dissolved organic carbon (DOC), total particulate carbon (TPC), and transparent exopolymer particles (TEP, gel-forming polysaccharides representing a major component of biogenic exudate material). Over a 2 week growth period, SSA particle concentrations increased by a factor of less than 2 when only bacteria were present and by a factor of about 3 when bacteria and phytoplankton were present. Production of jet generated SSA particles of diameter less than 200 nm increased with time, while production of all particle diameters increased with time when frits were used. The implications of a marine biological activity dependent SSA flux are discussed.

Description: The OSIRIS instrument on the Odin satellite, launched in 2001 and currently operational, measures limb scattered sunlight from which profiles of stratospheric aerosol extinction are retrieved. SAGE II was launched in 1984 and provided measurements of stratospheric aerosol extinction until mid 2005. This provides approximately four years of mission overlap which has allowed us to consistently extend the SAGE II version 7.00 record to the present using OSIRIS aerosol extinction retrievals. In this work we first compare coincident aerosol extinction observations during the overlap period by interpolating the SAGE II 525nm and 1020nm channels to the OSIRIS extinction wavelength of 750nm. In the tropics to mid latitudes mean differences are typically less than 10%, although larger biases are seen at higher latitudes and at altitudes outside the main aerosol layer. OSIRIS aerosol extinction retrievals at 750nm are used to create a monthly time series zonally averaged in 5 degree bins and qualitatively compared to SAGE II 525nm observations averaged in the same way. The OSIRIS time series is then translated to 525nm with an Angstrom exponent relation and bias corrected. For most locations, this provides agreement during the overlap time period to better than 15%. Uncertainty in the resulting OSIRIS time series is estimated through a series of simulation studies over the range of aerosol particle size distributions observed by in-situ balloon instruments, and is found to be approximately 20% for background and moderately volcanic aerosol loading conditions for the majority of OSIRIS measurement conditions.

Description: An intensive field campaign was conducted on Deokjeok Island off the west coast of the Korean Peninsula during spring 2009 to characterize the optical and hygroscopic properties of Asian continental outflows. A slightly high wavelength dependence of light absorption coefficient, α of 1.6 ± 0.05 (average ± 1·standard deviation), and a low humidity-dependent light scattering enhancement factor at 80% relative humidity, f (80%) (2.0 ± 0.2), were obtained when air masses originated from the northern part of China (N China), compared to those obtained when air masses originated from the eastern part of China (E China) (α = 1.4 ± 0.1; f (80%) = 2.4 ± 0.2). The relatively high α and low f (80%) during the N China compared to those during the E China were consistent with a relatively high mass ratio of organic aerosol to sum of SO 4 2− , NO 3 − and NH 4 + during the N China (1.01 ± 0.17) compared to the E China episode (0.25 ± 0.13). This result indicates the importance of organic aerosol on aerosol optical and hygroscopic properties of haze plumes. Single scattering albedo (SSA) of dry PM 2.5 (0.92 ± 0.01) and mass scattering efficiency (MSE) of dry PM 2.5 at 550 nm wavelength during the E China episode (3.6 ± 0.3 m 2 g −1 ) were higher than those previously obtained at the air mass source regions in China (SSA = ~0.8; MSE = ~3.0 m 2 g −1 ), implying that optical properties of PM 2.5 were significantly altered during long-range atmospheric transport.

Description: This study utilizes a version of the Advanced Regional Prediction System (ARPS) with a canopy sub-model (ARPS-CANOPY) to evaluate the sensitivity of cold-air pool evolution to forest canopy density and valley geometry, and elucidate the underlying processes. Numerical experiments are conducted with forest canopy structures spanning from bare ground to dense canopies and terrain configurations ranging from small, shallow valleys to broad, deep valleys. In a set of experiments in which forest canopy density is varied, the minimum potential temperature in the cold-air pool is found to be as much as 15K warmer with a dense canopy than with no canopy. Analysis of the thermodynamic budget reveals weaker cooling rates in the dense canopy case than the sparse canopy case, with differences in cooling rates persisting through the duration of the simulation. An additional experiment in which cooling of canopy elements and the influence of the canopy on the ground radiation budget are neglected highlights the important role the canopy plays in limiting radiative loss from the ground surface along the sidewall and valley floor. In experiments in which valley geometry is varied, the cold-air pool is found to be strongest in a medium valley (10 km wide, 187.5 m deep), and weakest in a small valley (2 km wide, 37.5 m deep). Analysis of along-slope buoyancy suggests that downslope-flow driven cooling in a medium valley is more efficient than in a large valley (30 km wide, 500 m deep).

Description: Under a changing environment, seasonal droughts have been exacerbated with devastating impacts. However, the understanding of drought mechanism and predictability is limited. Based on the hindcasts from multiple climate models, the predictability and forecast skill for drought over China are investigated. The 3-month standardized precipitation index (SPI3) is used as the drought index, and the predictability is quantified by using a perfect model assumption. Ensemble hindcasts from multiple climate models are assessed individually and the grand multi-model ensemble is also evaluated. Drought forecast skill for model ensemble mean is higher than individual ensemble members, and NMME grand ensemble performs the best. Predictability is higher than forecast skill, indicating the room for improving drought forecast. Drought predictability and forecast skill are positively correlated in general, but they vary depending on seasons, regions and forecast leads. Higher drought predictability and forecast skill are found over regimes where ENSO has significant impact. For the ENSO-affected regimes, both drought predictability and forecast skill in ENSO years are higher than that in neutral years. This study suggests that predictability not only provides a measure for selecting climate models for ensemble drought forecast in ENSO-affected regimes, but also serves as an indicator for forecast skill especially when in-situ and/or remote sensing measurements for the hindcast verifications are considered unreliable.

Description: Arid and semi-arid regions located in subtropical zones are projected to experience the most adverse impacts of climate change. During the warm season, observations and IPCC global climate models (GCMs) generally support a “wet gets wetter, dry gets drier” hypothesis in these regions, which acts to amplify the climatological transitions in the context of the annual cycle. In this study, we consider changes in U.S. early warm season precipitation in the observational record and regional climate model (RCM) simulations driven by two “well performing” dynamically downscaled CMIP3 models (HadCM3 and MPI ECHAM5) that have a robust climatological representation of the North American Monsoon System (NAMS). Both observations and model results show amplification in historical seasonal transitions of temperature and precipitation associated with NAMS development, with WRF-MPI better representing the observed signal. Assuming the influence of remote Pacific sea surface temperature forcing (SST) associated with the El Niño Southern Oscillation and Pacific Decadal Variability (ENSO-PDV) on U.S. regional climate remains the same in the 21 st century, similar extreme trends are also projected by WRF-MPI for the next 30 years. A methodology is also developed to objectively analyze how climate change may be synergistically interacting with ENSO-PDV variability during the early warm season.. Our analysis suggests that interannual variability of warm season temperature and precipitation associated with Pacific SST forcing is becoming more extreme and the signal is stronger in the observed record.

Description: Present study documents the intraseasonal variability of sea surface temperature (SST) in the South China Sea (SCS) during boreal winter and its association with the East Asian winter monsoon (EAWM) variability. In northern Tropics, the largest intraseasonal variability of SST during boreal winter is found in the SCS, with two localized regions of large standard deviation, one extending westward from the Luzon Strait and the other extending southward from the coast of central Vietnam. Correspondingly, large intraseasonal variability in surface heat fluxes is observed in the above regions. Analysis shows that the formation of large intraseasonal SST anomalies in these regions is attributed largely to wind-related surface latent heat flux changes, with supplementary contribution from cloud-related surface shortwave radiation changes. Wind-induced Ekman advection has a negative effect and the Ekman upwelling pattern differs from the intraseasonal SST anomaly pattern. The intraseasonal variations of SST in the SCS display a close association with the East Asian winter monsoon (EAWM) change with a time lag of 3–5 days. In a weak (strong) phase of the EAWM, decrease (increase) in surface wind speed and suppression (enhancement) in surface latent heat flux lead to intraseasonal SST warming (cooling). This intraseasonal SST signal displays a southward propagation with the SST change in northern SCS leading that in southern SCS by about 2 days. A similar southward propagation is seen in surface wind speed and latent heat flux anomalies. The southward propagation of cloud and shortwave radiation anomalies is limited to northern part of the SCS.

Description: Emissions of methane (CH 4 ) and volatile organic compounds (VOCs) from oil and gas production may have large impacts on air quality and climate change. Methane and VOCs were measured over the Haynesville and Marcellus shale gas plays onboard the NCAR C-130 and NOAA WP-3D research aircraft in June-July of 2013. We used an eddy covariance technique to measure in situ fluxes of CH 4 and benzene from both C-130 flights with high-resolution data (10 Hz) and WP-3D flights with low-resolution data (1 Hz). Correlation (R=0.65) between CH 4 and benzene fluxes was observed when flying over shale gas operations and the enhancement ratio of fluxes was consistent with the corresponding concentration observations. Fluxes calculated by the eddy covariance method show agreement with a mass balance approach within their combined uncertainties. In general, CH 4 fluxes in the shale gas regions follow a lognormal distribution, with some deviations for relatively large fluxes (〉10 µg m −2 s −1 ). Statistical analysis of the fluxes shows that a small number of facilities (i.e. ~10%) are responsible for up to ~40% of the total CH 4 emissions in the two regions. We show that the airborne eddy covariance method can also be applied in some circumstances when meteorological conditions do not favor application of the mass balance method. We suggest that the airborne eddy covariance method is a reliable alternative and complementary analysis method to estimate emissions from oil and gas extraction.

Description: Convective features (CFs) observed by the Tropical Rainfall Measuring Mission (TRMM) satellite between 2004 and 2011 are analyzed to determine the relative roles of thermodynamics and aerosols as they modulate radar reflectivity and lightning. We studied the simultaneous impacts of normalized convective available potential energy (NCAPE) and warm cloud depth (WCD) as well as cloud condensation nuclei concentrations (D ≥ 40 nm; N40) on total lightning density (TLD), average height of 30 dBZ echoes (AVGHT30), and vertical profiles of radar reflectivity (VPRR) within individual CFs. The results show that TLD increases by up to 600% and AVGHT30 increases by up to 2–3 km with increasing NCAPE and N40 for fixed WCD. The partial sensitivity of TLD/AVGHT30 to NCAPE and N40 separately were comparable in magnitude, but account for a fraction of the total range of variability (i.e., when the influences of NCAPE and N40 are considered simultaneously). Both TLD and AVGHT30 vary inversely with WCD such that maxima of TLD and AVGHT30 are found for the combination of high NCAPE , high N40 , and shallower WCD . The relationship between lightning and radar reflectivity was shown to vary as a function of N40 for a fixed thermodynamic environment. Analysis of VPRRs shows that reflectivity in the mixed phase region is up to 5.0-5.6 dB greater for CFs in polluted environments compared to CFs in pristine environments (holding thermodynamics fixed). This analysis favors a merged hypothesis for the simultaneous roles of thermodynamics and aerosols as they influence deep convective clouds in the Tropics.

Description: The aim of this study is to investigate the influence of ozone depletion and recovery on the Southern Annular Mode (SAM) and stratosphere-troposphere coupling. Using the NIWA-UKCA chemistry-climate model, we compare reference runs that include forcing due to greenhouse gases and ozone depleting substances to sensitivity simulations in which ozone depleting substances are fixed at their 1960 levels. We find that ozone depletion leads to an increased frequency of extreme anomalies and increased persistence of the SAM in the stratosphere as well as stronger, more persistent stratosphere-troposphere coupling. Currently the stratosphere provides an appreciable amount of predictability to the troposphere on time scales of one or two months, however we find that this effect reduces over time as stratospheric ozone recovers to pre-ozone hole levels towards the latter part of this century.

Description: Despite the importance of the coupling between vegetation dynamics and root-zone soil moisture in land–atmosphere interactions, there is no land data assimilation system (LDAS) that currently addresses this issue, limiting the capacity to positively impact weather and seasonal forecasting. We develop a new LDAS that can improve the skill of an ecohydrological model to simulate simultaneously surface soil moisture, root-zone soil moisture, and vegetation dynamics by assimilating passive microwave observations that are sensitive to both surface soil moisture and terrestrial biomass. This LDAS first calibrates both hydrological and ecological parameters of a land surface model, which explicitly simulates vegetation growth and senescence. Then, it adjusts the model states of soil moisture and leaf area index (LAI) sequentially using a genetic particle filter. We can adjust the subsurface soil moisture, which is not observed directly by satellites, because we simulate the interactions between vegetation dynamics and subsurface water dynamics. From a point-scale evaluation, we succeed in improving the performance of our land surface model and generate ensembles of the model state whose distribution reflects the combined information in the land surface model and satellite observations. We show that the adjustment of the subsurface soil moisture significantly improves the capacity to simulate vegetation dynamics in seasonal forecast timescales. This LDAS can contribute to the generation of ensemble initial conditions of surface and subsurface soil moisture and LAI for a probabilistic framework of weather and seasonal forecasting.

Description: Effectively modeling the influence of terrestrial snow on climate in general circulation models (GCMs) is limited by imperfect knowledge and parameterization of arctic and sub-arctic climate processes, and a lack of reliable observations for model evaluation and improvement. This study uses a number of satellite-derived datasets to evaluate how well the current generation of climate models from the fifth Coupled Model Intercomparison Project (CMIP5) simulate the seasonal cycle of climatological snow cover fraction (SCF) and surface albedo over the Northern Hemisphere snow season (September – June). Using a variety of metrics, the CMIP5 models are found to simulate SCF evolution better than that of albedo. The seasonal cycle of SCF is well reproduced despite substantial biases in simulated surface albedo of snow-covered land (α sfc_snow ), which affect both the magnitude and timing of the seasonal peak in α sfc_snow during the fall snow accumulation period, and the springtime snow ablation period. Insolation-weighting demonstrates that the biases in α sfc_snow during spring are of greater importance for the surface energy budget. Albedo biases are largest across the boreal forest, where the simulated seasonal cycle of albedo is biased high in 15/16 CMIP5 models. This bias is explained primarily by unrealistic treatment of vegetation masking and subsequent overestimation (more than 50% in some cases) of peak α sfc_snow , rather than by biases in SCF. While seemingly straightforward corrections to peak α sfc_snow could yield significant improvements to simulated snow albedo feedbacks, changes in α sfc_snow could potentially introduce biases in other important model variables such as surface temperature.

Description: Understanding critical processes that contribute to the organization of mesoscale convective systems (MCSs) is important for accurate weather forecasts and climate predictions. In this study, we investigate the effects of wind shear at different vertical levels on the organization and properties of convective systems using the Weather Research and Forecasting (WRF) model with spectral-bin microphysics. Based on a control run for a MCS with weak wind shear (Ctrl), we find that increasing wind shear at the lower troposphere (L-shear) leads to a more organized quasi-line convective system. Strong wind shear in the middle troposphere (M-shear) tends to produce large vorticity and form a mesocyclone circulation and an isolated strong storm that leans toward supercellular structure. By increasing wind shear at the upper vertical levels only (U-shear), the organization of the convection is not changed much, but the convective intensity is weakened. Increasing wind shear in the middle troposphere for the selected case results in a significant drying, and the drying is more significant when conserving moisture advection at the lateral boundaries, contributing to the suppressed convective strength and precipitation relative to Ctrl. Precipitation in the L- and U-shear does not change much from Ctrl. Evident changes of cloud macro- and micro-physical properties in the strong wind shear cases are mainly due to large changes in convective organization and water vapor. The insights obtained from this study help us better understand the major factors contributing to convective organization and precipitation.

Description: The Sahelian Sudan is an arid to semi-arid region that depends on the seasonal rainfall as the main source of water, but its rainfall has large interannual variability. Such dry regions usually have their main moisture sources elsewhere; thus the rainfall variability is directly related to the moisture transport. This study seeks to identify source regions of water vapor for Sahelian Sudan during the monsoon period, from July to September. We have used the Lagrangian trajectory model FLEXPART driven by ERA-interim results for the time period 1998 to 2008. The results show that most of the air masses that reach this region during the monsoon period have their major origins over the Arabian Peninsula, Central Africa or are associated with the tropical easterly jet. Flow associated with ITCZ contributes almost half of the total precipitated water; most of it comes from Central Africa. This suggests that moisture recycling is the major contributor, compared to Oceanic sources. The flows from the northeast (Arabian Peninsula and north Asia) and east (Horn of Africa and north Indian Ocean) contribute about one third of the precipitated water. The rest of precipitated water comes from the Mediterranean, subtropical Atlantic and western Sahel, all with smaller contribution. Our results also indicate that different sub-regions of Sahelian Sudan have different moisture sources. Such result needs to be taken into account in seasonal forecasting practices.

Description: A combination of ray theory and 2D time-dependent simulations is used to investigate the linear effects of a time-dependent, vertically and horizontally inhomogeneous background horizontal wind field on the propagation, refraction and reflection of small-scale gravity wave packets. Interactions between propagating waves of different scales are likely to be numerous and important. We find that a static medium scale wave wind field of sufficient amplitude can channel and/or critical level filter a small scale wave or cause significant reflection, depending upon both waves’ parameters. However, the inclusion of a time-dependent phase progression of the medium scale wave can reduce energy loss through critical level filtering by up to ~70% and can also reduce reflection by up to ~60% for the cases simulated. We also find that the relative direction of propagation between the small scale and medium scale wave can significantly affect small-scale wave filtering. When the phases are progressing in the same horizontal direction, the small-scale wave is far more likely to become trapped and ultimately critical level filtered than if the phases are propagating in opposite horizontal directions unless reflection occurs first. Considerations of time-dependent winds associated with medium scale propagating waves and their directionality are important for assessing the propagation and dispersion of small-scale waves over large horizontal distances.

Description: A theoretical study is presented to investigate lightning strikes to towers, with focus on wave interactions occurring at the return-stroke front due to the arrival of current pulses that propagate upward on the channel after being transmitted from the tower. The lightning channel is represented as a transmission line including corona and nonlinear losses. Analyses for the hypothetical case of a lossless channel considering matched tower and channel impedances show that the arrival of current pulses at the upward moving return-stroke front leads to an increase in corona currents leaving the channel. This transient process generates current pulses whose arrival at the tower top can be interpreted as the effect of a current reflection at the upward moving front, even though no impedance discontinuity exists at that point if return-stroke and leader channel properties are assumed the same. In the more realistic case of a lossy channel considering unmatched channel and tower impedances, the nonlinear channel resistance modifies the current pulses that propagate along the channel so that they merge smoothly with the return-stroke front. In this case, the interaction of the current pulses transmitted from the tower to the channel with the upward-moving return-stroke front does not lead to features that can be clearly interpreted as the result of current reflections at that point in the evaluated conditions. Finally, it is argued that the current reflection coefficient at the tower top should be viewed as a current dependent parameter as opposed to a constant, linear value.

Description: Measurements made by microwave imaging radiometers can be used to retrieve several environmental parameters over the world's oceans. In this work, we calculate the uncertainty in retrievals obtained from the Special Sensor Microwave Imager Sounder (SSMIS) instrument caused by uncertainty in the input parameters to the retrieval algorithm. This work applies to the Version-7 retrievals of surface wind speed, total column water vapor, total column cloud liquid water, and rain rate produced by Remote Sensing Systems. Our numerical approach allows us to calculate an estimated input-induced uncertainty for every valid retrieval during the SSMIS mission. Our uncertainty estimates are consistent with the differences observed between SSMIS wind speed and vapor measurements made by SSMIS on the F16 and F17 satellites, supporting their accuracy. The estimates do not explain the larger differences between the SSMIS measurements of wind speed and vapor and other sources of these data, consistent with the influence of more sources of uncertainty.

Description: Frozen soil was simulated at six seasonally frozen and seven permafrost stations over the northern Tibetan Plateau using the Variable Infiltration Capacity (VIC) model for the period of 1962 – 2009. The VIC model resolved the seasonal cycle and temporal evolution of the observed soil temperatures and liquid soil moisture well. The simulated long-term changes during 1962–2009 indicated mostly positive trends for both soil temperature and soil moisture, and negative trends for soil ice content at annual and monthly time scales, although differences existed among the stations, soil layers and seasons. Increases in soil temperature were due mainly to increases in daily air temperature maxima and internal soil heat conduction, while decreases in soil ice content were related to warming of frozen soil. For liquid soil moisture, increases in the cold months can be attributed to increases in soil temperature and enhanced soil ice melt while changes in the warm months were the results of competition between positive precipitation and negative soil temperature effects. Precipitation and liquid soil moisture were strongly correlated with evapotranspiration and runoff, but had various degrees of correlations with baseflow during May-September. Seasonally frozen stations displayed longer and more active hydrological processes than permafrost stations. Slight enhancement of the surface hydrological processes at the study stations was indicated, due to the combined effects of precipitation changes, which were dominant, and frozen soil degradation.

Description: In the long term, precipitation in the central U.S. decreases by about 25% during the seasonal transition from June to July. This precipitation decrease is observed to have intensified since 1979, and such intensification could have enhanced spring drought occurrences in the central U.S., in which conditions quickly evolve from being abnormally dry to exceptionally dry. Various atmospheric and land reanalysis datasets were analyzed to examine the trend in the June-to-July seasonal transition. It was found that the intensified deficit in precipitation was accompanied by increased downward shortwave radiation flux, tropospheric subsidence, enhanced evaporative fraction, and elevated planetary boundary layer height, all of which could and did lead to surface drying. The change in tropospheric circulation was characterized by an anomalous ridge over the western U.S. and a trough on either side – a pattern known to suppress rainfall in the central U.S. The pattern of these trends calculated from 1979 to 2011 shows similarity with the evolution of the 2012 record drought from spring to summer.

Description: A detailed time-resolved chemical characterization of ambient non refractory submicron aerosols (NR-PM 1 ) was conducted for the first time in India. The measurements were performed during the winter (November 2011 to January 2012) in a heavily polluted city of Kanpur, which is situated in the Indo-Gangetic Plain (IGP). Real-time measurements provided new insights into the sources and evolution of organic aerosols (OA) that could not be obtained using previously deployed filter-based measurements at this site. The average NR-PM 1 loading was very high (〉100 µg/m 3 ) throughout the study, with OA contributing approximately 70% of the total aerosol mass. Source apportionment of the OA using positive matrix factorization (PMF) revealed large contributions from fresh and aged biomass burning OA throughout the entire study period. A back trajectory analysis showed that the polluted air masses were affected by local sources and distant source regions where the burning of paddy residues occurs annually during winter. Several fog episodes were encountered during the study, and the OA composition varied between foggy and non-foggy periods, with higher oxygen to carbon (O/C) ratios during the foggy periods. The evolution of OA and their elemental ratios (O:C and H:C) were investigated for the possible effects of fog processing.

Description: Quantifying the sensitivity of warm rain to aerosols is important for constraining climate model estimates of aerosol indirect effects. In this study, the precipitation sensitivity to cloud droplet number concentration ( N d ) in satellite retrievals is quantified by applying the precipitation susceptibility metric to a combined CloudSat/MODIS dataset of stratus and stratocumulus that cover the tropical and subtropical Pacific Ocean and Gulf of Mexico. Consistent with previous observational studies of marine stratocumulus, precipitation susceptibility decreases with increasing liquid water path (LWP), and the susceptibility of the mean precipitation rate R is nearly equal to the sum of the susceptibilities of precipitation intensity and of probability of precipitation. Consistent with previous modeling studies, the satellite retrievals reveal that precipitation susceptibility varies not only with LWP but also with N d . Puzzlingly, negative values of precipitation susceptibility are, however, found at low LWP and high N d . There is marked regional variation in precipitation susceptibility values that cannot simply be explained by regional variations in LWP and N d . This suggests other controls on precipitation apart from LWP and N d and that precipitation susceptibility will need to be quantified and understood at the regional scale when relating to its role in controlling possible aerosol-induced cloud lifetime effects.

Description: We investigate the observed trends and interannual variability in surface ozone over the United States using the Global Modeling Initiative chemical transport model. We discuss the roles of meteorology, emissions, and transport from the stratosphere in driving the interannual variability in different regions and seasons. We demonstrate that a hindcast simulation for 1991–2010 can reproduce much of the observed variability and the trends in summertime ozone, with correlation coefficients for seasonally and regionally averaged median ozone ranging from 0.46 to 0.89. Reproducing the interannual variability in winter and spring in the western United States may require higher resolution models to adequately represent stratosphere-troposphere exchange. Hindcast simulations with fixed versus variable emissions show that changes in anthropogenic emissions drive the observed negative trends in monthly median ozone concentrations in the eastern United States during summer, as well as the observed reduction in the amplitude of the seasonal cycle. The simulation underestimates positive trends in the western United States during spring, but excluding the first four years of data removes many of the statistically significant trends in this region. The reduction in the slope of the ozone versus temperature relationship before and after major emission reductions is also well represented by the model. Our results indicate that a global model can reproduce many of the important features of the meteorologically induced ozone variability as well as the emission-driven trends, lending confidence to model projections of future changes in regional surface ozone.

Description: This study presents a comprehensive evaluation of five widely used multisatellite precipitation estimates (MPEs) against 1°x1° gridded-rain gauge dataset as ground truth over India. One decade observations are used to assess the performance of various MPEs (climate prediction center (CPC)-South Asia dataset, CPC morphing technique (CMORPH), precipitation estimation from remotely sensed information using artificial neural networks (PERSIANN), tropical rainfall measuring mission's (TRMM) multi-satellite precipitation analysis (TMPA-3B42) and global precipitation climatology project (GPCP)). All MPEs have high detection skills of rain with larger probability of detection (POD) and smaller ‘misses’ values. However, the detection sensitivity differs from one product (and also one region) to the other. While the CMORPH has the lowest sensitivity of detecting rain, CPC shows highest sensitivity and often over detects rain, as evidenced by large POD and false alarm ratio (FAR) and small ‘misses’ values. All MPEs show higher rain sensitivity over eastern India than western India. These differential sensitivities are found to alter the biases in rain amount differently. All MPEs show similar spatial patterns of seasonal rain bias and root mean square error (RMSE), but their spatial variability across India is complex and pronounced. The MPEs overestimate the rainfall over the dry regions (northwest and southeast India) and severely underestimate over mountainous regions (west coast and northeast India), whereas the bias is relatively small over the core monsoon zone. Higher occurrence of virga rain due to sub-cloud evaporation and possible missing of small-scale convective events by gauges over the dry regions are the main reasons for the observed overestimation of rain by MPEs. The decomposed components of total bias show that the major part of overestimation is due to false precipitation. The severe underestimation of rain along the west coast is attributed to the predominant occurrence of shallow rain and underestimation of moderate to heavy rain by MPEs. The decomposed components suggest that the missed precipitation and hit bias are the leading error sources for the total bias along the west coast. All evaluation metrics are found to be nearly equal in two contrasting monsoon seasons (southwest and northeast), indicating that the performance of MPEs doesn't change with the season, at least over southeast India. Among various MPEs, the performance of TMPA is found to be better than others, as it reproduced most of the spatial variability exhibited by the reference.

Description: Cloud masks serve as a basis for estimates of cloud amount, which is an essential parameter for studying the Earth's radiation budget. The most commonly-used cloud mask is a simple thematic classification, which includes qualitative information on the presence of clouds in the satellite's instantaneous field of view (IFOV). Cloud mask classes have to be ‘translated’ into a quantitative measure, in order to be used for cloud amount calculations. The assignment of cloud fractions to cloud mask classes is a subjective process, and increases uncertainty in cloud amount estimates. We evaluated this degree of uncertainty using the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask product. Together with the operational MODIS cloud mask interpretation, we investigated two extreme alternatives: ‘rigorous’ (only ‘confident cloudy’ IFOVs were 100% cloudy), and ‘tolerant’ (only ‘confident clear’ IFOVs were 0% cloudy). Results showed that in Europe the range of uncertainty was 14.3% in Europe, and controlled by the frequency of small convective clouds. Comparison with surface-based observations suggests that the ‘rigorous’ interpretation of the cloud mask is more accurate than that used operationally for MODIS Level 3 product generation. The ‘rigorous’ approach resulted in the smallest bias (−0.7%), the smallest RMSE (4.6%), the small standard deviation (6%), and the strongest correlation (0.935). These results suggest that for climatological applications the ‘rigorous’ scenario should be considered as a more accurate ‘best guess’ over land.

Description: High-altitude meteorological processes in the Himalaya are influenced by complex interactions between the topography and the monsoon and westerly circulation systems. In this study, we use the atmospheric model WRF configured with high-spatial resolution to understand seasonal patterns of near-surface meteorological fields and precipitation processes in the Langtang catchment in the central Himalaya. Using a unique high-altitude observational network, we evaluate a simulation from 17 June 2012 to 16 June 2013 and conclude that, at 1-km horizontal grid spacing, the model captures the main features of observed meteorological variability in the catchment. The finer representation of the complex terrain and explicit simulation of convection at this grid spacing give strong improvements in near-surface air temperature and small improvements in precipitation, in particular in the magnitudes of daytime convective precipitation and at higher elevations. The seasonal differences are noteworthy, including a reversal in the vertical and along-valley distributions of precipitation between the monsoon and winter seasons, with peak values simulated at lower altitudes (~3000 m a.s.l) and in the upper regions (above 5000 m a.s.l.) in each season, respectively. We conclude that there is great potential for improving the local accuracy of climate change impact studies in the Himalaya by using high-resolution atmospheric models to generate the forcing for such studies.

Description: Accurate prediction of total lightning flash rate in thunderstorms is important to improve estimates of nitrogen oxides (NO x ) produced by lightning (LNO x ) from the storm scale to the global scale. In this study, flash rate parameterization schemes from the literature are evaluated against observed total flash rates for a sample of eleven Colorado thunderstorms, including nine storms from the Deep Convective Clouds and Chemistry (DC3) experiment in May-June 2012. Observed flash rates were determined using an automated algorithm that clusters very high frequency (VHF) radiation sources emitted by electrical breakdown in clouds and detected by the northern Colorado lightning mapping array (LMA). Existing schemes were found to inadequately predict flash rates and were updated based on observed relationships between flash rate and simple storm parameters, yielding significant improvement. The most successful updated scheme predicts flash rate based on the radar-derived mixed-phase 35-dBZ echo volume. Parameterizations based on metrics for updraft-intensity were also updated but were found to be less reliable predictors of flash rate for this sample of storms. The 35-dBZ volume scheme was tested on a dataset containing radar reflectivity volume information for thousands of isolated convective cells in different regions of the US. This scheme predicted flash rates to within 5.8% of observed flash rates on average. These results encourage the application of this scheme to larger radar datasets and its possible implementation into cloud-resolving models.

Description: The central United States experiences a wide array of hydrological extremes, with the 1993, 2008, 2013, and 2014 flooding events and the 1988 and 2012 droughts representing some of the most recent extremes, and is an area where water availability is critical for agricultural production. This study aims to evaluate the ability of a set of global impact models (GIMs) from the WaterMIP project to reproduce the regional hydrology of the central United States for the period 1963–2001. Hydrological indices describing annual daily maximum, medium and minimum flow and their timing are extracted from both modeled daily runoff data by nine GIMs and from observed daily streamflow measured at 252 river gauges. We compare trend patterns for these indices, and their ability to capture runoff volume differences for the 1988 drought and 1993 flood. In addition, we use a subset of 128 gauges and corresponding gridcells to perform a detailed evaluation of the models on a gauge-to-gridcell basis. Results indicate that these GIMs capture the overall trends in high, medium, and low flows well. However, the models differ from obervations with respect to the timing of high and medium flows. More specifically, GIMs that only include water balance tend to be closer to the observations than GIMs that also include the energy balance. In general, as it would be expected, the performance of the GIMs is the best when describing medium flows, as opposed to the two ends of the runoff spectrum. With regards to low flows, some of the GIMs having considerably large pools of zeros or low values in their time series, undermining their ability in capturing low flow characteristics and weakening the ensemble's output. Overall, this study provides a valuable examination of the capability of GIMs to reproduce observed regional hydrology over a range of quantities for the central United States.

Description: Atmospheric rivers (ARs) often trigger extreme precipitation events in British Columbia (BC), Canada. Here we analyze how well the autumn AR events with the highest probability for extreme precipitation over BC, henceforth called AR-extreme events, are simulated in five Coupled Model Intercomparison Project Phase 5 (CMIP5) global climate models (GCMs) and how these AR-extreme events are projected to change by the end of the century. We examine the daily synoptic patterns of integrated water vapor transport ( IVT ) over the Pacific Ocean that favor the formation of AR-extreme events. Our analysis and comparison with AR-extreme events in four reanalysis products for the period 1979–2010 reveal that the GCMs more successfully resolve their seasonality and inter-annual variability than their frequencies and amount of precipitation brought to BC. For the CMIP5 scenario's RCP4.5 and RCP8.5, the frequency of AR-extreme events will increase for the period 2070–2100 with the largest increase in December. All models project an increase in total precipitation over BC, due to the increase in frequency and intensity of the AR-extreme events, however, the dominant factor is the increase in frequency, especially of those events with precipitation exceeding 20 mm/day. The path of the ARs during the AR-extreme events is projected to move northward, bringing stronger IVT and more precipitation to the north coast of BC, while the south coast may become drier than at the present-day. The shift in the ARs is driven by the northward shift in the Aleutian Low Pressure System, especially in RCP8.5.

Description: A widely used approach in observational and modeling studies of NO x produced by lightning is to relate NO x production to the number of flashes, without regard for the distribution of lightning flash sizes. Recent studies have begun to consider channel length and flash size, which is now observable with VHF Lightning Mapping Array (LMA) data. This study uses a capacitor model for flash energy based on the flash coverage area, which defines a size scale. This flash area is then filled with channel using a fractal method and compared to other methods that estimate length directly from the VHF source locations. In the presence of instrument measurement errors, area- and fractal-based estimates are shown to be more stable estimators of flash length and than connect-the-dots approaches, and therefore are better suited for comparison to NO x production. A geometric interpretation of using vertical profiles of VHF source density to weight the altitude distribution of total channel length is developed. An example of the time series of moments of the lightning flash size distribution is shown for an example case and some meteorological interpretation is given.

Description: California near-surface air temperatures are influenced by large-scale, regional and local factors. In that sense, a numerical model experiment was carried out to analyze the contribution of large-scale (changes in atmospheric and oceanic conditions) and regional (increased urbanization) factors on the observed California South Coast Air Basin regional summer daily maximum temperature warming pattern from 1950 to 2013. The simulations were performed with past (1950–54) and present (2009–13) land-cover and climate conditions. The past land cover was derived from historical digital maps, the present land cover was updated with high-resolution airborne remote sensing data. Results show that both factors contribute to the total change in daily maximum temperatures. Changes due to large-scale climate conditions dominate in coastal (due to warming SSTs) and non-urban regions, while changes due to urbanization have an impact mainly in urban areas, especially inland where large-scale warming weakens. Increased urbanization has also reduced sea-breeze intensity due to changes in surface roughness. The model was able to reproduce the regional observed warming pattern, as it incorporates urban heat island effects, otherwise underestimated by large-scale climate change only.

Description: During the Stratosphere-Troposphere Analyses of Regional Transport 2008 Experiment (START08) the NCAR/NSF Gulfstream V aircraft observed high concentrations of NO and NO y in the upper troposphere downwind of a weakening squall line in northern Texas, suggesting either convective transport of polluted boundary layer air to the upper troposphere or lightning production of nitrogen oxides in the convection. These hypotheses are tested by computing three-dimensional back trajectories using winds from a high-resolution simulation of the event with the Weather Research and Forecasting (WRF) Model. The WRF model simulation reproduces the storm structure and evolution with good fidelity. The back trajectories reveal two distinct layers of outflow air from different mesoscale convective systems (MCSs). Most air in the upper layer is transported northward from an MCS in southern Texas, while the lower layer is from both the northern squall line and the southern MCS. In both layers inconsistencies between observed concentrations of CO, NO, and O 3 , and predictions from a simple mixing model suggest that there is significant production of NO by lightning in the convective systems. This is consistent with lightning observations from the National Lightning Detection Network (NLDN). Additionally, the model simulation appears to slightly underestimate the depth of vertical transport by the MCS.

Description: This study investigated the air-sea interaction over the Kuroshio in the East China Sea by focusing on the response and feedback of the ocean to typical spring weather events. The Weather Research and Forecasting Model was coupled with the HYbrid Coordinate Ocean Model for use in the study. The study period comprised a sequence of typical weathers in the area: prevailing southwesterly winds, the passage of a cold front and the ensuing cold-air outbreak, and the development of a Taiwan low. The air-sea interaction operated on a diurnal timescale under conditions of moderate wind speeds, high insolation, and a shallow oceanic mixed layer. The sea surface temperature and upper ocean heat content increased progressively prior to the frontal passage. The model reproduced the retreat of Kuroshio in response to the strong wind during the cold-air outbreak. The diurnal cycle vanished at high wind speeds. Wind stirring eroded the upper seasonal thermocline and deepened the oceanic mixed layer. The upper ocean heat content decreased because of entrainment cooling and surface heat losses. Surface restratification was subsequently suppressed in the thick and weakly stratified remnant layer. The consequently insufficient recovery of upper ocean heat content may have preconditioned a stagnation of the Taiwan low. The recovery of upper ocean heat content was discussed to derive the implication for climate simulations.

Description: We present the first coordinated study using two lidars at two separate locations to characterize a 1-h mesoscale gravity wave event in the mesopause region. The simultaneous observations were made with the STAR Na Doppler lidar at Boulder, CO, and the USU Na Doppler lidar and temperature mapper at Logan, UT on 27 November 2013. The high precision possessed by the STAR lidar enabled these waves to be detected in vertical wind. The mean wave amplitudes are ~0.44 m/s in vertical wind and ~1% in relative temperature at altitudes of 82–107 km. Those in the zonal and meridional winds are 6.1 and 5.2 m/s averaged from 84–99 km. The horizontal and vertical wavelengths inferred from the mapper and lidars are ~219±4 and 16.0±0.3 km, respectively. The intrinsic period is ~1.3 h for the airglow layer, Doppler shifted by a mean wind of ~17 m/s. The wave packet propagates from Logan to Boulder with an azimuth angle of ~135° clockwise from North and an elevation angle of ~ 3° from the horizon. The observed phase difference between the two locations can be explained by the travelling time of the 1-h wave from Logan to Boulder, which is about ~2.4 h. The wave polarization relations are examined through the simultaneous quantifications of the three wind components and temperature. This study has developed a systematic methodology for fully characterizing mesoscale gravity waves, inspecting their intrinsic properties, and validating the derivation of horizontal wave structures by applying multiple instruments from coordinated stations.

Description: The climatic impacts of dust on East Asian precipitation, summer monsoon, and sea surface temperature (SST) were investigated by a regional coupled atmosphere–ocean-land model. The regional and annual mean dust clear-sky (all-sky) direct radiative forcings were predicted to be −6.65 W m −2 (−1.78 W m −2 ) at the surface and 3.79 W m −2 (8.65 W m −2 ) at the TOA (top-of-atmosphere). The climatic effects of dust include a cooling effect below 700 hPa and a warming effect above, leading to more stabilized lower troposphere and anomalously cyclonic wind over Japan and surrounding oceans. Sensitivity tests show that the 'surface cooling' effect by dust could reduce evaporation over land and enhance stability of the lower atmosphere, leading to reduced vapor content and precipitation in China in the spring. However, the upward movements either by the 'Elevated Heat Pump' (EHP) effect of dust or by the atmospheric convergence, and the downward movement by secondary circulation dominated Northern, Southern, and Central China, respectively, along with enhanced evaporation and weakened lapse rate over China, leading to increased precipitation in downwind areas of dust source regions and Southern China, and to decreased precipitation in Central China in summer. Results from this simulation also show that dust aerosol tends to weaken the East Asia Summer Monsoon by reducing the land-sea-temperature-contrast. In addition, dust could also perturb SST by the local net heat flux rebalance as well as the northward heat transport from surrounding oceans. The anomalous northward surface wind contributes even larger on SST in JJA.

Description: The effective prediction and estimation of hydro-meteorological variables are important for water resources planning and management. In this study, we propose a multivariate conditional model for streamflow prediction and the refinement of spatial precipitation estimates. This model consists of high-dimensional vine copulas, conditional bivariate copula simulations, and a quantile-copula function. The vine copula is employed because of its flexibility in modeling the high-dimensional joint distribution of multivariate data by building a hierarchy of conditional bivariate copulas. We investigate two cases to evaluate the performance and applicability of the proposed approach. In the first case, we generate one-month-ahead streamflow forecasts that incorporate multiple predictors including antecedent precipitation and streamflow records in a basin located in South China. The prediction accuracy of the vine-based model is compared with that of traditional data-driven models such as the support vector regression (SVR) and the adaptive neural-fuzzy inference system (ANFIS). The results indicate that the proposed model produces more skillful forecasts than SVR and ANFIS. Moreover, this probabilistic model yields additional information concerning the predictive uncertainty. The second case involves refining spatial precipitation estimates derived from the tropical rainfall measuring mission precipitation (TRMM) product for the Yangtze River basin by incorporating remotely sensed soil moisture data and the observed precipitation from meteorological gauges over the basin. The validation results indicate that the proposed model successfully refines the spatial precipitation estimates. Although this model is tested for specific cases, it can be extended to other hydro-meteorological variables for predictions and spatial estimations.

Description: The objective of this research was to test, by means of an experiment in a high-voltage laboratory, the effect of an array of hydrometeors on the processes involved in the streamer-leader formation of lightning. Because the common types of hydrometeors present whenever lightning initiation in thunderstorms occurs are ice particles (graupel, hail, or ice crystals), we used, in this experiment, conductive particles similar to hail in size, with various spacing between them, but all under normal atmospheric pressure and room temperature. The laboratory array was suspended on dielectric threads in a uniform electric field of 1 MV m -1 in the middle of the gap between the high-voltage and ground electrodes. During the first phase of the experiment, we studied the formation of a bidirectional arc discharge from the array, and the effects of the array's size on the electrical characteristics and on the speed of development of the discharge. We continued with the same objectives in the second phase of the experiment, adding high-speed video observations with a recording speed of 10 Mfps, to observe all stages of the streamer-leader formation.

Description: Partition of the energy between sensible heat and latent heat indicates that surface temperatures are affected by soil moisture deficits. Since transpiration by plants is the largest contributor to the land's total latent heat, the coupling of temperature and soil moisture will depend on the response of vegetation to soil moisture deficit and those are influenced by the soil moisture regimes. Utilizing daily precipitation and temperature data from China for a period of 1961–2010, this study computes average annual climatic water balance (AACWB) for defining soil moisture regimes and then quantitatively investigates the summer soil moisture-temperature coupling. With precipitation deficits (indicated by standardized precipitation index with the selected optimal time scale of 3 months) as proxy of soil moisture deficits, results indicate that the relationship between summer precipitation deficits and hot extremes tends to be enhanced when the negative AACWB draws closer towards zero while tends to be weakened with the increase of positive AACWB. For the region with the negative AACWB closing zero, the enhanced relationship should be attributed to the increase of the proportion of latent heat compared to the absorbed total energy. However, the weakened relationship with the increase of positive AACWB may be owing to the different responses of vegetation to precipitation deficit that the transpiration in the region with lower positive AACWB is less when responding to precipitation deficit. However, the physiological mechanisms behind vegetation response to soil moisture deficits still need to be further analyzed. By quantifying relevant biological and hydrological processes and their interaction, it is expected that the uncertainties in future-climate scenarios be reduced, that would then allow the development of early warning and adaptation measures prior to the occurrence of hot extremes. Further, the summer precipitation deficit-temperature coupling is strongest along the strip stretching from southwest to northeast in China.

Description: Climate model simulations are routinely compared to observational data sets for evaluation purposes. The resulting differences can be large and induce artifacts if propagated through impact models. They are usually termed ‘model biases’, suggesting that they exclusively stem from systematic models errors. Here we explore for Switzerland the contribution of two other components of this mismatch, which are usually overlooked: interpolation errors and natural variability. Precipitation and temperature simulations from the RCM COSMO-CLM were compared to two observational data sets, for which estimates of interpolation errors were derived. Natural variability on the multi-decadal time scale was estimated using three approaches relying on homogenized time series, multiple runs of the same climate model and bootstrapping of 30-year meteorological records. We find that although these methods yield different estimates, the contribution of the natural variability to RCM-observation differences in 30-year means is usually small. In contrast, uncertainties in observational data sets induced by interpolation errors can explain a substantial proportion of the mismatch of 30-year means. In those cases, we argue that the model biases can hardly be distinguished from interpolation errors, making the characterization and reduction of model biases particularly delicate. In other regions, RCM biases clearly exceed the estimated contribution of natural variability and interpolation errors, enabling bias characterization and robust model evaluation. Overall, we argue that bias correction of climate simulations needs to account for observational uncertainties and natural variability. We particularly stress the need for reliable error estimates to accompany observational datasets.

Description: Seasonal changes in near-surface aerosol mass loading and their chemical characteristics over the marine environment of Bay of Bengal (BoB) have been investigated, based on the ship-based experiments carried out as part of three major field campaigns in pre-monsoon, winter and monsoon seasons. These spatio-temporal properties of in-situ measured near-surface aerosols were compared with the columnar aerosol optical depth (AOD), Angstrom exponent, effective radius, mass concentration, fine mode AOD and large mode AOD retrieved by MODIS. The spatial heterogeneity in the chemical nature of aerosols for the three contrasting seasons has been addressed. In all the seasons, BoB is found to be polluted by anthropogenic aerosols as revealed by their chemical composition. North BoB exhibits highest near-surface aerosol loading, columnar AOD and aerosol mass concentration irrespective of the season. While the high AOD in winter is due to fine mode anthropogenic aerosols, that in monsoon is due to large-sized sea-salt aerosols. The fine mode AOD over BoB contributes 45% in monsoon, 69% in pre-monsoon and 73% in winter. The column AOD in winter and pre-monsoon is ~50% of its monsoonal value. The e-fold scale distances of total aerosol mass loading and those of various chemical species over BoB are also investigated. The column mass concentration has been estimated from the in-situ measured near-surface aerosol mass loading and compared with that retrieved from MODIS. Regions where low near-surface aerosol mass loading was observed, the satellite-retrieved columnar loading was found to be over-estimated suggesting seasonal and regional differences in scale height.

Description: In contrast to the impacts of anthropogenic aerosols and greenhouse gases, little is known about the impact of urban land-surface forcing (ULSF) on large-scale atmospheric circulation. This study explores atmospheric responses to idealized ULSF in eastern China during the boreal spring using the Community Atmosphere Model version 5.1 coupled with the Community Land Model version 4. Results show that the ULSF leads to an increased air temperature in northern China both near the surface and in the lower troposphere. Related to a strong thermal feedback loop, a mid-upper tropospheric cooling is found in eastern China while a relatively strong warming occurs in the mid-high latitudes, which acts to enhance the meridional temperature gradient to the north of the source region and then shifts the East Asian subtropical jet stream (EASJ) southward. A weakened southwesterly in the lower troposphere in southern China slows down moisture transportation to northern China, and the southward-shifted EASJ induces strong anomalous sinking motion to the north of the Yangtze River Valley (YRV). The associated changes in moisture and vertical airflow result in moisture divergence along the YRV and convergence in southern China. Thus, the spring rain belt is shifted southward, as characterized by below-normal rainfall extending from the Huai River Valley to South Korea and above-normal rainfall from southern China to the south coast of Japan. In addition, analysis of the upper tropospheric wave activity signifies that large-scale atmospheric responses due to the ULSF also exert an important influence on local climate.

Description: Using ERA-Interim and MERRA re-analysis datasets, we investigated the effects of the central pacific (CP) El Niño on the Southern Hemispheric (SH) stratosphere particularly during the austral spring. SH stratosphere warming is at a maximum in September rather than in November and December, as suggested by previous studies. SH stratospheric temperature anomalies become significant beginning in July and reach a peak of approximately 4K in September, reflecting a weakened SH vortex and a strengthened SH stratospheric Brewer-Dobson circulation. The anomalous Eliassen-Palm flux and its divergence in the SH mid-latitudes are most significantly enhanced in August, leading to the SH maximum stratospheric temperature anomalies approximately one month later. In the middle latitudes of the SH, the poleward and upward propagation of enhanced planetary waves (PWs) during the austral winter (July-September) causes anomalous SH polar warming and tropical cooling in the stratosphere. The wavenumber 1 (WN1) pattern is responsible for PW anomalies in August, whereas the WN2 pattern is responsible those in September. Eddy heat flux during CP El Niño is also anomalously enhanced in extra-tropical SH stratosphere in both August and September and subsequently weaken during the following months.

Description: Terra MODerate Resolution Imaging Spectroradiometer (MODIS) is one of the key sensors in the NASA's Earth Observing System (EOS), which has successfully completed fifteen years of on-orbit operation. Terra MODIS continues to collect valuable information of the Earth's energy radiation from visible to thermal infrared wavelengths. The instrument has been well characterized over its lifetime using On Board Calibrators (OBC) whose calibration references are traceable to the NIST standards. In this paper, we focus on the electronic crosstalk effect of Terra MODIS band 29, a Thermal Emissive Band (TEB) whose center wavelength is 8.55 µm. Previous works have established the mechanism to describe the effect of the electronic crosstalk in the TEB channels of Terra MODIS. This work utilizes the established methodology to apply to band 29. The electronic crosstalk is identified and characterized using the regularly scheduled lunar observations. The moon being a near pulse-like source allowed easy detection of extraneous signals around the actual moon surface. First, the crosstalk transmitting bands are identified along with their amplitudes. The crosstalk effect then is characterized using a moving average mechanism that allows a high fidelity of the magnitude to be corrected. The lunar based analysis unambiguously shows that the crosstalk contamination is becoming more severe in recent years and should be corrected in order to maintain calibration quality for the affected spectral bands. Finally, two radiometrically well characterized sites, Pacific Ocean and Libya 1 desert, are used to assess the impact of crosstalk effect. It is shown that the crosstalk contamination induces a long-term upward drift of 1.5 K in band 29 brightness temperature of MODIS Collection 6 L1B, which could significantly impact the science products. The crosstalk effect also induces strong detector-to-detector differences, which result in severe stripping in the Earth view images. With crosstalk correction applied, both the long-term drift and detector differences are significantly reduced.

Description: Atmospheric profiles of temperature (T), vapor density (ρ v ), and relative humidity (RH) retrieved from ground-based microwave radiometer (MWR) measurements are compared with radiosonde soundings at Wuhan, China. The MWR retrievals were averaged in the ±30-min period centered at sounding times of 00 and 12 UTC. A total of 403 and 760 profiles under clear and cloudy skies were selected. Based on the comparisons, temperature profiles have better consistency than the ρ v and RH profiles, lower levels are better than upper levels, and the cloudy are better than the clear-sky profiles. Three cloud types (low, middle, and high) were identified by matching the infrared radiation thermometer (IRT) detected cloud-base temperature to the MWR retrieved temperature-height profiles. Temperature profile under high cloud has the highest correlation coefficient (R) and the lowest bias and RMS, but under low cloud is in the opposite direction. The ρ v profile under middle cloud has the highest R and the lowest bias, but under high cloud has the lowest R, the largest bias and RMS. Based on the radiosonde soundings, both clear and cloudy wind speeds and drifting distances increase with height, but increases much faster under clear than cloudy above 4 km. The increased wind speeds and drifting distances with height have resulted in decreased correlation coefficient and increased temperature biases and RMSs with height for both clear and cloudy skies. The differences in R, bias and RMS between clear and cloudy skies are primarily resulted from their wind speeds and drifting distances.

Description: Solar Radiation Management (SRM) has been proposed as a means to partly counteract global warming. The Geoengineering Model Intercomparison Project (GeoMIP) has simulated the climate consequences of a number of SRM techniques. Thus far, the effects on vegetation have not yet been thoroughly analyzed. Here, the vegetation response to the idealized GeoMIP G1 experiment from eight fully coupled earth system models (ESMs) is analyzed, in which a reduction of the solar constant counterbalances the radiative effects of quadrupled atmospheric CO 2 concentrations (abrupt4xCO2). For most models and regions, changes in net primary productivity (NPP) are dominated by the increase in CO 2 , via the CO 2 fertilization effect. As SRM will reduce temperatures relative to abrupt4xCO2, in high latitudes this will offset increases in NPP. In low latitudes, this cooling relative to the abrupt4xCO2 simulation decreases plant respiration while having little effect on gross primary productivity, thus increasing NPP. In Central America and the Mediterranean, generally dry regions which are expected to experience increased water stress with global warming, NPP is highest in the G1 experiment for all models due to the easing of water limitations from increased water-use efficiency at high-CO 2 concentrations and the reduced evaporative demand in a geoengineered climate. The largest differences in the vegetation response are between models with and without a nitrogen-cycle, with a much smaller CO 2 fertilization effect for the former. These results suggest that until key vegetation processes are integrated into ESM predictions, the vegetation response to SRM will remain highly uncertain.

Description: We present a simple chemical definition to demark the edge of the mesospheric polar vortices. Because this vortex definition does not rely on the wind field it is useful in the mesosphere where wind observations are sparse and reanalysis winds are unreliable. The chemical definition is also insensitive to double jets that complicate vortex identification in the mesosphere. The algorithm is based on horizontal gradients of carbon monoxide (CO) and mirrors the widely used vortex edge definition in the stratosphere based on potential vorticity (PV) gradients. Here, the approach is used to identify the Arctic vortex in the mesosphere during a 10-year (2004–2014) record of Microwave Limb Sounder data. Vortex size and shape comparisons are made where the CO and PV methods overlap in the upper stratosphere. A case study is presented during the NH 2008–2009 winter that demonstrates the fidelity of the CO gradient method on individual days and emphasizes the impact of double jets on methods to identify the polar vortex. We recommend transitioning from a PV or stream function-based vortex definition in the stratosphere to using a CO gradient definition above 0.1 hPa (~60 km). The CO gradient method identifies a coherent region of high CO at 80 km that is confined to mid-to-high latitudes 99.8% of the time during Arctic winter. Taking advantage of the CO gradient method to identify the polar vortex adds ~20 km of reliable vortex information (from 60–80 km) in a region of the atmosphere where reanalyses are most suspect.

Description: The severe 2013/14 cold winter has been examined in the context of the previous 55 winters using the NCEP reanalysis data for the period 1960–2014. North America is dominated by pronounced cold anomalies over the Great Plains and Great Lakes in December 2013 and February 2014, but exhibits an east–west contrast pattern with warm anomalies over most of the North American West in January 2014. A relevant temperature index, defined as land surface temperature anomalies averaged over (40°-60°N, 105°-80°W), reveals a warming trend as well as interannual variability with a significant power peak of 6.0 years. While 2013/14 was the second coldest winter during 1960–2014, it is the coldest one in the linearly detrended series, with a negative anomaly of 2.63 standard deviations. This indicates that the long-term warming has made the 2013/14 winter less severe than it could have been. The temperature and circulation variability in association with the zonally symmetric variability of the polar vortex projects weakly on the corresponding anomalies in the 2013/14 winter, whereas the variability associated with the principal mode of North American surface temperature projects strongly on the corresponding anomalies in the winter. This mode is associated with a sea surface temperature (SST) pattern of significant anomalies over the North Pacific and North Atlantic mid-high latitudes. The anomalous atmospheric circulation shows an anticyclonic anomaly over the Gulf of Alaska-Bering Sea and a cyclonic anomaly downstream over North America. It bears resemblance to the North Pacific Oscillation/Western Pacific (NPO/WP) pattern, and drives the SST in the North Pacific. Over western-central Canada and the northern US, below-average heights are associated with above-normal precipitation, implying enhanced upward vertical motion and variation of local cloud forcing, leading to a variation of the surface energy budget dominated by surface longwave radiation anomalies. Over North America, there is less downwelling longwave radiation at the surface when the atmosphere is cold, which is offset by the corresponding reduction in outgoing longwave radiation.

Description: An accurate understanding of the instantaneous, dynamic land surface emissivity is necessary for a physically based, multi-channel passive microwave precipitation retrieval scheme over land. In an effort to assess the feasibility of the physical approach for land surfaces, a semi-empirical emissivity model is applied for calculation of the surface component in a test area of the US Southern Great Plains (SGP). A physical emissivity model, using land surface model data as input, is used to calculate emissivity at the 10 GHz frequency, combining contributions from the underlying soil and vegetation layers, including the dielectric and roughness effects of each medium. An empirical technique is then applied, based upon a robust set of observed channel covariances, extending the emissivity calculations to all channels. For calculation of the hydrometeor contribution, reflectivity profiles from the Tropical Rainfall Measurement Mission Precipitation Radar (TRMM-PR) are utilized along with coincident brightness temperatures (Tbs) from the TRMM radiometer (TMI), and cloud resolving model profiles. Ice profiles are modified to be consistent with the higher frequency microwave Tbs. Resulting modeled top of the atmosphere (TOA) Tbs show correlations to observations of 0.9, biases of 1K or less, RMS errors on the order of 5K, and improved agreement over the use of climatological emissivity values. The synthesis of these models and datasets leads to creation of a simple prototype Tb database that includes both dynamic surface and atmospheric information physically consistent with the LSM, emissivity model, and atmospheric information.

Description: Forecast uncertainties and physical mechanisms of a quasi-linear extreme-rain-producing mesoscale convective system (MCS) along the Mei-yu front in East China, during the midnight-to-morning hours of 8 July 2007, are studied using ensembles of 24-h convection-permitting simulations with a nested grid spacing of 1.11 km. The simulations reveal a strong sensitivity to uncertainties in the initial state despite the synoptic environment being favorable for heavy rainfall production. Linear changes of a less skillful member's initial state toward that of a skillful member lead to a monotonic improvement in the precipitation simulation, with the most significant contribution arising from changes in the moisture field. Sensitivity to physics parameterizations representing sub-grid-scale processes fail to account for the larger simulation errors (missing the MCS) with the physics variation examined, but could result in a large spread in the location and amount of accumulative rainfall. A robust feature of the best-performing members that reasonably simulate the MCS-associated heavy rainfall is the presence of a cold dome ahead of the Mei-yu front generated by previous convection. The cold dome promotes nocturnal convective initiation by lifting high equivalent potential temperature air in the southwesterly flow to its level of free convection. The skillful members reproduce the convective backbuilding and echo-band training processes that are observed during this event and many other heavy rainfall events over China. In contrast, the less skillful members that miss the development of the MCS either do not simulate the previous convection or produce a cold dome that is too shallow to initiate the MCS.

Description: This study aims to investigate some characteristics of the moist processes of the Madden-Julian oscillation (MJO), by making use of joint HDO (or δ D) and H 2 O vapor measurements. The MJO is the main intraseasonal mode of the tropical climate, but is hard to properly simulate in global atmospheric models. The joint use of δ D-H 2 O diagnostics yields additional information compared to sole humidity measurements. We use mid-tropospheric Infrared Atmospheric Sounding Interferometer (IASI) satellite δ D and H 2 O measurements to determine the mean MJO humidity and δ D evolution. Moreover, by making use of high temporal resolution data, we determine the variability in this evolution during about eight MJO events from 2010 to 2012 (including those monitored during the CINDY/DYNAMO campaign). This data has a higher spatio-temporal coverage than previous δ D measurements, enabling the sampling of individual MJO events. IASI measurements over the Indian Ocean confirm earlier findings that the moistening before the precipitation peak of an MJO event is due to water vapor slightly enriched in HDO. There is then a HDO depletion around the precipitation peak that also corresponds to the moister environment. Most inter-event variability determined in the current study occurs 5 to 10 days after the MJO event. In 75% of the events, humidity decreases while the atmosphere remains depleted. In a quarter of the events, humidity increases simultaneously with an increase in δ D. After this, the advection of relatively dry and enriched air brings back the state to the mean. Over the maritime continent, δ D-H 2 O cycles are more variable on timescales shorter than the MJO and the inter-event variability is larger than over the Indian Ocean. The sequence of moistening and drying processes as revealed by the q- δ D cycles can be used as a benchmark to evaluate the representation of moist processes in models. This is done here by comparing observations to simulations of the isotope enabled LMDZ GCM nudged with reanalysis wind fields. These simulations also give informations to investigate possible physical origins of the observed q- δ D cycles.

Description: This study investigates the seasonality in anthropogenic aerosol optical depth (AOD) distributions and their effects on clouds and precipitation in East Asia with the Community Atmospheric Model version 5. The differences between the model experiments with and without anthropogenic emissions exhibit a northward shift of the maximal AOD change in East Asia from March to July and then a southward withdrawal from September to November, which are induced by East Asian monsoon circulation. Associated with the shift, the direct and semi-direct effects of the anthropogenic aerosols are the most pronounced in spring and summer, with a maximum center in North China during summer and a secondary center in South China during spring. The cloud liquid water path and shortwave cloud forcing changes, however, are the weakest in North China during summer. The indirect effect is the strongest in South China during spring, which is related to the large amount of middle-low level clouds in cold seasons in East China. A positive feedback between aerosol induced surface cooling and low-level cloud increase is identified in East China, which acts to enforce the aerosol indirect effect in spring. Accordingly, the climate response to the anthropogenic aerosols is also characterized by a northward shift of reduced precipitation from spring to summer, leading to a spring drought in South China and a summer drought in North China. The spring drought is attributed to both direct and indirect effects of the anthropogenic aerosols, while the summer drought is primarily determined by the aerosols' direct effect.

Description: This study describes the large-scale atmospheric flow patterns that favor mixing events within western Long Island Sound (wLIS), and how inter-annual and inter-decadal variations in surface winds relate to bottom dissolved oxygen (DOb) variability. DOb data from the wLIS Coastal Observing System (LISCOS) buoy were used in conjunction with the surface winds at La Guardia Airport (LGA) and National Buoy Data Center (NBDC) buoy to identify criteria for water column ventilation and mixing from June-September (JJAS). It is shown that mixing for a 36 hour period after onset is favored when a majority of the surface wind observations for a day (starting at 00 UTC) are from 30°-110° (NE to ESE) and 〉= 4 m s −1 . This criterion was used to develop a synoptic climatology and the trend in potential mixing events from 1950–2009. These mixing events were categorized based on three synoptic patterns: high pressure, low pressure, and a hybrid high and low. High pressure patterns, which include a hybrid system with a high building from the north/west and low to the south, result in the largest percentage of potential mixing events (76.9%). The number of potential mixing events increases from the 1950s to 1990s (full season and July-August) primarily from an increasing number of high pressure systems; however, the seasonal DOb decreased during this period. There was a slight decrease in the number of July-August potential mixing events from 1990–2009, mainly from a decrease in the number of low pressure systems.

Description: A recent empirical study of Stanhill et al. [2014], which was based on the Angstrom-Prescott relationship between global radiation and sunshine duration, was evaluated. The parameters of this relationship seemed to be rather stable across the dimming and brightening periods. Thus, the authors concluded that the variation in global radiation is more influenced by changes in cloud cover and sunshine duration than by the direct aerosol effects. In our study, done for the Potsdam station (one of six globally distributed stations, the source of one of the longest observational records and closely located to former hotspots of aerosol emission), we tested and rejected the hypothesis that the dimming of global radiation directly caused by aerosols is negligible. The residuals of the Angstrom-Prescott regression reveal a statistically significant positive temporal trend and a temporal level segmentation. The latter was consistent with the temporal emission patterns around Potsdam. The trend in the residuals only disappeared when the model intercept varied according to the temporal level segmentation. The magnitude of the direct aerosol effect on the level changes in global radiation derived from the modified Angstrom-Prescott relationship was in the range indicated in previous studies. Thus from here, a specific request cannot be made for a revision of current climate models state-of-the-art representation of both the cooling effect directly caused by aerosols and the temperature sensitivity to the increase of greenhouse gases.

Description: In this study, we investigated the impacts of the triggering function of the deep convection scheme on diurnal rainfall variation in the middle latitudes by using the single-column version of the Community Atmospheric Model (SCAM). Using the climate statistics of a long-term ensemble analysis of SCAM simulations, we quantified and validated the diurnal rainfall climatological regimes at the Atmospheric Radiation Measurement Southern Great Plains (SGP) site. The results showed that the averaged diurnal rainfall cycle simulated using the default Zhang-Mcfarlane (ZM) scheme of the SCAM peaks near noon, which is far earlier than the observed nighttime peak phase. This bias was due to the ZM scheme, which produced spurious daytime rainfall, even during days in which only light rainfall was observed. By contrast, using a weather-focused scheme, the Simplified Arakawa-Schubert (SAS) scheme, we successfully simulated the nocturnal peak of the diurnal cycle. Experiments conducted on the ZM and SAS schemes featuring different triggering functions revealed that, the relaxation of launching parcels above the planetary boundary layer (PBL) and the inclusion of convective inhibition (CIN) were crucial designs for the model to capture the nocturnal rainfall events of the SGP. The inclusion of CIN reduces spurious weak convective events, and the allowance of launching parcels being above the PBL better captures convective cloud base. The results of this study highlight the modulatory effect of low-level inhomogeneity on the diurnal variation of convection over mid-latitudes and the importance of the triggering function of the deep convection scheme in capturing those variations.

Description: To determine the complex dependencies of currents and electric fields within the Global Electric Circuit (GEC) on the underlying physics of the atmosphere, a new modeling framework of the GEC has been developed for use within global circulation models. Specifically, the Community Earth System Modeling (CESM) framework has been utilized. A formulation of atmospheric conductivity based on ion production and loss mechanisms (including galactic cosmic rays, radon, clouds and aerosols), conduction current sources, and ionospheric potential changes due to the influence of external current systems are included. This paper presents a full description of the calculation of the electric fields and currents within the model, which now includes several advancements to GEC modeling as it incorporates many processes calculated individually in previous articles into a consistent modeling framework. This framework uniquely incorporates effects from the troposphere up to the ionosphere within a single GEC model. The incorporation of a magnetospheric potential, which is generated by a separate magnetospheric current system, acts to modulate or enhance the surface level electric fields at high latitude locations. This produces a distinct phasing signature with the GEC potential that is shown to depend on the observation location around the globe. Lastly, the model output for Vostok and Concordia, two high latitude locations, is shown to agree with the observational data obtained at these sites over the same time period.

Description: Climate change in response to a change in external forcing can be understood in terms of fast response to the imposed forcing and slow feedback associated with surface temperature change. Previous studies have investigated the characteristics of fast response and slow feedback for different forcing agents. Here, we examine to what extent fast response and slow feedback derived from time-mean results of climate model simulations can be used to infer total climate change. To achieve this goal, we develop a multivariate regression model of climate change in which the change in a climate variable is represented by a linear combination of its sensitivity to CO 2 forcing, solar forcing, and change in global mean surface temperature. We derive the parameters of the regression model using time-mean results from a set of HadCM3L climate model step-forcing simulations, and then use the regression model to emulate HadCM3L-simulated transient climate change. Our results show that the regression model emulates well HadCM3L-simulated temporal evolution and spatial distribution of climate change, including surface temperature, precipitation, runoff, soil moisture, cloudiness, and radiative fluxes under transient CO 2 and/or solar forcing scenarios. Our findings suggest that temporal and spatial patterns of total change for the climate variables considered here can be represented well by the sum of fast response and slow feedback. Furthermore, by using a simple 1-D heat-diffusion climate model, we show that the temporal and spatial characteristics of climate change under transient forcing scenarios can be emulated well using information from step-forcing simulations alone.

Description: The cloud detection algorithm for passive sensors is usually based on a fuzzy logic system with thresholds determined from previous observations. In recent years, haze and high aerosol concentrations with high AOD occur frequently in China and may critically impact the accuracy of the MODIS cloud detection. Thus, we comprehensively explore this impact by comparing the results from MODIS/Aqua (passive sensor), CALIOP/CALIPSO (lidar sensor), and CPR/CloudSat (microwave sensor) of the A-Train suite of instruments using an averaged AOD as an index for an aerosol concentration value. Case studies concerning the comparison of the three sensors indicate that MODIS cloud detection is reduced during haze events. In addition, statistical studies show that an increase in AOD creates an increase in the percentage of uncertain flags and a decrease in hit rate, a consistency index between consecutive sets of cloud retrievals. On average, AOD values lower than 0.1 give hit rate values up to 80.0% and uncertainty values lower than 16.8%, while AOD values greater than 1.0 reduce the hit rate below to 66.6% and increase the percentage of uncertain flags up to 46.6%. Therefore, we can conclude that the ability of MODIS cloud detection is weakened by large concentrations of aerosols. This suggests that use of the MODIS cloud mask, and derived higher level products, in situations with haze requires caution. Further improvement of this retrieval algorithm, is desired as haze studies based on MODIS products are of great interest in a number of related fields.

Description: Multi-year applications of an online-coupled meteorology-chemistry model allow an assessment of the variation trends in simulated meteorology, air quality, and their interactions to changes in emissions and meteorology, as well as the impacts of initial and boundary conditions (ICONs/BCONs) on simulated aerosol-cloud-radiation interactions over a period of time. In this work, the Weather Research and Forecasting model with Chemistry version 3.4.1 (WRF/Chem v. 3.4.1) with the 2005 Carbon Bond mechanism coupled with the Volatility Basis Set module for secondary organic aerosol formation (WRF/Chem-CB05-VBS) is applied for multiple years (2001, 2006, and 2010) over continental U.S. This work also examines the changes in simulated air quality and meteorology due to changes in emissions and meteorology and the model's capability in reproducing the observed variation trends in species concentrations from 2001 to 2010. In addition, the impacts of the chemical ICONs/BCONs on model predictions are analyzed. ICONs/BCONs are downscaled from two global models, the modified Community Earth System Model/Community Atmosphere model version 5.1 (CESM/CAM) v5.1 and the Monitoring Atmospheric Composition and Climate model (MACC). The evaluation of WRF/Chem-CB05-VBS simulations with the CESM ICONs/BCONs for 2001, 2006, and 2010 shows that temperature at 2-m (T2) is underpredicted for all three years likely due to inaccuracies in soil moisture and soil temperature, resulting in biases in surface relative humidity, wind speed, and precipitation. With the exception of cloud fraction, other aerosol-cloud variables including aerosol optical depth, cloud droplet number concentration, and cloud optical thickness are underpredicted for all three years, resulting in overpredictions of radiation variables. The model performs well for O 3 and PM 2.5 for all three years comparable to other studies from literature. The model is able to reproduce observed annual average trends in O 3 and PM 2.5 concentrations from 2001 to 2006 and from 2006 to 2010, but is less skillful in simulating their observed seasonal trends. The 2006 and 2010 results using CESM and MACC ICONs/BCONs are compared to analyze the impact of ICONs/BCONs on model performance and their feedbacks to aerosol, clouds, and radiation. Comparing to the simulations with MACC ICONs/BCONs, the simulations with the CESM ICONs/BCONs improve the performance of O 3 mixing ratios (e.g., the normalized mean bias for maximum 8-hr O 3 is reduced from −17% to −1% in 2010), PM 2.5 in 2010, and sulfate in 2006 (despite a slightly larger NMB for PM 2.5 in 2006). The impacts of different ICONs/BCONs on simulated aerosol-cloud-radiation variables are not negligible, with larger impacts in 2006 compared to 2010.

Description: We examined high-speed video and electric field records of 23 subsequent strokes following the previous stroke channel in 3 natural negative lightning flashes, which were obtained at the Lightning Observatory in Gainesville (LOG), Florida. Five strokes exhibited faintly luminous formations (FLFs) occurring in a single, pre-return-stroke frame and ranging from 130 to 908 m in length between the lower end of downward leader and the prospective strike point. The FLFs were inferred to be not streamers (as in first strokes), but manifestations of an increase in conduction current in the defunct channel between the leader and ground in response to the increasing electric field produced by the descending leader. Further, in eight strokes, we observed residual channel luminosity persisting over many frames (for 4.7 to 18 ms), through the pre-return-stroke frame. The residual luminosity was apparently associated with a stronger channel heating and a larger channel radius (and, hence, a lower temperature decay rate), both associated with the relatively long preceding continuing current. Presence of either FLF or residual channel luminosity did not appear to significantly influence the mode and velocity of propagation of the descending leader.

Description: Observational constraints on the change in the radiative energy budget caused by the presence of aerosols, i.e. the aerosol direct radiative effect (DRE), have recently been made using observations from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite (CALIPSO). CALIPSO observations have the potential to provide improved global estimates of aerosol DRE compared to passive sensor-derived estimates due to CALIPSO's ability to perform vertically-resolved aerosol retrievals over all surface types and over cloud. In this study, uncertainties in CALIPSO-inferred aerosol DRE are estimated using multiple years of observations from the Atmospheric Radiation Measurement (ARM) program's Raman lidars (RL) at midlatitude and tropical sites. We find that CALIPSO is unable to detect all radiatively-significant aerosol, resulting in an underestimate in the magnitude of the aerosol DRE by 30–50% at the two ARM sites. The undetected aerosol is likely the consequence of random noise in CALIPSO measurements and therefore will affect global observations as well. This suggests that the global aerosol DRE inferred from CALIPSO observations are likely too weak. Also examined is the impact of the ratio of extinction-to-backscatter (i.e. the lidar ratio) whose value CALIPSO retrievals must assume to obtain the aerosol extinction profile. It is shown that if CALIPSO can reproduce the climatological value of the lidar ratio at a given location, then the aerosol DRE there can be accurately calculated (within about 3%).

Description: Decadal hemispheric WRF-CMAQ simulations from 1990-2010 were conducted to examine the meteorology and air quality responses to the aerosol direct radiative effects. The model's performance for the simulation of hourly surface temperature, relative humidity, wind speed and direction was evaluated through comparison with observations from NOAA's National Climatic Data Center (NCDC) Integrated Surface Data. The inclusion of aerosol direct radiative effects improves the model's ability to reproduce the trend in daytime temperature range (DayTR) which over the past two decades was increasing in eastern China but decreasing in eastern U.S. and Europe. Trends, spatial, and diurnal variations of the surface-level gaseous and particle concentrations to the aerosol direct effect were analyzed. The inclusion of aerosol direct radiative effects was found to increase the surface-level concentration of SO 2 , NO 2 , O 3 , SO 4 2- , NO 3 - and PM 2.5 in eastern China, eastern U.S. and Europe by 1.5-2.1%, 1-1.5%, 0.1-0.3%, 1.6-2.3%, 3.5-10.0%, 2.2-3.2% respectively on average over the entire 21-yr period. However, greater impacts are noted during polluted days with increases of 7.6-10.6%, 6.2-6.7%, 2.0-3.0%, 7.8-9.5%, 11.1-18.6% and 7.2-10.1% respectively. Due to the aerosol direct radiative effects, stabilizing of the atmosphere associated with reduced PBL height and ventilation leads to an enhancement of pollution. Consequently, the continual increase of AOD in eastern China leads to an increasing trend in the air quality feedback which exacerbates air pollution, while emission reductions in eastern US and Europe result in a declining trend in both AODs and feedbacks which make the air pollution control strategies more effective.

Description: The CloudSat 2C-ICE data product is derived from a synergetic ice cloud retrieval algorithm that takes as input a combination of CloudSat radar reflectivity ( Z e ) and CALIPSO lidar attenuated backscatter profiles. The algorithm uses a variational method for retrieving profiles of visible extinction coefficient, ice water content and ice particle effective radius in ice or mixed-phase clouds. Because of the nature of the measurements and to maintain consistency in the algorithm numerics, we choose to parameterize (with appropriately large specification of uncertainty) Z e and lidar attenuated backscatter in the regions of a cirrus layer where only the lidar provides data and where only the radar provides data, respectively. To improve the Z e parameterization in the lidar-only region, the relations among Z e , extinction, and temperature have been more thoroughly investigated using Atmospheric Radiation Measurement (ARM) long-term millimeter cloud radar (MMCR) and Raman lidar measurements. This Z e parameterization provides a first order estimation of Z e as a function extinction and temperature in the lidar-only regions of cirrus layers. The effects of this new parameterization have been evaluated for consistency using radiation closure methods where the radiative fluxes derived from retrieved cirrus profiles compare favorably with CERES measurements. Results will be made publicly available for the entire CloudSat record (since 2006) in the most recent product release known as R05.

Description: Atmospheric gravity waves (GWs) significantly influence global circulation. Deep convection, particularly that associated with typhoons, is believed to be an important source of gravity waves. Stratospheric gravity waves induced by Typhoon Mindulle (2004) were detected by the Atmospheric Infrared Sounder (AIRS). Semicircular GWs with horizontal wavelengths of 100–400 km were found over Taiwan through an inspection of AIRS radiances at 4.3 μ m. Characteristics of the stratospheric gravity waves generated by Typhoon Mindulle were investigated using the Weather Research and Forecasting (WRF) model. The initial and boundary data were determined by the high-resolution European Center for Medium-Range Weather Forecasts (ECMWF) re-analysis data. The WRF simulation reproduces the main features of Typhoon Mindulle and the significant GWs. The simulated GWs with horizontal wavelengths of 100–400 km match the AIRS observations: they propagate upward and eastward, and the westward components are mostly filtered in the stratosphere. By comparing the measured waves with a WRF simulation in the absent of orography (WRF-FLAT), we find that the orographic gravity waves (OGWs) generated by the flow of Typhoon Mindulle over the Central Mountain Range (CMR) in Taiwan account for approximately 50% of the total wave momentum flux in the troposphere. The dominant orientation of the OGW wave fronts is parallel to the CMR rideline. When entering into the stratosphere, OGW propagation is determined by the position of the typhoon center relative to the CMR.

Description: A new parameterization for quantifying the mixing state of aerosol populations has been applied for the first time to samples of ambient particles analyzed using spectro-microscopy techniques. Scanning transmission x-ray microscopy/near edge x-ray absorption fine structure (STXM/NEXAFS) and computer controlled scanning electron microscopy/energy dispersive x-ray spectroscopy (CCSEM/EDX) were used to probe the composition of the organic and inorganic fraction of individual particles collected on June 27 th and 28 th during the 2010 Carbonaceous Aerosols and Radiative Effects (CARES) study in the Central Valley, California. The first field site, T0, was located in downtown Sacramento, while T1 was located near the Sierra Nevada Mountains. Mass estimates of the aerosol particle components were used to calculate mixing state metrics, such as the particle-specific diversity, bulk population diversity, and mixing state index, for each sample. The STXM data showed evidence of changes in the mixing state associated with a build-up of organic matter confirmed by collocated measurements and the largest impact on the mixing state was due to an increase in soot dominant particles during this build-up. The mixing state from STXM was similar between T0 and T1 indicating that the increased organic fraction at T1 had a small effect on the mixing state of the population. The CCSEM/EDX analysis showed the presence of two types of particle populations; the first was dominated by aged sea salt particles and had a higher mixing state index (indicating a more homogeneous population), the second was dominated by carbonaceous particles and had a lower mixing state index.

Description: A detailed examination is made in both observations and the Community Earth System Model (CESM) of relationships among top-of-atmosphere (TOA) radiation, water vapor, temperatures and precipitation for 2000-2014 to assess the origins of radiative perturbations and climate feedbacks empirically. The 30-member large ensemble coupled runs are analyzed along with one run with specified sea surface temperatures for 1994 to 2005 (to avoid volcanic eruptions). The vertical structure of the CESM temperature profile tends to be top-heavy in the model, with too much deep convection and not enough lower stratospheric cooling as part of the response to tropospheric heating. There is too much absorbed solar radiation (ASR) over the southern oceans and not enough in the tropics, and ENSO is too large in amplitude in this version of the model. However, the co-variability of monthly mean anomalies produces remarkably good replication of most of the observed relationships. There is a lot more high frequency variability in radiative fluxes than in temperature, highlighting the role of clouds and transient weather systems in the radiation statistics. Over the Warm Pool in the tropical western Pacific and Indian oceans, where non-local effects from the Walker circulation driven by the ENSO events are important, several related biases emerge: in response to high SST anomalies there is more precipitation, water vapor and cloud, and less ASR and Outgoing Longwave Radiation (OLR) in the model than observed. Different model global mean trends are evident, however, and possibly hinting at too much positive cloud feedback in the model.

Description: The geographical shift of global anthropogenic aerosols from the developed countries to the Asian continent since the 1980s could potentially perturb the regional and global climate due to aerosol-cloud-radiation interactions. We use an atmospheric general circulation model with different aerosol scenarios to investigate the radiative and microphysical effects of anthropogenic aerosols from different regions on the radiation budget, precipitation, and large-scale circulations. An experiment contrasting anthropogenic aerosol scenarios in 1970 and 2010 shows that the altered cloud reflectivity and solar extinction by aerosols results in regional surface temperature cooling in East and South Asia, and warming in US and Europe respectively. These aerosol induced temperature changes are consistent with the relative temperature trends from 1980 to 2010 over different regions in the reanalysis data. A reduced meridional streamfunction and zonal winds over the tropics as well as a poleward shift of the jet stream suggest weakened and expanded tropical circulations, which are induced by the redistributed aerosols through a relaxing of the meridional temperature gradient. Consequently, precipitation is suppressed in the deep tropics and enhanced in the sub-tropics. Our assessments of the aerosol effects over the different regions suggest that the increasing of Asian pollution accounts for the weakening of the tropics circulation, while the decreasing of pollution in Europe and US tends to shift the circulation systems southward. Moreover, the aerosol indirect forcing is predominant over the total aerosol forcing in magnitude, while aerosol radiative and microphysical effects jointly shape the meridional energy distributions and modulate the circulation systems.

Description: One of the challenges in evaluating and applying regional climate models (RCMs) is the non-linear behavior of atmospheric processes, which is still poorly understood. The non-linearities induce chaos which leads to an internal variability in the model. Therefore, an ensemble of RCM simulations has been run and a budget study for potential temperature has been applied to investigate the internally generated variability. Hence, the physical processes associated with diabatic and dynamical terms inducing the inter-member variability have been analyzed. The study is applied over the Arctic on an ensemble of 20 members, differing in their initial conditions, simulated with the RCM HIRHAM5 during summer 2012. This time period is of particular importance because of the melting sea ice and its influence on atmospheric circulation and the resulting effect on the inter-member variability. The amplitude of the inter-member variability of the simulations fluctuates strongly both temporally and spatially. During the beginning of August 2012 the inter-member variability is strongest and coincides with the great Arctic cyclone event. The most important contributions for the inter-member variability tendency are the horizontal and vertical ‘baroclinic’ terms. Both terms have largest absolute values along the coastlines of the Arctic Ocean which are associated with the Arctic frontal zone leading to the cyclone maximum over the Arctic Ocean during summer.

Description: Reconstruction of unknown atmospheric releases using measured concentrations is an ill-posed inverse problem. Due to insufficient measurements and dispersion model uncertainties, reliable interpretation of a retrieved source is limited by lack of resolution, non-uniqueness and instability in the inverse solution. The study presents an optimality analysis, in terms of resolution, stability and reliability, of an inverse solution given by a recently proposed inversion technique, called as Renormalization . The inversion technique is based on an adjoint source-receptor framework and construction of a weight function which interprets a priori information about the unknown release apparent to the monitoring network. The properties of weight function provides a perfect data resolution, maximum model resolution and minimum variance (or stability) for the retrieved source. The reliability of the retrieved source is interpreted in view of the information derived from the geometry of the monitoring network. The inversion technique and resolution features are evaluated for a point source reconstruction using measurements from a recent dispersion experiment (Fusion Field Trials 2007) conducted at Dugway Proving Ground, Utah. With the real measurements, the point release is reconstructed within an average distance of 23 m from the true release where the average distance of the nearest receptor from the true source was 32 m. In all the trials, the point release is retrieved within 3–60 m euclidean distance from their true location. The source strength is retrieved within a factor of 1.5 to the true release mass. The posterior uncertainty in the release parameters is observed to be within 20% of their mean value. The source localization features are resolved to its maximum extent feasible with the design of the monitoring network. The sensitivity studies are conducted to highlight the importance of receptors reporting zero concentration measurements and variations in the resolution features of the source retrieval with respect to the various arrangements of the receptors.

Description: Due to the harshness and inaccessibility of desert regions, the uncertainties concerning the processes of dust mobilization at the surface, airborne transport, and sedimentation are still considerable, limiting the ability to perform model simulations. In June 2011 a comprehensive data set of ground-based and airborne in-situ measurements and remote sensing observations was acquired within the Fennec/LADUNEX field campaign in the western Sahara region. Here, we evaluate the ability of the state-of-the-art Lagrangian particle dispersion model FLEXPART, newly fitted with a dust mobilization capability, to simulate dust transport in this region. We investigate a case where a large Mesoscale Convective System (MCS) triggered dust emissions in central Mali, which subsequently moved as a large cold-pool dust front towards northern Mauritania. Specifying dust mobilization for this case is shown to be an important obstacle to simulating dust transport during this event, since neither the MCS nor the associated cold pool causing dust emission are represented in the meteorological analysis. Obtaining a realistic dust transport simulation for this case therefore requires an inversion approach using a manual specification of the dust sources supported by satellite imagery. When compared to in-situ and remote-sensing data from two aircraft, the Lagrangian dust transport simulations represent the overall shape and evolution of the dust plume well. While accumulation and coarse mode dust are well represented in the simulation, giant mode particles are considerably underestimated. Our results re-emphasize that dust emission associated with deep moist convection remains a key issue for reliable dust model simulations in northern Africa.

Description: We examined atmospheric responses to 35,000+ oceanic eddies in the Kuroshio Extension (KE) region during the period of 2006–2009. Using satellite data, we showed that cold (warm) eddies cause surface winds to decelerate (accelerate), and reduce (increase) latent and sensible heat fluxes, cloud liquid water, water vapor content, and rain rate; all of these changes are quantified. Both the linear correlation between wind divergence and downwind SST gradient and the correspondence between vorticity and crosswind SST gradient support the vertical momentum mixing mechanism, which indicates that SST perturbations modify surface winds by changing the vertical turbulent mixing in the marine atmospheric boundary layer (MABL). High-resolution NCEP Climate Forecast System Reanalysis (CFSR) data can reproduce the atmospheric responses to the oceanic eddies in the MABL albeit with some differences in intensity. In addition, the CFSR data reveal that the atmospheric responses to these oceanic eddies are not confined in the MABL. MABL deepens (shoals) over the warm (cold) eddies; enhanced (reduced) vertical transport of transient zonal momentum occurs over the warm (cold) eddies from the sea surface to about 850-hPa level; vertical velocity anomalies over oceanic eddies penetrate beyond the MABL into free atmosphere; there exists a positive correlated relationship between SST and convective rain rate anomalies, indicative of ocean eddies' impact on the free troposphere. However, the composites of cloud liquid water and rain rate are different from the results based on the satellite data.

Description: The impact of size distribution of mineral dust aerosol on radiative transfer was investigated using the AERONET retrieved aerosol size distributions. Three methods for determining the aerosol optical properties using size distributions were discussed. The first is referred to as a bin method in which the aerosol optical properties are determined for each bin of the size distribution; The second is named as an assembly mean method in which the aerosol optical properties are determined with an integration of the aerosol optical parameters over the observed size distribution; The third is a normal parameterization method based on an assumed size distribution. The bin method was used to generate the benchmark results in the radiation calculations against the methods of the assembly mean and parameterizations based on two size distribution functions, namely log-normal and Gamma were examined. It is seen that the assembly mean method can produce aerosol radiative forcing with accuracy of better than 1%. The accuracies of the parameterizations based on log-normal and Gamma size distributions are about 25% and 5%, respectively. Both the log-normal and Gamma size distribution can be determined by two parameters, the effective radius and effective variance. The better results from the Gamma size distribution can be explained by a third parameter of skewness which is found to be useful for judging how close the assumed distribution is to the observation result. The parameterizations based on the two assumed size distributions are also evaluated in a climate model. The results show that the reflected solar fluxes over the desert areas determined by the scheme based on the Gamma size distribution are about 1 W m −2 less than those from the scheme based on the log-normal size distribution, bringing the model results closer to the observations.

Description: In this paper, we evaluate the impact of mineral dust (MD) on snow radiative properties in the European Alps at ground, aerial and satellite scale. A field survey was conducted to acquire snow spectral reflectance measurements with an ASD Field-spec Pro spectroradiometer. Surface snow samples were analyzed to determine the concentration and size distribution of MD in each sample. An overflight of a four-rotor Unmanned Aerial Vehicle (UAV) equipped with an RGB digital camera sensor was carried out during the field operations. Finally, Landsat 8 Operational Land Imager (OLI) data covering the Central European Alps were analyzed. Observed reflectance evidenced that MD strongly reduced the spectral reflectance of snow, in particular from 350 to 600 nm. Reflectance was compared with that simulated by parameterizing the Snow, Ice and Aerosol Radiation (SNICAR) radiative transfer model. We defined a novel spectral index, the Snow Darkening Index (SDI), that combines different wavelengths showing nonlinear correlation with measured MD concentrations (R 2 =0.87, RMSE=0.037). We also estimated a positive instantaneous radiative forcing that reaches values up to 153 W/m 2 for the most concentrated sampling area. SDI maps at local scale were produced using the UAV data, while regional SDI maps were generated with OLI data. These maps show the spatial distribution of MD in snow after a natural deposition from the Saharan desert. Such post-depositional experimental data are fundamental for validating radiative transfer models and Global Climate Models (GCM) that simulate the impact of MD on snow radiative properties.

Description: Future changes in aridity, defined as the ratio of annual precipitation to potential evapotranspiration (PET), are analyzed using simulations from the Community Earth System Model (CESM) Large Ensemble (LE) and the phase 5 of the Coupled Model Intercomparison Project (CMIP5) during the period 1980–2080. Both CESM and CMIP5 ensembles can reproduce the observed temporal and spatial variability of aridity. On the interannual time scale, annual average PET is sensitive to the variability of relative humidity, net surface energy flux, and surface air temperature while the precipitation variability is the dominant component of annual average aridity sensitivity. For the long-term trends, differences between the two ensembles illustrate that the impact of the internal variability is smaller than that of the model structural uncertainty with the trends from the CMIP5 ensemble of models having a much larger spread than those from the single model CESM-LE. The annual mean aridity averaged over global land increases (becomes drier) by 6.4% in 2055–2080 relative to 1980–2005. Aridity trends differ by region in the ensemble mean. In the future, increasing precipitation leads to decreasing aridity over northwest China and central (or tropical) Africa, while decreasing precipitation leads to drying (increasing aridity) in the sub-tropics, northern and southern Africa and the Amazon. Increases in PET can lead to increasing aridity even in regions with increasing precipitation.

Description: Marine stratocumulus clouds (MSC) cover large areas over the oceans and possess super sensitivity of their cloud radiative effect to changes in aerosol concentrations. Aerosols can cause transitions between regimes of fully cloudy closed cells and open cells. The possible role of aerosols in cloud cover has a big impact on the amount of reflected solar radiation from the clouds, thus potentially constitutes very large aerosol indirect radiative effect, which can exceed 100 Wm −2 . It is hypothesized that continentally-polluted clouds remain in closed cells regime for longer time from leaving continent and hence for longer distance away from land, thus occupying larger ocean areas with full cloud cover. Attributing this to anthropogenic aerosols would imply a very large negative radiative forcing with a significant climate impact. This possibility is confirmed by analyzing a detailed case study based on geostationary and polar-orbiting satellite observations of the microphysical and dynamical evolution of MSC. We show that large area of closed cells was formed over the northeast Atlantic Ocean downwind of Europe in a continentally polluted air mass. The closed cells undergo cleansing process that was tracked for 3.5 days that resulted with a rapid transition from closed to open cells once the clouds started drizzling heavily. The mechanism leading to the eventual breakup of the clouds due to both meteorological and aerosol considerations are elucidated. We termed this cleansing and cloud breakup process maritimization. Further study is needed to assess the climatological significance of such situations.

Description: Drought properties and the socio-economic impact it makes, are expected to increase in the coming years due to climate change. Here, we review the possible impacts of changes in climate variability on the properties of different drought types. The downscaled and bias-corrected data from five General Circulation Models (GCMs) was used to produce an ensemble of precipitation, temperature, and wind speed, through a relative entropy approach and was used for drought analysis. A novel multivariate drought index known as Multivariate Drought Index (MDI) was then employed for an integrated quantification of all physical forms of drought. We studied the spatial patterns of drought properties, and performed multivariate frequency analysis for each planning region in Texas to recognize the distribution of potential drought hazard areas under climate change impact by formulating a Drought Hazard Index (DHI). A drought vulnerability assessment was also carried out by taking into consideration various socio-economic factors, leading to the development of socio-economic Drought Vulnerability Index (DVI). A set of composite drought risk maps that combines hazard and vulnerability analysis were developed. This study also explored the cause-effect relationship between the drought events and several hydro climatic triggers. A transfer entropy measure was used to quantify the causal relationships, thus indicating the predominant future drought triggers. Overall, the findings are expected to help achieve an effective drought mitigation strategy for the state of Texas.