George Tselioudis

Global patterns of cloud optical thickness variation with temperature

Abstract The International Satellite Cloud Climatology Project (ISCCP) dataset is used to correlate variations of cloud optical thickness and cloud temperature in todays atmosphere. The analysis focuses on low clouds in order to limit the importance of changes in cloud vertical extent, particle size, and water phase. Coherent patterns of change are observed on several time and space scales. On the planetary scale, clouds in colder, higher latitudes are found to be optically thicker than clouds in warmer, lower latitudes. On the planetary scale, winter clouds are, for the most part, optically thicker than summer clouds. The logarithmic derivative of cloud optical thickness with temperature is used to describe the sign and magnitude of the optical thickness-temperature correlation. The seasonal, latitudinal, and day-to-day variations of this relation are examined for Northern Hemisphere clouds in 1984. The analysis is done separately for clouds over land and ocean. In cold continental clouds, optical thick...

Major Characteristics of Southern Ocean Cloud Regimes and Their Effects on the Energy Budget

Clouds over the Southern Ocean are often poorly represented by climate models, but they make a significant contributiontothetop-of-atmosphere(TOA)radiationbalance,particularlyintheshortwaveportionoftheenergy spectrum. This study seeks to better quantify the organization and structure of Southern Hemisphere midlatitude clouds by combining measurements from active and passive satellite-based datasets. Geostationary and polarorbitersatellitedatafromtheInternationalSatelliteCloudClimatologyProject(ISCCP)areusedtoquantifylargescale,recurring modesofcloudiness,andactiveobservationsfrom CloudSatand Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) are used to examine vertical structure, radiative heating rates, and precipitation associated with these clouds. It is found that cloud systems are organized into eight distinct regimes and that ISCCP overestimates the midlevel cloudiness of these regimes. All regimes contain a relatively high occurrenceoflowcloud,with79%ofallcloudlayersobservedhavingtopsbelow3 km,butmultiple-layered clouds systems are present in approximately 34% of observed cloud profiles. The spatial distribution of regimes varies according to season, with cloud systems being geometrically thicker, on average, during the austral winter. Those regimes found to be most closely associated with midlatitude cyclones produce precipitation the most frequently, although drizzle is extremely common in low-cloud regimes. The regimes associated with cyclones have the highest in-regime shortwave cloud radiative effect at the TOA, but the low-cloud regimes, by virtue of their high frequency of occurrence over the oceans, dominate both TOA and surface shortwave effects in this region as a whole.

The Radiative, Cloud, and Thermodynamic Properties of the Major Tropical Western Pacific Cloud Regimes

This study investigates the radiative, cloud, and thermodynamic characteristics of the atmosphere separated into objectively defined cloud regimes in the tropical western Pacific (TWP). A cluster analysis is applied to 2 yr of daytime-only data from the International Satellite Cloud Climatology Project (ISCCP) to identify four major cloud regimes in the TWP region. A variety of data collected at the Department of Energy’s Atmospheric Radiation Measurement Program (ARM) site on Manus Island is then used to identify the main characteristics of the regimes. Those include surface and top-of-the-atmosphere radiative fluxes and cloud properties derived from a suite of ground-based active remote sensors, as well as the temperature and water vapor distribution measured from radiosondes. The major cloud regimes identified in the TWP area are two suppressed regimes—one dominated by the occurrence of mostly shallow clouds, the other by thin cirrus—as well as two convectively active regimes— one exhibiting a large coverage of optically thin cirrus clouds, the other characterized by a large coverage with optically thick clouds. All four of these TWP cloud regimes are shown to exist with varying frequency of occurrence at the ARM site at Manus. It is further shown that the detailed data available at that site can be used to characterize the radiative, cloud, and thermodynamic properties of each of the regimes, demonstrating the potential of the regime separation to facilitate the extrapolation of observations at one location to larger scales. A variety of other potential applications of the regime separation are discussed.

Cloud and Radiation Variations Associated with Northern Midlatitude Low and High Sea Level Pressure Regimes

Abstract A global meteorological dataset, a global satellite dataset, and a radiative transfer model are combined to map the cloud types in low, near-normal, and high sea level pressure regimes in the northern midlatitudes, and to calculate the radiative balance in those regimes. The prominent cloud feature is a background cloud field that is present most of the time and is modulated by changes in dynamic regime. It consists of a low cloud deck, which becomes optically thicker in the warm seasons over ocean and in the cold seasons over land, and a population of optically thin middle-to-high-top clouds that is mostly middle-top in the cold and mostly high-top in the warm seasons. This background cloud field is modulated by the emergence of a population of optically thick high-top clouds in the low pressure regime and by an increase in the optical thickness of the low clouds in the high pressure regime. The top-of-the-atmosphere (TOA) shortwave flux differences between dynamic regimes show that more sunligh...

Evaporation–Precipitation Variability over the Mediterranean and the Black Seas from Satellite and Reanalysis Estimates

Abstract Satellite retrievals of surface evaporation and precipitation from the Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data (HOAPS-3) dataset are used to document the distribution of evaporation, precipitation, and freshwater flux over the Mediterranean and Black Seas. An analysis is provided of the major scales of temporal and spatial variability of the freshwater budget and the atmospheric processes responsible for the water flux changes. The satellite evaporation fluxes are compared with fields from three different reanalysis datasets [40-yr ECMWF Re-Analysis (ERA-40), ERA-Interim, and NCEP]. The results show a water deficit in the Mediterranean region that averages to about 2.4 mm day−1 but with a significant east–west asymmetry ranging from 3.5 mm day−1 in the eastern part to about 1.1 mm day−1 in the western part of the basin. The zonal asymmetry in the water deficit is driven by evaporation differences that are in turn determined by variability in the air–sea humidity differe...

Increases in tropical rainfall driven by changes in frequency of organized deep convection

Increasing global precipitation has been associated with a warming climate resulting from a strengthening of the hydrological cycle. This increase, however, is not spatially uniform. Observations and models have found that changes in rainfall show patterns characterized as ‘wet-gets-wetter’ and ‘warmer-gets-wetter’. These changes in precipitation are largely located in the tropics and hence are probably associated with convection. However, the underlying physical processes for the observed changes are not entirely clear. Here we show from observations that most of the regional increase in tropical precipitation is associated with changes in the frequency of organized deep convection. By assessing the contributions of various convective regimes to precipitation, we find that the spatial patterns of change in the frequency of organized deep convection are strongly correlated with observed change in rainfall, both positive and negative (correlation of 0.69), and can explain most of the patterns of increase in rainfall. In contrast, changes in less organized forms of deep convection or changes in precipitation within organized deep convection contribute less to changes in precipitation. Our results identify organized deep convection as the link between changes in rainfall and in the dynamics of the tropical atmosphere, thus providing a framework for obtaining a better understanding of changes in rainfall. Given the lack of a distinction between the different degrees of organization of convection in climate models, our results highlight an area of priority for future climate model development in order to achieve accurate rainfall projections in a warming climate.

Global Weather States and Their Properties from Passive and Active Satellite Cloud Retrievals

AbstractIn this study, the authors apply a clustering algorithm to International Satellite Cloud Climatology Project (ISCCP) cloud optical thickness–cloud top pressure histograms in order to derive weather states (WSs) for the global domain. The cloud property distribution within each WS is examined and the geographical variability of each WS is mapped. Once the global WSs are derived, a combination of CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) vertical cloud structure retrievals is used to derive the vertical distribution of the cloud field within each WS. Finally, the dynamic environment and the radiative signature of the WSs are derived and their variability is examined. The cluster analysis produces a comprehensive description of global atmospheric conditions through the derivation of 11 WSs, each representing a distinct cloud structure characterized by the horizontal distribution of cloud optical depth and cloud top pressure. Matching those distinct WSs ...

Clouds, Aerosol, and Precipitation in the Marine Boundary Layer: An ARM Mobile Facility Deployment

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Simulations of a Cold Front by Cloud-Resolving, Limited-Area, and Large-Scale Models, and a Model Evaluation Using In Situ and Satellite Observations

Abstract The Global Energy and Water Cycle Experiment has identified the poor representation of clouds in atmospheric general circulation models as one of the major impediments for the use of these models in reliably predicting future climate change. One of the most commonly encountered types of cloud system in midlatitudes is that associated with cyclones. The purpose of this study is to investigate the representation of frontal cloud systems in a hierarchy of models in order to identify their relative weaknesses. The hierarchy of models was classified according to the horizontal resolution: cloud-resolving models (5-km resolution), limited-area models (20-km resolution), coarse-grid single-column models (300 km), and an atmospheric general circulation model (>100 km). The models were evaluated using both in situ and satellite data. The study shows, as expected, that the higher-resolution models give a more complete description of the front and capture many of the observed nonlinear features of the front...

Temperature Dependence of Low Cloud Optical Thickness in the GISS GCM: Contributing Mechanisms and Climate Implications

Abstract A current-climate simulation of the Goddard Institute for Space Studies (GISS) GCM, which includes interactive cloud optical properties that depend on the predicted cloud water content, is analyzed to document the variations of low cloud optical thickness with temperature in the model atmosphere. It is found that low cloud optical thickness decreases with temperature in the warm subtropical and tropical latitudes and increases with temperature in the cold midlatitude regions. This behavior is in agreement with the results of two observational studies that analyzed satellite data from the International Satellite Cloud Climatology Project and Special Sensor Microwave/Imager datasets. The increase of low cloud optical thickness with temperature in the midlatitudes is due to vertical extent and cloud water increases, whereas the decrease with temperature in the warm latitudes is due to decreases in cloud water content and happens despite increases in cloud vertical extent. The cloud processes that pr...