Eric M. Wilcox

The Frequency of Extreme Rain Events in Satellite Rain-Rate Estimates and an Atmospheric General Circulation Model

Abstract The frequency distributions of surface rain rate are evaluated in the Tropical Rainfall Measuring Mission (TRMM) and Special Sensor Microwave/Imager (SSM/I) satellite observations and the NOAA/GFDL global atmosphere model version 2 (AM2). Instantaneous satellite rain-rate observations averaged over the 2.5° latitude × 2° longitude model grid are shown to be representative of the half-hour rain rate from single time steps simulated by the model. Rain-rate events exceeding 10 mm h−1 are observed by satellites in most regions, with 1 mm h−1 events occurring more than two orders of magnitude more frequently than 10 mm h−1 events. A model simulation using the relaxed Arakawa–Schubert (RAS) formulation of cumulus convection exhibits a strong bias toward many more light rain events compared to the observations and far too few heavy rain events. A simulation using an alternative convection scheme, which includes an explicit representation of mesoscale circulations and an alternative formulation of the cl...

Estimate of the impact of absorbing aerosol over cloud on the MODIS retrievals of cloud optical thickness and effective radius using two independent retrievals of liquid water path

Two independent satellite retrievals of cloud liquid water path (LWP) from the NASA Aqua satellite are used to diagnose the impact of absorbing biomass burning aerosol overlaying boundary-layer marine water clouds on the Moderate Resolution Imaging Spectrometer (MODIS) retrievals of cloud optical thickness (tau) and cloud droplet effective radius (r(sub e)). In the MODIS retrieval over oceans, cloud reflectance in the 0.86-micrometer and 2.13-micrometer bands is used to simultaneously retrieve tau and r(sub e). A low bias in the MODIS tau retrieval may result from reductions in the 0.86-micrometer reflectance, which is only very weakly absorbed by clouds, owing to absorption by aerosols in cases where biomass burning aerosols occur above water clouds. MODIS LWP, derived from the product of the retrieved tau and r(sub e), is compared with LWP ocean retrievals from the Advanced Microwave Scanning Radiometer-EOS (AMSR-E), determined from cloud microwave emission that is transparent to aerosols. For the coastal Atlantic southern African region investigated in this study, a systematic difference between AMSR-E and MODIS LWP retrievals is found for stratocumulus clouds over three biomass burning months in 2005 and 2006 that is consistent with above-cloud absorbing aerosols. Biomass burning aerosol is detected using the ultraviolet aerosol index from the Ozone Monitoring Instrument (OMI) on the Aura satellite. The LWP difference (AMSR-E minus MODIS) increases both with increasing tau and increasing OMI aerosol index. During the biomass burning season the mean LWP difference is 14 g per square meters, which is within the 15-20 g per square meter range of estimated uncertainties in instantaneous LWP retrievals. For samples with only low amounts of overlaying smoke (OMI AI less than or equal to 1) the difference is 9.4, suggesting that the impact of smoke aerosols on the mean MODIS LWP is 5.6 g per square meter. Only for scenes with OMI aerosol index greater than 2 does the average LWP difference and the estimated bias in MODIS cloud optical thickness attributable to the impact of overlaying biomass burning aerosol exceed the instantaneous uncertainty in the retrievals.

Scale Dependence of the Thermodynamic Forcing of Tropical Monsoon Clouds: Results from TRMM Observations

Clouds exert a thermodynamic forcing on the ocean‐atmosphere column through latent heating, owing to the production of rain, and through cloud radiative forcing, owing to the absorption of terrestrial infrared energy and the reflection of solar energy. The Tropical Rainfall Measuring Mission (TRMM) satellite provides, for the first time, simultaneous measurements of each of these processes on the spatial scales of individual clouds. Data from TRMM are used to examine the scale dependence of the cloud thermodynamic forcing and to understand the dominant spatial scales of forcing in monsoonal cloud systems. The tropical Indian Ocean is chosen, because the major monsoonal cloud systems are located over this region. Using threshold criteria, the satellite data are segmented into rain cells (consisting of only precipitating pixels) and clouds (consisting of precipitating as well as nonprecipitating pixels), ranging in scales from 10 3 km2 to 10 6 km2. For each rain cell and cloud, latent heating is estimated from the microwave imager and radiative forcing is estimated from the Cloud and the Earth’s Radiant Energy System radiation budget instrument. The sizes of clouds and rain cells over the tropical Indian Ocean are distributed lognormally. Thermodynamic forcing of clouds increases with rain cell and cloud area. For example, latent heating increases from about 100 Wm 22 for a rain cell of 10 3 km2 to as high as 1500 W m22 for a rain cell of 10 6 km2. Correspondingly, the liquid water path increases tenfold from 0.3 to nearly 3 kg m22, the longwave cloud forcing from 30 to 100 W m22, and the diurnal mean shortwave cloud forcing from 250 to 2 150 Wm 22. Previous studies have shown that in regions of deep convection, large clouds and rain cells express greater organization into structures composed of convective core regions attached to stratiform anvil cloud and precipitation. Entrainment of moist, cloudy air from the stratiform anvil into the convective core helps to sustain convection against the entrainment of unsaturated air. Thus large clouds produce more rain, trap more terrestrial radiation, and reflect more solar energy than do smaller clouds. The combined effect of increased forcing and increased spatial coverage means that larger clouds contribute most of the total forcing. Rain cells larger than 10 5 km2 make up less than 2% of the rain cell population, yet contribute greater than 70% of the latent heating. Similarly, the clouds larger than 105 km2, in which the largest rain cells are embedded, make up less than 3% of clouds, yet are the source of greater than 90% of the total thermodynamic forcing. Significant differences are apparent between the scales of latent heating and radiative forcing, as only about 25% of cloud area is observed to precipitate. The fraction of clouds that contain some rain increases dramatically from about 5% for the smaller scale (10 3 km2) to as high as 90% for the largest scale considered here (10 6 km2). The fractional area of the precipitating cloud ranges from 0.2 to 0.4 with a hybrid-scale dependence. Greater than one-half of radiative forcing is provided by nonprecipitating anvil portions of deep convective cloud systems. The results presented here have significant implications for the parameterization of clouds and rain in GCMs and washout of solute trace gases and aerosols in chemistry and transport models.

Strong radiative heating due to wintertime black carbon aerosols in the Brahmaputra River Valley

values of BC mass concentration were 9–41 mgm � 3 , with maxima over 50 mgm � 3 during evenings and early mornings. Median BC concentrations were higher than in mega cities of India and China, and significantly higher than in urban locations of Europe and USA. The corresponding mean cloud-free aerosol radiative forcing is � 63.4 Wm � 2 at the surface and +11.1 Wm � 2 at the top of the atmosphere with the difference giving the net atmospheric BC solar absorption, which translates to a lower atmospheric heating rate of � 2 K/d. Potential regional climatic impacts associated with large surface cooling and high lower-atmospheric heating are discussed. Citation: Chakrabarty, R. K., M. A. Garro, E. M. Wilcox, and H. Moosmuller (2012), Strong radiative heating due to wintertime black carbon aerosols in the Brahmaputra River Valley, Geophys. Res. Lett., 39, L09804, doi:10.1029/2012GL051148.

Black carbon solar absorption suppresses turbulence in the atmospheric boundary layer

Significance The cooling effect of aerosols on climate and the modification of clouds by aerosols have been widely debated, because quantifying their effects is important for constraining current climate change. Here we present measurements of turbulence from unmanned aerial vehicles. We find that absorption of sunlight by black carbon (BC) aerosols suppresses turbulence in the lower atmosphere, with important consequences for the environmental impacts of BC emissions from anthropogenic fossil fuel and biomass burning. A mechanism is proposed that links the suppressed turbulence to taller clouds. These results highlight the importance of understanding and observing the role of turbulence in studies of aerosol impacts on clouds. Suppressed turbulence also exacerbates the visibility and human health impacts of pollution. The introduction of cloud condensation nuclei and radiative heating by sunlight-absorbing aerosols can modify the thickness and coverage of low clouds, yielding significant radiative forcing of climate. The magnitude and sign of changes in cloud coverage and depth in response to changing aerosols are impacted by turbulent dynamics of the cloudy atmosphere, but integrated measurements of aerosol solar absorption and turbulent fluxes have not been reported thus far. Here we report such integrated measurements made from unmanned aerial vehicles (UAVs) during the CARDEX (Cloud Aerosol Radiative Forcing and Dynamics Experiment) investigation conducted over the northern Indian Ocean. The UAV and surface data reveal a reduction in turbulent kinetic energy in the surface mixed layer at the base of the atmosphere concurrent with an increase in absorbing black carbon aerosols. Polluted conditions coincide with a warmer and shallower surface mixed layer because of aerosol radiative heating and reduced turbulence. The polluted surface mixed layer was also observed to be more humid with higher relative humidity. Greater humidity enhances cloud development, as evidenced by polluted clouds that penetrate higher above the top of the surface mixed layer. Reduced entrainment of dry air into the surface layer from above the inversion capping the surface mixed layer, due to weaker turbulence, may contribute to higher relative humidity in the surface layer during polluted conditions. Measurements of turbulence are important for studies of aerosol effects on clouds. Moreover, reduced turbulence can exacerbate both the human health impacts of high concentrations of fine particles and conditions favorable for low-visibility fog events.

Biomass Burning, Land-Cover Change, and the Hydrological Cycle in Northern Sub-Saharan Africa

The Northern Sub-Saharan African (NSSA) region, which accounts for 20%-25%of the global carbon emissions from biomass burning, also suffers from frequent drought episodes and other disruptions to the hydrological cycle whose adverse societal impacts have been widely reported during the last several decades. This paper presents a conceptual framework of the NSSA regional climate system components that may be linked to biomass burning, as well as detailed analyses of a variety of satellite data for 2001-2014 in conjunction with relevant model-assimilated variables. Satellite fire detections in NSSA show that the vast majority (greater than 75%) occurs in the savanna and woody savanna land-cover types. Starting in the 2006-2007 burning season through the end of the analyzed data in 2014, peak burning activity showed a net decrease of 2-7% /yr in different parts of NSSA, especially in the savanna regions. However, fire distribution shows appreciable coincidence with land-cover change. Although there is variable mutual exchange of different land cover types, during 2003-2013, cropland increased at an estimated rate of 0.28% /yr of the total NSSA land area, with most of it (0.18% /yr) coming from savanna.During the last decade, conversion to croplands increased in some areas classified as forests and wetlands, posing a threat to these vital and vulnerable ecosystems. Seasonal peak burning is anti-correlated with annual water-cycle indicators such as precipitation, soil moisture, vegetation greenness, and evapotranspiration, except in humid West Africa (5 deg-10 deg latitude),where this anti-correlation occurs exclusively in the dry season and burning virtually stops when monthly mean precipitation reaches 4 mm/d. These results provide observational evidence of changes in land-cover and hydrological variables that are consistent with feedbacks from biomass burning in NSSA, and encourage more synergistic modeling and observational studies that can elaborate this feedback mechanism.

Spatial and Temporal Scales of Precipitating Tropical Cloud Systems in Satellite Imagery and the NCAR CCM3

Abstract Testing general circulation model parameterizations against observations is traditionally done by comparing simulated and observed, time-averaged quantities, such as monthly mean cloud cover, evaluated on a stationary grid. This approach ignores the dynamical aspects of clouds, such as their life cycle characteristics, spatial coverage, temporal duration, and internal variability. In this study, a complementary Lagrangian approach to the validation of modeled tropical cloudiness is explored. An automated cloud detection and tracking algorithm is used to observe and track overcast decks of cloud in a consecutive set of hourly Meteosat-5 images and the National Center for Atmospheric Research Community Climate Model version 3 (NCAR CCM3). The algorithm is applied to the deep convective cloud systems of the tropical Indian Ocean region during a 49-day period of the 1999 winter monsoon season. Observations of precipitation are taken from the Tropical Rainfall Measuring Mission (TRMM) satellite in add...

Aerosol interactions with African/Atlantic climate dynamics

Mechanistic relationships exist between variability of dust in the oceanic Saharan air layer (OSAL) and transient changes in the dynamics of Western Africa and the tropical Atlantic Ocean. This study provides evidence of possible interactions between dust in the OSAL region and African easterly jet–African easterly wave (AEJ–AEW) system in the climatology of boreal summer, when easterly wave activity peaks. Synoptic-scale changes in instability and precipitation in the African/Atlantic intertropical convergence zone are correlated with enhanced aerosol optical depth (AOD) in the OSAL region in response to anomalous 3D overturning circulations and upstream/downstream thermal anomalies at above and below the mean-AEJ level. Upstream and downstream anomalies are referred to the daily thermal/dynamical changes over the West African monsoon region and the Eastern Atlantic Ocean, respectively. Our hypothesis is that AOD in the OSAL is positively correlated with the downstream AEWs and negatively correlated with the upstream waves from climatological perspective. The similarity between the 3D pattern of thermal/dynamical anomalies correlated with dust outbreaks and those of AEWs provides a mechanism for dust radiative heating in the atmosphere to reinforce AEW activity. We proposed that the interactions of OSAL dust with regional climate mainly occur through coupling of dust with the AEWs.

Multi-spectral Remote Sensing of Sea Fog with Simultaneous Passive Infrared and Microwave Sensors

Mitigating the impacts of sea fog can be facilitated with accurate detection of sea fog events in satellite observations. While low stratus clouds and fog layers are apparent in both visible and infrared passive imager observations, some challenges remain in positively identifying cases of fog. Low liquid water clouds typically appear bright in visible imagery, but are difficult to distinguish from high clouds based only on visible reflectance. Emission from cloud tops in common infrared imagery is increasingly difficult to distinguish from surface emission as the cloud top altitude decreases. Prior work suggests that knowledge of the surface temperature can increase the reliability of satellite fog detection based on visible and infrared satellite observations. However, that work relied on output from a complex weather model. Here we exploit the co-location of sea surface temperature retrievals from passive microwave imagery with visible and infrared brightness temperature observations on the NASA Aqua polar-orbiting satellite to test a method for sea fog detection that does not rely on ancillary data from weather models or surface station data. The method exploits sea surface temperature retrievals based on microwave emission even in the presence of non-precipitating clouds. Two examples of sea fog are explored that have been verified as true cases of fog in the literature. They are found to have substantial areas of cloud where the 11 μm emission temperature of the cloud is very close to the sea surface temperature value. This differs substantially from an example of subtropical stratus and stratocumulus cloud, which has been similarly explored and found to have cloud tops substantially colder relative to the sea surface compared to the fog cases. This result suggests that the multispectral approach including microwave sea surface temperature retrievals may be able to discriminate between some types of fog and other types of low water clouds that are not fog. Application of the method is currently limited by the poor spatio-temporal sampling of polar orbiting satellites and a lack of data in the coastal zone. Validation of the method will require an independent identification of open-ocean fog cases as ground-truth. Combining the microwave sea surface temperature retrievals with geostationary visible and infrared observations may be able to overcome the sampling bias of polar-orbiting satellites, which limit views of any location to only once or twice per day. Greater than 10 years of microwave sea surface temperature retrievals allow for the possibility of constructing a global climatology of sea fog if the distinguishing features of sea fog found in this study can be validated with many more cases.