Response of Deep Convective Clouds to Increases in ... - ceres

Deep convective clouds frequently occur with heavyaerosol burdens off the west coast of Africa during theseason of biomass burning.CALIPSO 532-nm attenuated backscatter, 13 August 2009 1430 UTC

Three years of CERES SSF from Terra used toobtain the properties of deep convective cloudscollocated with large aerosol burdens.CERES: Clouds and Earth’s Radiant Energy SystemSSF: Single Satellite Footprint• The SSF combines retrievals of aerosol and cloudproperties based on MODIS radiances with radiativefluxes derived from the CERES broadband radiometers.• Each CERES footprint is ~20 km resolution at the Earth’ssurface.

Deep convective clouds frequent the Pacific Warm Pool, theNorth Indian Ocean, the west coast of Africa, and SouthAmerica.Deep convectivesystems were soughtbetween 60° S to 60° N.Sherwood (2002) andJiang et al. (2008)reported finding effectsof aerosols on deepconvective clouds overSouth America.

Deep convective clouds and heavy aerosol burdensoccur primarily in the Northern Indian Ocean, Africa,and South America.

Properties of deep convective clouds collocated with largeaerosol burdens are compared with those collocated with smallaerosol burdens.SMALLLARGEFollowing Loeb andSchuster (2008), for eachday obtain meanproperties of deepconvective clouds andmean aerosol burdens ineach 2°× 2° lat.-lon. region.SMALLLARGEDivide the 2°× 2° regionswithin a 10°× 10° regioninto those with meanaerosol optical depthsgreater than the mean forthe 10° region and thosewith optical depths lessthan the mean.Correlate differences incloud properties withdifferences in aerosoloptical depths.

Deep convective clouds show nochanges with increased aerosol burdens.Difference = Large − Small

Ice crystal diameterand cloud opticaldepth increase withdecreasing cloudtemperature.

Deep convective cloudsexhibit relationships amongtheir properties.The least-squares fits were calculatedusing statistically independent FOVs.The means and standard errors wereobtained for equal populations (10 thpercentiles) of the cloud optical depthand 11-μm brightness temperature.The coldest bin consistently had thelargest optical depths and the smallestice crystal diameters in all regions andall seasons.The largest optical depth binsconsistently showed a decrease in icecrystal diameter with increasing cloudoptical depth.

Deep convective cloudsexhibit relationships amongtheir properties.The least-squares fits were calculatedusing statistically independent FOVs.The means and standard errors wereobtained for equal populations (10 thpercentiles) of the cloud optical depthand 11-μm brightness temperature.The coldest bin consistently had thelargest optical depths and the smallestice crystal diameters in all regions andall seasons.The largest optical depth binsconsistently showed a decrease in icecrystal diameter with increasing cloudoptical depth.

Monthly mean ice particle diametersappear to decrease with increasingmonthly mean aerosol burdens.The changes in ice particle diametersare also consistent with the lowertemperatures observed during theseason with increased aerosol loading.Sherwood (2002) also found lowercloud top temperatures linked tosmaller ice crystal sizes.Perhaps previous studies claimingaerosol effects were observing theincrease in small ice crystals withdecreasing temperatures.

Monthly mean ice particle diametersappear to decrease with increasingmonthly mean aerosol burdens.The changes in ice particle diametersare also consistent with the lowertemperatures observed during theseason with increased aerosol loading.Sherwood (2002) also found lowercloud top temperatures linked tosmaller ice crystal sizes.Perhaps previous studies claimingaerosol effects were observing theincrease in small ice crystals withdecreasing temperatures.

SummaryDeep convective clouds appear to be unaffected by collocated elevated burdens ofaerosols in regions where deep convective clouds and large aerosol burdens occurfrequently: South America and western Africa during biomass burning and theNorthern Indian Ocean during the winter and summer monsoons.Ice crystal diameter and cloud optical depth appear to increase with decreasingcloud temperature presumably due to large numbers of supercooled cloud dropletsbeing lofted above the freezing level by strong updrafts in deep convective clouds(e.g. Garrett et al. 2003 and Garrett et al. 2007, and Heymsfield et al. 2009).Previous studies claiming evidence for the effect of aerosols on clouds (Sherwood2002; Jiang et al. 2008, and Jiang et al. 2009) focused on South America whereclouds have lower cloud top temperatures and smaller ice crystals during theseason of biomass burning.Perhaps, the effect of aerosols is to suppress warm cloud precipitation therebyfacilitating deeper convection forming clouds with lower temperatures andsmaller ice crystals (Andrae et al. 2004).