HR: 1340h
AN: A33C-0926    [Abstracts]
TI: Effects of Threshold Cloud Retrievals on Estimates of the Aerosol Indirect Radiative Forcing
AU: Matheson, M A
EM: mmatheso@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, 104 COAS Admin Building Oregon State University, Corvallis, OR 97331-5503 United States
AU: * Coakley, J A
EM: coakley@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, 104 COAS Admin Building Oregon State University, Corvallis, OR 97331-5503 United States
AU: Tahnk, W R
EM: tahnk@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, 104 COAS Admin Building Oregon State University, Corvallis, OR 97331-5503 United States
AB: Advanced Very High Resolution Radiometer (AVHRR) 4-km imager pixels are collected over the northeastern Atlantic off the coast of the Iberian Peninsula for May-August, 1995-1999. An aerosol retrieval scheme is used to determine aerosol optical depth in cloud-free pixels. A retrieval scheme that accounts for partly cloudy pixels is used to derive cloud visible optical depth, cloud droplet effective radius, and pixel-scale fractional cloud cover. Many retrieval algorithms classify pixels that are only partly covered by clouds as being overcast if the radiances in that pixel exceed a prescribed threshold. In order to simulate these threshold retrieval schemes, radiances in pixels that are identified as having a fractional cloud cover greater than 0.20 are used to recalculate cloud properties assuming the pixel is overcast. The threshold retrieval scheme retrieves cloud optical depths that are smaller and droplet effective radii that are larger than those retrieved by the partly cloudy pixel retrieval scheme. Aerosol optical depths in 1° latitude-longitude regions on a given day are compared with the cloud properties in the same 1° region on the same day. Results are composited for 5° latitude-longitude regions to increase the likelihood that observed changes in cloud properties are caused by the observed changes in aerosol burdens. In most of the 5° regions, there is a statistically significant decrease in droplet effective radius and increase in cloud visible optical depth as aerosol optical depth increases. The decrease in droplet radii and increase in cloud optical depths for a given change in aerosol optical depth are larger using the threshold retrieval scheme than when using the partly cloudy pixel retrieval scheme. These differences indicate that using a threshold retrieval scheme may lead to an overestimation of the magnitude of aerosol indirect radiative forcing. Cloud liquid water, which is proportional to the product of cloud optical depth and droplet effective radius, remains constant or decreases as aerosol optical depth increases. The average decrease in cloud liquid water is greater using the partly cloudy pixel retrieval than using the threshold retrieval. Processes that counter the gain of liquid water predicted to occur due to drizzle suppression might be more important than threshold studies report.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0319 Cloud optics
DE: 3311 Clouds and aerosols
DE: 3360 Remote sensing
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005