HR: 0800h
AN: A11C-0078    [Abstracts]
TI: AVHRR Observations of the Aerosol Indirect Effect for Summertime Stratiform Clouds in the Northeastern Atlantic
AU: * Matheson, M A
EM: mmatheso@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, COAS Admin 104, Corvallis, OR 97331-5503 United States
AU: Coakley, J A
EM: coakley@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, COAS Admin 104, Corvallis, OR 97331-5503 United States
AU: Tahnk, W R
EM: tahnk@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, COAS Admin 104, 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, 1994-2001. A 3-channel retrieval scheme is used to derive the cloud properties: visible optical depth, droplet effective radius, cloud-top height, and pixel-scale fractional cloud cover. A 2-channel aerosol retrieval scheme is used to determine aerosol optical depth in cloud-free pixels. Aerosol optical depths in one-degree latitude-longitude regions on a given day are compared with the cloud properties in the same and adjacent one-degree regions on the same day. Results are composited for small (e.g. 5 x 5 degree) regions to study the influence of geographically controlled trends. For all years in most regions, there is a statistically significant decrease in droplet effective radius and an increase in cloud visible optical depth as aerosol optical depth increases. Depending on the region, cloud liquid water path can increase, remain constant, or decrease as aerosol optical depth increases. The change in cloud liquid water is probably a function of the relative humidity of the free troposphere above the cloud. Preliminary radiative transfer calculations indicate that the aerosol indirect radiative forcing is about 70 percent larger than the aerosol direct radiative forcing. On the other hand, the indirect forcing calculated from measured cloud optical depths is smaller than that calculated using measured droplet effective radii and assuming constant liquid water. Retrieved aerosol optical depth increases as the percentage of cloudy pixels in a region increases. The increase in aerosol optical depth could result from the aerosol particles swelling in high humidity environments, from undetected sub-pixel scale clouds residing in the pixels used for the aerosol retrievals, or an enhancement in the illumination of the aerosols due to radiation escaping from the sides of nearby clouds. Cloud optical depth also increases as the percentage of cloudy pixels in a region increases. The co-occurrence of increases in aerosol and cloud optical depths with increasing cloud fraction could be misinterpreted as evidence of the aerosol indirect radiative forcing when, in fact, processes unrelated to the indirect forcing may be governing changes in aerosol and cloud optical depths.
DE: 1600 GLOBAL CHANGE (New category)
DE: 1640 Remote sensing
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
DE: 0345 Pollution--urban and regional (0305)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting