Date: Tuesday, August 14, 2007
Time: 10:30am
Location: NIA, Room 137
Additional Information: Presentation (.pdf)

Variation of Tropical Upper Tropospheric Clouds with
Sea Surface Temperature and Associated Radiative Effects
Hui Su, Jet Propulsion Laboratory

It has been intensely debated whether tropical cirrus clouds increase or decrease with
sea surface temperature (SST) and whether they provide a positive or negative climate
feedback. We address these questions using new cloud observations from NASA A-train
satellites and other related satellite measurements. In particular, cloud fraction and cloud
top pressure from the Atmospheric Infrared Sounder (AIRS) on Aqua and upper
tropospheric (UT) ice water content (IWC) from the Microwave Limb Sounder (MLS) on
Aura are analyzed, together with SST from the Advanced Microwave Scanning Radiometer (AMSR-E) on Aqua and precipitation from Tropical Rainfall Measuring Mission (TRMM). It is found that the daily mean UT cloud fraction averaged over tropical cloudy areas is nearly invariant with the mean under-cloud SST, while the daily mean IWC and ice water path (IWP, integrated IWC above 215 hPa) over tropical cloudy areas increase with the mean under-cloud SST at a rate about 20% per degree K, faster than the increase of daily mean cloudy-area precipitation with SST. The net radiative forcing of the observed UT clouds is to warm the Earth-atmosphere system. The amplitude of the net warming is about 7-17 W m-2 in the tropical average, with uncertainty largely arising from the estimate of fractional coverage of UT clouds. The increase of IWP with SST would yield an increase of net warming of ~0.1 W m-2 K-1, corresponding to a positive feedback, until the net warming reaches a maximum when IWP is increased by 50%. Doubling of IWP yields almost no change in the net UT cloud forcing, although the changes in longwave and shortwave cloud forcing individually are substantial, about 3.2 W m-2 averaged over the tropics. The associated upper tropospheric water vapor and temperature changes with the cloud changes will also be discussed.