Tropical upper tropospheric clouds: variation with sea surface temperature and radiative effects

Preprint requests:Hui.Su@jpl.nasa.gov
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Abstract. The variations of tropical upper tropospheric clouds (UTC) 21 with sea surface temperature (SST) are analyzed in terms of cloud fraction (CFR) from the Atmospheric Infrared Sounder (AIRS) on Aqua and ice water content (IWC) from the Microwave Limb Sounder (MLS) on Aura. The SST data are from the Advanced Microwave Scanning Radiometer (AMSR-E) on Aqua. It is found that the daily mean CFR 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 effect of the observed UTC is to warm the Earth30 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 UTC. The net UTC radiative effect (CRE) varies approximately monotonically with CFR, but non33 monotonically with IWP. The increase of IWP with SST would yield an increase of net warming of about 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 CRE, although the changes in LW and SW CRE individually are substantial, about 3.2 W m-2 averaged over the tropics.

Citation. Su, H., J. H. Jiang, Y. Gu, J. David Neelin, B. H. Kahn, D. Feldman, Y. L. Yung, J. W. Waters, N. J. Livesey and M. L. Santee: Tropical upper tropospheric clouds: variation with sea surface temperature and radiative effects J. Atmos. Sci., accepted.