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Journal Article

Radiative effects of convection in the tropical Pacific


Grassl,  Hartmut
MPI for Meteorology, Max Planck Society;

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Collins, W. D., Valero, F. P. J., Flatau, P. J., Lubin, D., Grassl, H., & Pilewskie, P. (1996). Radiative effects of convection in the tropical Pacific. Journal of Geophysical Research: Atmospheres, 101, 14999-15012. doi:10.1029/95JD02534.

Cite as: https://hdl.handle.net/21.11116/0000-0009-0E73-7
The radiative effects of tropical clouds at the tropopause and the ocean surface have been estimated by using in situ measurements from the Central Equatorial Pacific Experiment (CEPEX). The effect of clouds is distinguished from the radiative effects of the surrounding atmosphere by calculating the shortwave and longwave cloud forcing. These terms give the reduction in insolation and the increase in absorption of terrestrial thermal emission associated with clouds. At the tropopause the shortwave and longwave cloud forcing are nearly equal and opposite, even on daily timescales. Therefore the net effect of an ensemble of convective clouds is small compared to other radiative terms in the surface-tropospheric heat budget. This confirms the statistical cancellation of cloud forcing observed in Earth radiation budget measurements from satellites. At the surface the net effect of clouds is to reduce the radiant energy absorbed by the ocean. Under deep convective clouds the diurnally averaged reduction exceeds 150 W m(-2). The divergence of flux in the cloudy atmosphere can be estimated from the difference in cloud forcing at the surface and tropopause. The CEPEX observations show that the atmospheric cloud forcing is nearly equal and opposite to the surface forcing. Based upon the frequency of convection, the atmospheric forcing approaches 100 W m(-2) when the surface temperature is 303 K. The cloud forcing is closely related to the frequency of convective cloud systems. This relation is used in conjunction with cloud population statistics derived from satellite to calculate the change in surface cloud forcing with sea surface temperature. The net radiative cooling of the surface by clouds increases at a rate of 20 W m(-2)K(-1)during the CEPEX observing period.