Details

Autor(en) / Beteiligte

• Sherwood, Steven C.
• Ramanathan, V.
• Barnett, Tim P.
• Tyree, Mary K.
• Roeckner, Erich

Titel

Response of an atmospheric general circulation model to radiative forcing of tropical clouds

Ist Teil von

• Journal of Geophysical Research - Atmospheres, 1994-10, Vol.99 (D10), p.20829-20845

Ort / Verlag

Washington, DC: American Geophysical Union

Erscheinungsjahr

1994

Link zum Volltext

Beschreibungen/Notizen

• The effects of upper tropospheric cloud radiative forcing (CRF) on the atmosphere have been examined using a recent version of the atmospheric general circulation model (AGCM) developed by the Max Planck Institute of Meteorology and the University of Hamburg. This model reproduces satellite‐observed radiative forcing of clouds well overall, except that model maxima somewhat exceed those of observations. Three simulations have been performed where the clouds above 600 mbar have been rendered transparent to all radiation: first, throughout the tropics in the “NC” experiment; then only over oceans warmer than 25°C in the “NCW” experiment; and finally, only over the western Pacific warm oceans in the “NCWP” experiment. The local radiative effects of these clouds when they are present in the model are radiative heating of the middle to upper troposphere due to convergence of longwave and solar radiation; radiative cooling of the tropical atmosphere near and above the tropopause; a large reduction of solar radiation (50 to 100 W/m2) reaching the surface; and a slight increase (5 to 20 W/m2) in the downward longwave radiation at the surface. The removal of cloud radiative forcing significantly alters the circulation of the model atmosphere, as in previous AGCM studies, showing that a seemingly moderate heat source such as CRF is nonetheless capable of widespread influence over the global circulation and precipitation. The experiment responses include a significant weakening (in NCW) or rearrangement (in NCWP) of the Walker circulation. Zonal mean cloud cover, rainfall, and low‐level convergence change modestly in the experiments, while zonal departures of these from their tropical means shift considerably. Regions over the warmest oceans which lose CRF become much less cloudy, indicating a positive local feedback to convection. The experiment circulation changes are diagnosed in terms of simple energy budget arguments, which suggest that the importance of CRF is enabled by the small magnitude of the atmospheric moist energy transport in the tropics. They also suggest that the response of the zonal mean atmosphere may be strongly dependent on the response of zonal eddies and on interactions between surface fluxes and tropospheric lapse rates. The response of the zonal eddies itself should be relatively independent of these interactions.