10.4
A Simple Moist Model of the Hadley and Walker Circulation: Role of Cloud Radiative Forcing
Baijun Tian, SIO/Univ. of California, La Jolla, CA; and V. Ramanathan
Similar to Davey and Gill (1987), a simple moist model of the tropical Hadley and Walker circulations is constructed based on Gill's (1980) dry dynamics supplemented by a moisture equation. The model describes the first baroclinic mode of the troposphere in response to the thermodynamic forcing from the surface. We have included a realistic prescription of the moist static stability, a Newtonian cooling term that mimics more closely the clear sky radiative cooling and a prescription of the role of clouds in the atmospheric radiative cooling, the so-called atmospheric cloud radiative forcing (CRF). The CRF term was obtained from satellite observations of the radiation budget and radiative transfer models. Notwithstanding its simplicity, the model can simulate the fundamental climatological features of the Hadley and Walker circulations, such as the trades, the ascending branches in the ITCZ and the SPCZ and the descending branches in the cold tongue and subtropics. A major new insight from the present model is that the atmospheric CRF plays an important role in driving the tropical circulation. We will discuss the implications of this finding to our current understanding of the importance of the various thermodynamic forcing terms to the tropical circulation. The present model also offers a viable theoretical framework for numerous GCM studies, which have shown the Hadley and Walker circulations to be highly sensitive to the atmospheric CRF.
Session 10, Climate Model Diagnostics: Clouds and Radiation, Part II
Wednesday, 16 January 2002, 1:30 PM-3:00 PM
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