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We examine the importance of pressure gradients due to surface temperature gradients to low-level (p ≥ 700 mb) flow and convergence in the tropics over time scales ≳ 1 month. The latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall. Our approach is to consider a simple one-layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically—consistent with ECMWF analyzed data. The top of the layer is taken at 700 mb. The influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer. Horizontal convergence within the layer is taken up by cumulonimbus mass flux. However, the development of the cumulonimbus mass flux is associated with a short relaxation time O(½ hr) (roughly the development time for such convection). During this short time, horizontal convergence acts to redistribute mass so as to reduce horizontal pressure gradients. This effect proves important in the immediate neighborhood of the equator. Our results show that flows forced directly by surface temperature are often comparable to observed low-level flows in both magnitude and distribution.
Lindzen et al. (Tue,) studied this question.