The relationship between surface temperature, low-level convergence and precipitation (latent heating) in the tropics of a general circulation model of the atmosphere

The relative roles of precipitation (latent heating) and surface temperature forcing (sensible heat flux) in forcing the time mean low level convergence in the tropics and subtropics of a general circulation model (GCM) of the atmosphere are explored with a steady state model that is a linearization of the GCM about a zonally symmetric basic state. The zonal and meridional winds are also examined. A hierarchy of experiments in which the boundary conditions used in the GCM are successively simplified is used to isolate the signal due to the two different forcings. The GCM experiments include an experiment conducted with realistic continental distribution and sea surface temperature. A parallel experiment is conducted in which the realistic continental distribution and sea surface temperature are kept but the mountains are removed. An idealized land area experiment with a homogeneous vegetation cover is also conducted in an attempt to isolate the signal seen in the more realistic geometry experiments. The transient variability of the relationship between divergence, surface temperature and soil moisture is examined for the idealized land area.;Over the ocean the near surface convergence field is the result of constructive interference between the convergence forced by surface temperature forcing and latent heating. The zonal wind response is however dominated by the response to latent heating. Higher in the boundary layer both wind fields as well as the convergence field are dominated by the response to latent heating. Over the land the convergence field in the boundary layer is the result of destructive interference between the convergence field forced by the surface temperature forcing and the convergence field forced by the latent heating. The near surface response is dominated by the surface temperature forcing but higher in the boundary layer the latent heating response dominates.;The response to idealized heating profiles is explored in the linear model. The low level response is highly dependant on the height of the base of the heating. If the heating is placed at a low enough level the low level convergence response in the heating region is warm core and decreases in magnitude upward or equivalently increases in strength toward the surface. As the base of the heating is displaced upward the low level convergence response changes to cold core and increases with height up from the surface. If the base of the heating is displaced to a high enough level the cold core nature of the response below the base of the heating leads to a negligible response in the convergence field as the surface is approached.