Rayleigh friction, newtonian cooling, and the linear response to steady tropical heating

A series of studies are performed to examine the response of the tropical a tmosphere to a prescribed steady, large-scale, elevated heat source (i.e,, a region of persistent precipitation). Special emphasis ii placed on the su rface wind response in two idealized cases in which dissipation is achieved exclusively by Rayleigh friction or by Newtonian cooling. Starting from th e linearized equations on an equatorial beta plane, theoretical arguments a re presented that suggest there are qualitative differences in the solution s of these two models. A dry spectral primitive equation model of the atmos phere is employed and confirms the results obtained from the analytical stu dies. The results from both the analytical study and the numerical simulations ar e consistent in showing that Rayleigh friction acid Newtonian cooling play totally different roles in the tropical atmosphere. Newtonian cooling homog enizes the atmospheric motion in the vertical direction, and a strong, vert ically uniform wind is found below the base of the heat source. When Raylei gh friction dominates, the circulation driven by the heat source is confine d to the layer where the heat source is located. It is also shown that a st rong Hadley circulation is associated with reasonable strong Rayleigh frict ion, but not with Newtonian cooling alone. Finally, the numerical solution is found fur the case where Newtonian cooli ng acts uniformly in the vertical and Rayleigh friction is included in the lower atmosphere to mimic crudely the dissipation of momentum in the bounda ry layer. The introduction of the simple boundary layer dramatically reduce s the surface circulation that was supported in the Newtonian cooling alone case. Together these results suggest a significant surface circulation is unlikely to be driven by an elevated hear source if it resides above the to p of the boundary layer.