Heat transport and weakening of atmospheric stability induced by mesoscale flows

Abstract

We present an analytical evaluation and interpretation on how diabatic heating of the convective boundary layer (CBL) is transported upward into the midtroposphere by mesoscale flows, and how the air mixes with the environment and therefore weakens the atmospheric static stability. The thermodynamic imprint on the free atmosphere due to the irreversible processes such as mixing, dissipation, and diffusion, associated with the mesoscale flow, is more clearly shown when the forcing is periodic in time. Convective mixing in the CBL accounts for a thermodynamic perturbation of the order of a few degrees, while mixing associated with the mesoscale activity accounts for a perturbation of the order of half a degree. To isolate this last effect, we prescribe a periodic forcing with a 1 day period, so over 24 hours, the net diabatic input averages to zero, and the contribution due to the advection cancels out. In this formulation the perturbation is solely due to irreversible processes associated with the mesoscale. These perturbations are relevant, since they are smaller, but of the same order of magnitude as perturbations associated with mesoscale advection and the CBL mixing. A more complete evaluation of the relative contribution to the atmospheric perturbations due to the mesoscale activity was completed using an initial value problem approach. In this case, there is a net transport of the diabatic heat induced by the mesoscale flow. As a consequence, when the mesoscale flow persists for several days, the static stability of the atmosphere is eroded by the combined action of the diabatic heat, CBL mixing, and transport and mixing due to the mesoscale activity. In this paper we first evaluate the contribution of the irreversible processes using a periodic in time forcing. Then we examine the atmospheric impact due to a sequence of several sea breeze days, starting from rest at time zero and letting the flow evolve as an initial value problem. Results suggest that perturbations associated with mesoscale flows generated by landscape variability are of climatological importance and need to be introduced in a parametric form in coarser large-scale models, as presently is done with turbulent subgrid CBL processes. Copyright 2000 by the American Geophysical Union.