Heat transport and weakening of atmospheric stability induced by mesoscaleflows

Citation
Ga. Dalu et al., Heat transport and weakening of atmospheric stability induced by mesoscaleflows, J GEO RES-A, 105(D7), 2000, pp. 9349-9363
Citations number
21
Categorie Soggetti
Earth Sciences
Volume
105
Issue
D7
Year of publication
2000
Pages
9349 - 9363
Database
ISI
SICI code
Abstract
We present an analytical evaluation and interpretation on how diabatic heat ing of the convective boundary layer (CBL) is transported upward into the m idtroposphere by mesoscale flows, and how the air mixes with the environmen t and therefore weakens the atmospheric static stability. The thermodynamic imprint on the free atmosphere due to the irreversible processes such as m ixing, dissipation, and diffusion, associated with the mesoscale flow, is m ore clearly shown when the forcing is periodic in time. Convective mixing i n the CBL accounts for a thermodynamic perturbation of the order of a few d egrees, while mixing associated with the mesoscale activity accounts for a perturbation of the order of hall: a degree. To isolate this last effect, w e prescribe a periodic forcing with a 1 day period, so over 24 hours, the n et diabatic input averages to zero, and the contribution due to the advecti on cancels out. Tn this formulation the perturbation is solely due to irrev ersible processes associated with the mesoscale. These perturbations are re levant, since they are smaller, but of the same order of magnitude as pertu rbations associated with mesoscale advection and the CBL mixing. A more com plete evaluation of the relative contribution to the atmospheric perturbati ons due to the mesoscale activity was completed using an initial value prob lem approach. In this case, there is a net transport of the diabatic heat i nduced by the mesoscale flow. As a consequence, when the mesoscale flow per sists for several days, the static stability of the atmosphere is eroded by the combined action of the diabatic heat, CBL mixing, and transport and mi xing due to the mesoscale activity. In this paper we first evaluate the con tribution of the irreversible processes using a periodic in time forcing. T hen we examine the atmospheric impact due to a sequence of several sea bree ze days, starting from rest at time zero and letting the flow evolve as an initial value problem. Results suggest that perturbations associated with m esoscale flows generated by landscape variability are of climatological imp ortance and need to be introduced in a parametric form in coarser large-sca le models, as presently is done with turbulent subgrid CBL processes.