Heat transport and weakening of atmospheric stability induced by mesoscale flows

Abstract

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 I 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.

Synoptic Circulation and Land Surface Influences on Convection in the Midwest U.S. “Corn Belt” during the Summers of 1999 and 2000. Part II: Role of Vegetation Boundaries

“…The energy fluxes help determine the thermodynamics of the PBL (Hong and Pan 2000;Barros and Hwu 2002), as indicated by a number of stability indices [e.g., lifted index (LI), "cap strength," CAPE; see Peppler and Lamb 1989;Eltahir and Pal 1996;Schreiber et al 1996;Doran and Zhong 2000;Donner and Phillips 2003]. Moreover, large height differences between crops and trees aerodynamically influence PBL characteristics (Banta and White 2003), potentially inducing mesoscale vertical circulations across LULC boundaries that promote convection, or nonclassical mesoscale circulations (NCMCs) (e.g., Segal et al 1989;Travis 1997;Dalu et al 2000;Carleton et al 2001). Experiments with regional atmospheric models for the central United States suggest a preferred horizontal length scale of 10-20 km (Baidya Roy et al 2003) over which the energy and moisture contrasts associated with land surface heterogeneities become organized into NCMCs (Segal and Arritt 1992), although the relatively few ob-servational studies undertaken have not unambiguously detected these circulations (Hubbe et al 1997).…”

Comments on “Atmospheric Disturbance Caused by Human Modification of the Landscape”

“…2, could be produced, thus possibly influencing the location and intensity of precipitation when the next synoptic-scale disturbance entered the domain. Dalu et al (2000) have shown that, even disregarding water vapor transport, persistent mesoscale flow should have an impact on atmospheric stability that is significant compared to boundary layer turbulence and diabatic heating. Since soil wetness is a major control on the partitioning of surface fluxes into sensible and latent heat, these impacts on precipitation might also systematically alter the surface heterogeneity that is the driving force for the landscape-generated mesoscale circulations, thus resulting in positive or negative feedbacks.…”

Analytical evaluation of mesoscale fluxes and pressure field

“…Some thermally-generated mesoscale circulations are due to horizontal inhomogeneities in ground wetness, vegetation cover, snow cover, cloud cover as studied by many authors, e.g. [8,[10][11][12][13]. In addition there are also anthropogenic modifications of soil and vegetation, such as irrigated or non-irrigated fields, crops at different stages of growth, inhabited areas, and so on [14][15][16][17].…”