Nonlinear Influence of Mesoscale Land Use on Weather and Climate

This paper demonstrates that the influence of mesoscale landscape spatial variability on the atmosphere must be parameterized (or explicitly modeled) in larger-scale atmospheric model simulations including general circulation models. The mesoscale fluxes of heat that result from this variability are shown to be of the same order of magnitude but with a different vertical structure than found for the turbulent fluxes. These conclusions are based on experiments in which no phase changes of water were permitted. When, for example, cumulus clouds organized in response to the landscape pattern develop, the mesoscale influence on larger-scale climate is likely to be even more important. To parameterize surface thermal inhomogeneities, the influence of landscape must be evaluated using spectral analysis or an equivalent procedure. For horizontal scales much less than the local Rossby radius, based on the results of Dalu and Pielke, the surface heat fluxes over the different land surfaces can be proportionately summed and an average grid-area value used as proposed by Avissar and Pielke. Moisture fluxes can probably be represented in the same fashion as for heat fluxes. For larger-scale spatial variability, however, the mesoscale fluxes must also be included as shown in this paper. While the linear effect could be parameterized using a procedure such as presented in Dalu and Pielke, where the spectral analysis is used to fractionally weight the contributions of the different spatial scales, the complete vertical mesoscale heat flux requires the incorporation of nonlinear advective effects. To include the nonlinear contribution of each scale, numerical model simulations for the range of observed surface and overlying atmospheric conditions must be performed.