a Parameterization of Mesoscale Heat Fluxes for General Circulation Models
Abstract
This study addresses the parameterization of mesoscale heat fluxes generated by landscape discontinuities. The realistic representation of the planetary boundary layer (PBL) is one of the most important aspects of climate modelling (e.g., Shukla and Mintz, 1982; Avissar, 1992). Former research in the field of planetary boundary layer modelling is explored. It is suggested that current representations of boundary layer processes in largescale models (e.g., General Circulation Models (GCMs)) are unrealistic when there are land discontinuities 10200 km wide. These land discontinuities generate mesoscale circulations which can be as strong as seabreezes. Given the relevance of climate change to human welfare and the environment, current efforts should be directed towards improvement in boundary layer modelling. A set of equations by Avissar and Chen (1993) is reviewed. They suggested a set of equations for GCMs which includes the impact of both turbulent and mesoscale processes on the mean variables. They derived prognostic equations for the mesoscale fluxes, which present a closure problem. Thus, they emphasized a need to develop a parameterization of these fluxes. A mesoscale atmospheric model was used to evaluate the impact of subgridscale landscape discontinuities on the vertical profiles of resolved temperature, moisture, and moist static energy in the planetary boundary layer of GCMs. Profiles of the mean variables were produced with a threedimensional (3D) version of the model by averaging horizontally the various atmospheric variables over a domain about the size of a single grid element in a GCM. They were compared to corresponding vertical profiles produced with a onedimensional (1D) version of the model, which simulates the PBL, as in a GCM, over a single horizontal grid element. The differences between the horizontally averaged atmospheric variables produced with the 3D simulations and the 1D simulations emphasize the impact of subgrid scale landscape discontinuities on GCM resolved variables. Landscape discontinuities, characterized by horizontal contrasts of surface wetness and size of land patches, were simulated under various backgroundwind conditions. Differences of air temperature, specific humidity, and moist static energy as large as 4 K, 6 g kg^{1 }, and 10 kJ kg^{1} were obtained, respectively, in some cases. These differences were not affected significantly by moderate winds, but were sensitive to the spatial distribution of surface wetness. Similarity theory was then used to develop a parameterization of mesoscale heat fluxes induced by landscape discontinuities in GCMs. For this purpose, Buckingham Pi Theory, a systematic method for performing dimensional analysis, was used to derive a set of dimensionless groups, which describes the largescale atmospheric background conditions, the spatial variability of surface sensible heat flux, and the characteristic structure of the landscape. These dimensionless groups were used to calculate the coefficients of a fourthorder Chebyshev polynomial, which represents the vertical profiles of dimensionless mesoscale heat fluxes obtained for a broad range of largescale atmospheric conditions and different landscapes. The numerous threedimensional numerical experiments performed to evaluate the proposed parameterization suggests that it is quite robust.
 Publication:

Ph.D. Thesis
 Pub Date:
 1994
 Bibcode:
 1994PhDT.......100L
 Keywords:

 CLIMATE;
 PLANETARY BOUNDARY LAYER;
 Environmental Sciences; Physics: Atmospheric Science