Idealized simulations of circulations forced by land surface heterogeneity

Idealized large eddy simulations (LES) and mesoscale model simulations are used to investigate the conditions that govern the formation of circulations due to heterogeneities in surface heating. Many previous studies have investigated the formation of such circulations. However, there has been little work using a theoretical formulation to determine whether given conditions will result in a circulation.

Using gravity current theory, a non-dimensional group has been derived that predicts whether two adjacent patches of ground with differing surface heating will result in a circulation that is able to overcome the background wind. This non-dimensional group includes: the width of the patches making up the land surface heterogeneity, the wind speed perpendicular to the boundary between the patches, the difference in surface buoyancy flux between the patches, the acceleration due to gravity, and virtual potential temperature of the boundary layer.

The critical value of this non-dimensional group is investigated using idealized mesoscale (WRF) and LES (Bryan-Fritsch Cloud Model [CM1]). A “cool” patch of land with lower surface buoyancy flux (SBF) is bordered by a “warm” patch of land with higher SBF. The background wind speed, the magnitude of the SBF flux difference between the patches, and the width of the patches are varied to determine those values of the non-dimensional parameter that induce a circulation that is strong enough to overcome the background wind. The results from WRF and CM1 simulations are compared to determine the characteristics of circulations not resolved by a mesoscale model. This may lead to a method to parameterize the effects of unresolved circulations in mesoscale models or to communicate the resulting uncertainty in the meteorological conditions input to atmospheric transport and dispersion (AT&D) models.