A Study of the Effect of Wind Shear on a Heat Island Circulation Characteristic of an Urban Complex

Abstract

Simple linear models were used to study the effect of wind shear on the nocturnal urban heat island circulation. It was shown that the intensity of the urban heat circulation decreases by increasing the magnitude of the bulk shear through the boundary layer, if the heating rate is constant. However, if the difference between the mean temperature in the urban and environmental boundary layer and the stabilities are identical in two separate cases, the case with the stronger shear will have a stronger urban heat island circulation. When the magnitude of the vertical velocity was computed, it was found that the results were contingent on the boundary conditions used to solve the second-order equations for the motion. In the case where the perturbation vertical and horizontal velocities were set to zero at the surface, both the vertical scale and the intensity of the circulation approached infinity as the characteristic Richardson number approached zero. When the rigid lid concept was employed to limit the vertical scale of the circulation, a critical Richardson number existed in stable air for which the circulation had a finite maximum intensity. The value of the critical Richardson number was a function of the vertical scale of the rigid lid, and for a given vertical scale, there was more than one value for the critical Richardson number. The most intense circulation was found when the critical Richardson number was small.