Heat and momentum fluxes induced by thermal inhomogeneities with and without large-scale flow

The authors present an analytical evaluation of the vertical heat and momentum fluxes associated with mesoscale flow generated by periodic and isolated thermal inhomogeneities within the convective boundary layer (CBL). The influence of larger-scale wind flow is also included. The results show that, with little or no synoptic wind, the vertical velocity is in phase with the diabatic temperature perturbations and that the mesoscale heat flux is positive and of the same order as the diabatic heat flux within the CBL. Above the CBL, the heat flux is negative and penetrates into the free atmosphere through a depth comparable to the depth of the CBL. In the presence of synoptic flow, the mesoscale perturbation is in the form of propagating waves that penetrate deeply into the free atmosphere. As a result, there is a net downward flux of momentum, which is dissipated within the CBL by turbulence. Furthermore, mixing with the environment of the air particles displaced by the waves results in a net negative mesoscale heat flux, which contributes to the weakening of the stability of the free atmosphere. Strong synoptic advection can significantly weaken the horizontal temperature gradients in the CBL, thereby weakening the intensity of the mesoscale flow. Turbulent diffusion also weakens the temperature gradients and the intensity of the mesoscale flow at large wavenumbers when the wavelength is comparable to the CBL depth. Finally, when the synoptic wind is very strong, the mesoscale perturbation is very weak and vertically trapped.