Vertical Heat Fluxes Generated by Mesoscale Atmospheric Flow Induced by Thermal Inhomogeneities in the PBL

Abstract

An analytical evaluation of the vertical heat fluxes associated with the mesoscale flow generated by thermal inhomogeneities in the PBL in the absence of a synoptic wind is presented. Results show that the mesoscale fluxes are of the same order as the diabatic heat fluxes. In the sea-breeze case results show that in the lower layer of the atmosphere the heat flux is positive over the land and negative over the sea with an overall positive horizontal average. In the free atmosphere above the PBL the mesoscale vertical heat flux is negative over the land and over the sea; that is, the lower atmosphere becomes warmer while the free atmosphere above becomes cooler. As a result the mesoscale flow contributes to the weakening of the atmospheric stability within a region that extends a Rossby radius distance from the coastline, and up to an altitude larger than twice the depth of the convective PBL. The average momentum flux equals zero because the momentum removed over the sea is fed back into the atmosphere over the land. Sinusoidally periodic thermal inhomogeneities induce periodic atmospheric cells of the same horizontal scale. The intensity of mesoscale cells increases for increasing values of the wavenumber, reaches its maximum value when the wavelength of the forcing is of the order of the local Rossby radius, and then decreases as the wavelength of the forcing decreases, because of the destructive interference between mesoscale cells. The intensity of the vertical velocity and vertical fluxes is, however, only a weak function of the wavenumber, at large wavenumber. Therefore, the intensity of the mesoscale heat flux does not decrease substantially at high wavenumbers; however, the transport of cool air over small heated patches of land may cut off the temperature gradient in the atmosphere between the land and water early in the day, thereby reducing the duration of the mesoscale activity. Also horizontal diffusion of heat in the convective boundary layer can significantly weaken horizontal temperature gradients for large wavenumbers. Periodic square-wave thermal inhomogeneities are more effective than sinusoidal waves in generating mesoscale cells; that is, the intensity of the flow is generally stronger. When dealing with low resolution models, which do not resolve explicitly the mesoscale activity, the mesoscale heat fluxes have to be introduced in a parametric form, using this or a similar theory.