The dynamics of heat lows

A numerical model is used to investigate dynamical aspects of the structure and evolution of a heat low in an idealized flow configuration with an area of land surrounded by sea. Of particular interest is the evolution of the distributions of relative vorticity and potential vorticity. While the heat low has a minimum surface pressure in the late afternoon following strong solar heating of the land, the relative vorticity is strongest in the early morning hours following a prolonged period of low-level convergence. Thus the heat low is not approximately in quasi-geostrophic balance. The low-level convergence is associated with the sea-breeze and later with the nocturnal low-level jet. The effects of differing sea area, land area and Coriolis parameter on various aspects of the heat low are investigated. Although a cyclonic vortex, the heat low is characterized by an anticyclonic potential-vorticity anomaly relative to its environment throughout much of the lower troposphere on account of the greatly reduced static stability in the convectively well-mixed boundary layer; however, the surface temperature maximum over land corresponds with a cyclonic potential-vorticity anomaly at the surface. The reduced static stability in the mixed layer has the further consequence that the horizontal components of relative vorticity and horizontal potential-temperature gradient make a non-negligible contribution and of opposite sign to the potential vorticity in certain flow regions. Two processes associated with the flow evolution in the model appear to be fundamental to understanding a range of low-level atmospheric phenomena over the arid interior of Australia: these are the deep convective mixing over land during the daytime and the development of a nocturnal low-level jet, which leads to convergence in the trough. Such phenomena include the diurnal behaviour of dry cold fronts and the generation of nocturnal wind surges and bores. It is reasonable to assume that similar processes operate in other arid regions of the world where deep convective mixing over land produces local maxima of diabatic heating in the lower atmosphere.