A Theory of Cyclogenesis Forced by Diabatic Heating. Part I: A Quasi-geostrophic Approach

Abstract

A quasi-geostrophic theory of cyclogenesis forced by a low-level diabatic heating in a backsheared baroclinic flow is proposed. Existence of wind reversal in one direction of the basic flow is an essential criterion to obtain a forced baroclinic wave in the vicinity of the heating region. It was found that an analogy exists for a quasi-geostrophic flow over a mountain and a region of steady-state diabatic heating. The relationship is described by h(x, y) = (? g/T0N2)T(x, y), where h(x, y) is the mountain shape and T(x, y) is the temperature anomaly. The response of a backsheared baroclinic flow over a region of two-dimensional diabatic heating (cooling) is a coupled low?high (high?low) pressure pair located in the vicinity and on the downstream side of the heating (cooling), respectively. Physically, the growth of the pressure perturbation can be explained by a group velocity argument. The disturbance remains locally in the vicinity of the forcing due to zero phase speed of the forced baroclinic wave. The upshear phase tilt indicates that the disturbance is a baroclinic wave generated by diabatic forcing. The response of an east?west backsheared baroclinic flow over an isolated region of diabatic heating with circular contours is a growing cyclone located near the center of the heat source. A coupled high pressure forms downstream of the diabatic heating. The disturbance is confined in a shallow layer. The forced low resembles the geometry of the heat source in the early stage. One interesting finding is that an inverted ridge forms downstream of the low. When applied to East Coast cyclogenesis, a cyclone develops near the center of the region of maximum diabatic heating, i.e., near the western boundary of the Gulf Stream. The cutoff low remains in the vicinity of the diabatic heat source. Two regions of weaker high pressure form to the southeast and northwest corners of the low. To the south of the low, there exists a strong anticyclonic circulation. A confluent zone forms to the northeast of the low, while a diffluent zone forms to the southwest of the low. The genesis region and the flow pattern of the cyclone predicted by the theory are consistent with observations. With an easterly wind at the surface, the inverted trough?ridge couplet is more pronounced than with a northeasterly wind. The low starts to decay as it moves out of the concentrated heating region. The cyclone is produced hydrostatically by the less dense air above the heating region with the modification of the baroclinic effects.