A Theory of Cyclogenesis Forced by Diabatic Heating. Part II: A Semigeostrophic Approach

Abstract

A linear quasi-geostrophic theory of coastal cyclogenesis proposed by Lin has been extended by a semigeostrophic model. The response of an east?west backsheared, quasi-geostrophic baroclinic flow over an isolated heat source is a low pressure near the heating center and a weaker high pressure downstream, as found in part I of the theory. With the inclusion of nonlinear geostrophic advection, the low is weakened slightly and becomes asymmetric, while the high remains about the same strength. With the inclusion of nonlinear ageostrophic advection, the low is strengthened significantly by the warm air advection and becomes more asymmetric. With the addition of uniform friction, the cyclogenesis process is weakened. However, the cyclone is strengthened slightly by differential friction. It appears that the primary source of the cyclonic vorticity of the coastal cyclone is the hydrostatic response of the diabatic heating modified by the baroclinicity. With a mountain ridge included, there exists an inverted pressure ridge (trough) over (downstream of) the mountain. A damming effect is evidenced by a pool of cold air located upslope of the mountain ridge, which is formed by the cold air advection and upslope adiabatic cooling. The inverted pressure ridge between the mountain and the heat source is strengthened by the surface heating and is shifted farther upstream of the mountain. When the semigeostrophic model is applied to East Coast cyclogenesis with a northeasterly surface wind, a cyclone develops near the western boundary of the Gulf Stream. The low is skewed with a larger gradient located to the southeast corner. The mountain-induced high pressure is relatively weak since the surface wind is almost parallel to the mountain ridge. The cold-air damming is negligible and the Appalachians play a minor role in this type of coastal cyclogenesis. With an easterly surface wind, the cold-air damming is more pronounced. The inverted ridge or the damming area is shifted further upstream and more widespread in the region between the southern part of the Appalachians and the Gulf Stream. A confluent?diffluent couplet is produced to the northwest and southwest of the heat source. The results are consistent with observations.