The Interaction between Baroclinic and Diabatic Processes in a Numerical Simulation of a Rapidly Intensifying Extratropical Marine Cyclone

Abstract

This study addresses the relative contributions of adiabatic baroclinic and diabatic processes and their interaction in the evolution of a rapidly intensifying marine cyclone. Two numerical experiments were performed using a limited-area mesoscale model. The adiabatic simulation showed that the surface cyclone was associated with the quasi-geostrophic vertical motion forcing of a midtropospheric short wave for a period greater than 12 hours, suggesting the presence of deep baroclinic forcing during the evolution of the storm. The full-physics simulation produced major cyclogenesis with a central pressure of 967 mb and a deepening of 37 mb in 24 h. The model simulated the development of comma-shaped cloud patterns, which compared favorably with satellite observations of the storm. Further analysis showed that the rapid cyclogenesis was strongly related to moist frontogenesis at the warm front. During rapid storm intensification, the heavy precipitation, the generation of vorticity, strong surface frontogenesis, and large surface pressure falls all took place in the vicinity of the warm front. Quasi-geostrophic vertical velocity diagnosis of the full-physics simulation suggested a strong interaction between baroclinic and diabatic processes in the course of rapid development. The latent heat release significantly modified the frontal structure of the storm to reinforce its adiabatic secondary circulation. As a result, the adiabatic component of the vertical motion in the full-physics simulation was three times larger than that in the adiabatic simulation. Moreover, the upward and downward vertical motion induced by latent heat release was in phase with the secondary circulation associated with the adiabatic frontogenesis. The enhanced frontal circulation provided strong low-level moisture convergence to stimulate further frontal precipitation, establishing a positive feedback. Because of the large amount of latent heating associated with the warm frontal precipitation, diabatic heating was the dominant forcing mechanism for the vertical motion of the simulated storm during its rapid intensification. These results clearly indicate that it is not appropriate to treat the contribution of latent heat release (or other physical processes) to rapid development as a linear addition to the adiabatic dynamics, as has been done in most model sensitivity experiments. Rather, extratropical cyclogenesis should be viewed in the context of moist baroclinic instability with nonlinear interactions between the baroclinic dynamics and diabatic processes.