A Potential Vorticity-Based Study of the Role of Diabatic Heating and Friction in a Numerically Simulated Baroclinic Cyclone

Abstract

A particularly intense case of western Atlantic baroclinic cyclogenesis was investigated in this study. Specifically, the roles of latent heat of condensation and surface friction were examined from the potential vorticity or ?PV thinking? perspective. The methodology used for this study involves three key components: 1) a full-physics mesoscale model, which provides a continuous and dynamically consistent dataset and provides full user control over physical processes; 2) a partitioned PV integration, which temporally integrates the accumulation of PV due to various physical processes in the model's Eulerian framework: and 3) the piecewise inversion method of Davis and Emanuel, which calculates the balanced wind and mass field associated with particular PV anomalies. Potential vorticity features obtained through the partitioned integration technique were inverted to yield their direct contributions to the total circulation. In addition, sensitivity studies were carried out to determine the overall impact of various nonconservative processes on the cyclone development. Results of the PV integration showed that latent heating created a significant positive anomaly above the surface warm and bent-back fronts at the level of maximum heating. Inversion of this feature showed that it explained approximately 70% of the total balanced nondivergent circulation at low levels during the mature stage of the storm. The circulation associated with latent-heating-generated PV also enhanced the coupling between the surface and upper-level waves, both by hastening the eastward propagation of the surface wave and by slowing the eastward propagation of the upper-level wave. Comparison of the control experiment with a sensitivity test, in which latent heating was withheld, showed that latent heating also enhanced upper-level divergence, which expanded the downstream ridge and kept an upper-level small-scale PV anomaly coupled to the low-level disturbance. However, cyclogenesis still occurred in the absence of latent heating, due to a second, larger-scale upper PV anomaly that approached from the northwest. Surface friction caused the formation of mainly positive PV at low levels, primarily in the easterly flow of the warm frontal zone, where the dominant mechanism was frictional formation of southward-oriented horizontal vorticity in the presence of a strong southward temperature gradient. Inversion of this PV yielded a small cyclonic circulation centered on the surface low. However, a frictionless simulation produced a slightly stronger cyclone, due to indirect enhancement of the upper-level PV anomaly and the generation of low-level PV by thermal diffusion in the narrow warm sector of the storm.