Untangling the Effects of Latent Heat Release on an Extratropical Cyclone Using Potential Vorticity Analysis

Extratropical cyclones account for much of the precipitation and daily weather variation in the mid-latitudes, and many of the synoptic and mesoscale processes and linkages within extratropical cyclones are well understood, a product of decades of research. However, at finer scales, our understanding is more limited. While processes at these levels are localized at short time-scales, they affect the cyclone as a whole via upscale linkages. This research aims to expand our understanding of these linkages as they relate to the effect of latent heat release on extratropical cyclone development. With the projected changes to the climate system, warming will increase the vapor capacity of the atmosphere, leading to increased amounts of evaporation and condensation, and thus, greater latent heat fluxes overall. These changes may be most evident in the warm sector of the system (poleward and east of the circulation center). Here, the warm, humid air provides the bulk of the moisture and resulting precipitation during interaction with neighboring cooler air masses. What an increase in vapor content in the warm sector would do to cloud and precipitation processes in the cyclone as a whole is still unclear, but preliminary results indicate that the feedbacks are highly non-linear, and merit more focused study.

To this end, we have chosen to perform a case study of an extratropical cyclone off the East Coast of the United States from November 21-26, 2006. Stationary for approximately 3 days, the storm was characterized by a continual warm conveyor belt airflow that stretched from the tropics to the Canadian Maritime provinces. Given the persistent condensation and subsequent latent heat release of the tropical air as it was drawn northward, this storm is an optimal case to isolate the effects of latent heat release on cyclone and frontal structure. To deconvolve these effects, we simulate this case with the Weather Research and Forecasting (WRF) model run at a cloud system resolving horizontal grid spacing of 4 km. We run 2 separate instances of the model: the first under a full-physics condition, with latent heat release included, and the second with the effect of latent heat release removed From our synoptic analysis, we find that the latent heat release has not necessarily served to enhance the storm strength, an unexpected result. Therefore, we adopt a potential vorticity perspective in our further analysis of the storm, deconvolving the influence of latent heat release from the influence of dynamics on the evolution of the case storm. These results can help us to anticipate possible changes in extratropical storm dynamics that may occur in a warming climate.