Potential vorticity as a tool for assessing dynamical impacts of latent heat release in model forecasts

While the concept of "potential vorticity (PV) thinking" is widely used in the synoptic-dynamic research community, it has yet to gain widespread acceptance in the operational forecasting community in the United States. This may be in part because it has not been convincingly demonstrated to operational forecasters that using PV thinking provides them with information beyond that available via more traditional means. Today operational forecasters are confronted with an ever-increasing volume of numerical weather prediction (NWP) model guidance; and tools are needed to help them best assess this guidance and understand which physical processes are influencing model solutions. It is widely recognized that model quantitative precipitation forecasting (QPF) is poorly forecasted compared to other parameters, and this often results in the misrepresentation of dynamical feedbacks due to latent heating in model forecasts, especially in cases where convection is occurring. PV is not conserved in the presence of latent heating, therefore PV can be used to identify and track the dynamical feedback associated with latent heat release in NWP model output, making it a valuable diagnostic tool to the operational forecaster. Therefore, PV thinking provides a framework within which the forecaster can (i) potentially identify biases in NWP model forecasts due to the misrepresentation of latent heating and (ii) adjust model guidance using observational data and a conceptual understanding of the dynamical feedbacks associated with latent heating.

Since diabatic PV growth is often most pronounced in the lower-troposphere, PV thinking is particularly useful in identifying the impact of latent heating on surface cyclogenesis, low-level jets, and moisture transport. Its use will be motivated through the presentation of three case study examples that highlight the utility of PV thinking in diagnosing and understanding differences in the evolution of synoptic-scale and mesoscale phenomena in NWP model output. The first case will highlight the impact of an upstream precipitation feature on the rapid cyclogenesis and moisture transport in the 24–25 January 2000 East Coast cyclone. The second case will demonstrate how to use PV thinking to diagnose the impact of model convective scheme heating on a coastal cyclone/frontogenesis event offshore of the Carolinas from 17 February 2004. The final case will examine the diabatic enhancement of a low-level jet in the mid-Atlantic and northeastern United States in December 2004.