A Diagnostic Analysis of the Superstorm of March 1993

In this study, the synoptic evolution of the March 1993 superstorm is documented using conventional observations, and the predictability and deepening mechanisms of the storm are investigated using a newly implemented mesoscale version of the Canadian Regional Finite-Element (RFE) Mode. It is found that although the RFE model can predict well the large-scale background flow associated with the storm, it fails to predict various important mesoscale elements when it is initialized with fields that contain weak signals of the low-level circulations. However, when the storm is located close to the data-rich region, the model has considerable skill in predicting those mesoscale elements, such as a prefrontal squall line, upper- and low-level jets, as well as the quantitative aspects of their associated precipitation. Observational analysis reveals that the storm developed in a convectively unstable prestorm environment with ample moisture content. It first experienced an antecedent surface vorticity growth over the anomalously warm water of the Gulf of Mexico mainly through condensation processes. Then the storm deepened rapidly as two midlevel short-wave troughs approached, merged, and interacted with a thermally direct ageostrophic circulation in the entrance region of an upper-level jet streak. A potential vorticity (PV) diagnosis reveals that the intensification of the midlevel troughs is related to the pronounced descent of stratospheric PV-rich air associated with tropopause depressions. It is found that the tropopause depression, latent heat release, weak static stability, the jet streak-induced ageostrophic circulation, and surface sensible and latent heat fluxes, act together to determine the amplification and evolution of the storm, although their relative importance differs at different stages of the storm's development. In particular, latent heat release accounts for roughly 40% of the total deepening, and the heating-induced ageostrophic circulations provide an important coupling of the surface cyclogenesis with the above-mentioned processes.