Assessment of Symmetric Instability from Global Reanalysis Data

Slantwise convection, the process by which symmetric instability is released, has often been shown to be associated with banded clouds and precipitation, especially in the frontal zones within extratropical cyclones (e.g., Emanuel 1983a,b). Both theoretical and observational studies suggest that the latent heat release associated with slantwise convection can lead to a spinup of surface frontogenesis, which can further help in intensifying the cyclone in some cases (e.g.,Balasubramanian and Yau, 1994). However, most of these studies were for local areas or with short time durations. The climatology of the potential occurrence of symmetric instability (or slantwise convection) and its relationship with precipitating systems have not been investigated statistically. Using the 6 hourly ERA-interim analysis, we calculated two different indices for assessing symmetric instability, namely SCAPE (slantwise convective available potential energy) and VRS (vertically integrated extent of realisable symmetric instability), and investigated the likelihood of occurrence of symmetric instability around the globe. We also attempted to quantify the degree of association between these indices and the observed surface precipitation and the frequency of cyclone tracks. Despite large uncertainties, the time evolution of averaged SCAPE and VRS within rapidly deepening cyclones showed a significant drop in values after the onset of rapid intensification, supporting the idea that the release of symmetric instability might contribute to the intensification of storms.