The Use of Equivalent Potential Vorticity to Diagnose Regions of Conditional Symmetric Instability

Abstract

Conditional symmetric instability (CSI) is an important property of the atmosphere when diagnosing and predicting mesoscale bands of moderate to heavy precipitation within winter cyclones. Within regions of CSI, slantwise convection can increase snow totals over narrow regions. Typically, CSI is evaluated in a cross-sectional plane chosen normal to the middle-tropospheric thermal wind using Mg, the absolute geostrophic momentum, and ? e, the equivalent potential temperature. Regions where Mg, surfaces slope less than ?e, surfaces are subject to CSI. We describe an objective measure of CSI, called the equivalent potential vorticity (EPV), that makes evaluating CSI quick and effective. Cross sections of Mg, versus ?e, and EPV are compared for two cases to demonstrate the effectiveness of using EPV cross sections to diagnose CSI. The distinction between slantwise convection and upright convection is also demonstrated by these case studies.