Discussion about the Conditional Symmetric Instability in Operational Application

Conditional symmetric instability (CSI) is often used to explain the mesoscale precipitation bands. Many instability criteria can be used to diagnose the CSI. However, the forecasters are not sure whether these criteria are consistent and how to use them in operation. The similarities between criteria of conditional symmetric instability and criteria of conditional instability are illustrated. In operational application, slope criterion is used more often, i.e. the slope of geostrophic momentum surface is steeper than the isentropic surface while is more gradual than the surface of constant wet-bulb pseudo potential temperature. The criteria of equivalent potential vorticity (EPV) and moist symmetric instability (MSI) are consistent and two dimensional EPV is another expression of MSI. The criteria of EPV and MSI are only consistent when they are under the same two dimensional coordinates. It is wrong when applying EPV to three dimensions as the criterion of MSI.