A discussion on conditional symmetric instability in operational application

Conditional symmetric instability (CSI) is considered to be one of the mechanisms for the development of slantwise convection, and it is often used to explain mesoscale precipitation bands associated with fronts or rainbands observed on weather radar image. Many instability criteria can be used to diagnose conditional symmetric instability, such as the CSI slope criterion, the slantwise Convective Available Potential Energy (SCAPE), the moist symmetric instability (MSI), and the equivalent potential vorticity (EPV). However, forecasters are not sure whether these criteria are consistent and how to use them in operation. Aiming at the confusions mentioned above, the similarities between two criteria of conditional symmetric instability and two criteria of conditional instability are illustrated first in this paper because weather forecasters are more familiar with conditional instability. In operational application, the slope criterion is used more often, i.e. the slope of geostrophic momentum surface is steeper than the isentropic surface but more gradual than the surface of constant wet-bulb pseudo potential temperature. Since the environment that is conducive to slantwise convection is nearly saturated in a deep layer, the instability criterion of CSI is equivalent to MSI. Derivation and case study all indicate that the criteria of EPV and MSI are consistent and the two dimensional EPV is another expression of MSI. Comparing to subjective judgment of the slopes between constant geostrophic momentum surface and constant equivalent potential temperature surface, the EPV criterion solely based on its sign is more objective. It is important to note that the criteria of EPV and MSI are only consistent when they are under the same two-dimensional coordinate. It is wrong when using EPV as the criterion of MSI in three-dimensional coordinate.