The misuse and abuse of CSI can be lumped into several categories.

Elaboration of these points can be found in our manuscript.

1) Misterminology

• CSI as an instability We get the sense that some authors consider CSI as a lifting mechanism for slantwise convection. It is an instability. To say otherwise would be equivalent to saying that conditional instability is responsible for thunderstorms! Adequate lift and moisture must also be present to release the instability.

• CSI versus slantwise convection Some authors use the terms CSI and slantwise convection interchangeably. This is incorrect. CSI is an instability, whereas slantwise convection is the process that releases the instability. In fact, Thorpe and Rotunno (1989) argue that since the vertical heat flux is negative in some cases (i.e., the symmetric circulations act to cause warm air to sink and cold air to rise), the term slantwise convection is inappropriate. This argument, however, appears to be controversial.

• CSI versus PSI As illustrated on Chuck Doswell's page on CSI, the way many meteorologists assess CSI is to construct cross sections of geostrophic absolute momentum (Mg) and equivalent potential temperature (theta-e) and compare the slopes of the contours. If the theta-e lines slope more steeply than the Mg lines, then CSI is believed to be present (we call this the Mg-theta-e relationship). As in gravitational convection where theta-e decreasing with height is called potential instability, theta-e decreasing with height along an Mg surface (equivalent to the Mg-theta-e relationship) is called potential symmetric instability (PSI). Therefore, the test that most everyone applies for CSI is really a test for PSI.

• Use of geostrophic basic state As noted on Chuck Doswell's page on CSI, the instability is measured about a basic state. The absolute momentum surfaces are commonly constructed about a geostrophic basic state, therefore, geostrophic winds must be used in calculating Mg surfaces or the moist geostrophic potential vorticity (MPVg).

• MPVg versus Mg-theta-e relationship Because the Mg-theta-e relationship is dependent upon the orientation of the cross section, the use of negative MPVg (equivalent to the Mg-theta-e relationship in three dimensions) to assess moist symmetric instability is much preferred.

• Choice of thermodynamic variable As noted earlier, using theta-e to assess moist symmetric instability results in an assessment of PSI. To assess CSI, saturated equivalent potential temperature (theta-es or theta-e*) should be used. As we argue in our manuscript, CSI is a more appropriate measure of moist symmetric instability than PSI.

• Coexistence of moist gravitational and moist symmetric instabilities Blindly employing the tests for CSI will identify regions of conditional instability. Blindly employing the tests for PSI will identify regions of potential instability. Since the release of instability to gravitational convection is a more energetic process than that due to instability to slantwise convection, gravitational convection should dominate over slantwise convection, if it develops.

• Lack of instability The absence of moist symmetric instability does not preclude the formation of banded clouds and precipitation, because forced (rather than free) slantwise ascent could be occurring.

• Frontogenesis and moist symmetric instability Regions of moist symmetric instability are typically associated with frontogenesis. Because the symmetric stability modulates the ascent for a given frontogenetical forcing through the Sawyer-Eliassen equation, and frontogenesis acts as the lifting mechanism in the presence of the instability, determining whether banded precipitation is due to the instability or the frontogenesis is not possible.

• Coexistence of moist gravitational and moist symmetric instabilities How convection organizes in the presence of both gravitational and symmetric instabilities [termed convective-symmetric instability by Emanuel (1980) and Jascourt et al. (1988)], given adequate moisture and lift, is a wide-open question that should be addressed by future research projects.

  The figure below, adapted from a design by James Moore and Sean Nolan, shows a common stability profile across a frontal zone. The warm air above the frontal zone may be conditionally unstable and conditionally symmetrically unstable, while the frontal zone is neutrally symmetrically stratified. How convection will develop in this environment has not been addressed from an observational or theoretical perspective.

  

• Presence or absence of lightning There have been several examples of lightning occurring in the presence of moist symmetric instability. Therefore, the existence of lightning is not an adequate discriminator between moist gravitational instability and moist symmetric instability.

• Banding Idealized modeling and observational climatologies of precipitation structures indicate that even if a precipitation region meets the criteria for moist symmetric instability, the formation of banded structures cannot be assumed. Oftentimes nonbanded precipitation is observed in regions where the criteria for CSI is met.