The Use and Misuse of Conditional Symmetric Instability

Abstract

A commonly employed explanation for single-and multiple-banded clouds and precipitation in the extratropics is slantwise convection due to the release of moist symmetric instability (MSI), of which one type is conditional symmetric instability (CSI). This article presents a review of CSI with the intent of synthesizing the results from previous observational, theoretical, and modeling studies. This review contends that CSI as a diagnostic tool to assess slantwise convection has been, and continues to be, misused and overused. Drawing parallels to an ingredients-based methodology for forecasting deep, moist convection that requires the simultaneous presence of instability, moisture, and lift, some of the misapplications of CSI can be clarified. Many of these pitfalls have been noted by earlier authors, but are, nevertheless, often understated, misinterpreted, or neglected by later researchers and forecasters. Topics include the evaluation of the potential for slantwise convection, the relationship between frontogenesis and MSI, the coexistence of moist gravitational instability and MSI, the nature of banding associated with slantwise convection, and the diagnosis of slantwise convection using mesoscale numerical models. The review concludes with suggested directions for future observational, theoretical, and diagnostic investigation.

Environment and Early Evolution of the 8 May 2009 Derecho-Producing Convective System

“…We also used u e and u e * (saturated equivalent potential temperature) in the stability calculation, as discussed in Schultz and Schumacher (1999), and the results were almost identical, likely because the PV g is assessed in the upper troposphere, where the low absolute moisture content results in only slight differences in the three versions of potential temperature.…”

Sting jets in severe northern European wind storms

“…Browning (2004) and Clark et al (2005) suggested that the slantwise circulations in the cloud head may have been caused by the presence of conditional symmetric instability (CSI) (see Schultz and Schumacher (1999) for a review of CSI) by consistency with the scale of the circulations. Thus, they hypothesized, the release of CSI may play a role in the generation of the sting jet.…”

Non‐normal growth in symmetric shear flow

“…A thorough review (together with a rich reference list) of different types of SI and the 'use and misuse' of them appears in Schultz and Schumacher (1999). Theoretical treatment of linear SI dynamics (Stone, 1966(Stone, , 1970Hoskins, 1974;Bennetts and Hoskins, 1979;Emanuel, 1983a,b;Xu, 1986a,b) focused mainly on normal mode analysis, in which attention was focused on the necessary Richardson number condition for instability, and on the largest modal growth rate as a measure of the potential for occurrence of slantwise convection.…”