A Static Stability Index for Low-Topped Convection

Abstract

Static stability indexes provide a simple representation of a complex aspect of the atmosphere and are widely used in operational forecasting. However, their applicability is limited, since most are specifically designed to measure deep instability. In particular, they are ineffective at resolving instability that is capped by an inversion below the height of the 500-mb surface. Unstable thermal profiles that are capped between 700 and 500 mb have been identified as a significant forecasting problem in New Zealand. Referred to here as low-topped instability, these situations are easily overlooked by forecasters, due to a lack of relevant guidance. In this paper, an index capable of resolving low-topped instability is presented. Denoted TQ, it is based on the temperature and dewpoint at 850 mb and the temperature at 700 mb. A dataset of 90 events, displaying varying degrees of low-topped instability, was used to correlate TQ to observed weather. Using an independent dataset for verification, TQ was found to be effective at resolving unstable events, demonstrating skill that is statistically significant at the 2% level.