A static stability index for low-topped convection

Static stability indexes provide a simple representation of a complex aspec t of the atmosphere and are widely used in operational forecasting. However , their applicability is limited. since most are specifically designed to m easure deep instability. In particular, they are ineffective at resolving i nstability that is capped by an inversion below the height of the 500-mb su rface. Unstable thermal profiles that are capped between 700 and 500 mb hav e been identified as a significant forecasting problem in New Zealand. Refe rred to here as low-topped instability, these situations are easily overloo ked by forecasters, due to a lack of relevant guidance. In this paper, an i ndex capable of resolving low-topped instability is presented. Denoted TQ, it is based on the temperature and dewpoint at 850 mi, 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 indepe ndent dataset for verification, TQ was found to be effective at resolving u nstable events, demonstrating skill that is statistically significant at th e 2% level.