Evaluation of tropical cyclone genesis precursors with relative operating characteristics (ROC) in high-resolution ensemble forecasts

Identifying the environmental conditions that control a tropical cyclone (TC) genesis is a challenging problem. This study examines a new method to evaluate the environment precursors on TC genesis using high-resolution ensemble forecasts and relative operating characteristic (ROC) diagrams.

With an advanced research version of the Weather Research and Forecasting (WRF) model, high-resolution ensemble forecasts (at 5 km horizontal resolution) are conducted in various configurations with a bred vector method to form a set of 140 ensemble members for predicting Hurricane Ernesto's genesis. Basic evaluation shows that high-resolution ensemble forecasts are able to predict well-developed TCs, whereas the NCEP Global Ensemble Forecast System (GEFS) fails to do so. This set of 140 ensemble members is employed to study the environmental precursors on Hurricane Ernesto's genesis by contrasting the genesis and nongenesis cases. Specifically, ROC curves, composite figures for genesis and nongeneis cases, and Kolmogorov-Smirnov tests are applied to characterize the relationship between important environmental parameters near the beginning of the simulation and genesis likelihood 15-18 h later. It is found that the moist conditions at 850 hPa, vertical wind shear, the strength of the 850 hPa pre-existing wave and upper-level warming play notable roles in Ernesto's genesis.