Predictability and Dynamics of a Nonintensifying Tropical Storm: Erika (2009)

Abstract

Abstract In this study, the predictability of Tropical Storm Erika (2009) is evaluated by analyzing a 60-member convection-permitting ensemble initialized with perturbations from a real-time ensemble Kalman filter (EnKF) system. Erika was forecast to intensify into a hurricane by most operational numerical models, but in reality it never exceeded 50 kt (1 kt = 0.51 m s−1). There is a fairly large spread in the final intensities of the 60 ensemble members indicating large uncertainty in the deterministic prediction of Erika's intensity at 36–48-h lead times. An investigation into which factors prevented intensification of the weaker ensemble members provides insight that may aid in the forecasting of the intensity of future tropical cyclones under similar conditions. A variety of environmental and storm-related factors are examined, and the parameters that have the greatest relation to future intensity are determined based on ensemble sensitivity and correlation analysis. It appears that midlevel relative humidity, absolute vorticity, and the distribution of convection relative to the storm center all play a role in determining whether a given ensemble member intensifies or not. In addition, although differences in deep-layer shear among ensemble members are difficult to discern, many of the ensemble members that do not intensify fail to do so because of apparent dry air intrusions that wrap around the centers of the storms, particularly in the 700–500-hPa layer. In the presence of moderate shear, this dry air is able to penetrate the cores of the cyclones, thereby preventing further development.