Predictability and dynamics of the genesis of Hurricane Karl (2010)

Convective-permitting short-range ensemble forecasts initialized with the ensemble Kalman filter (EnKF) analysis and perturbations are used to study factors affecting the genesis of Hurricane Karl (2010). This limited-area EnKF was one of the ensemble prediction systems used in real-time during the Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) experiment in 2010. The EnKF system assimilated all non-radiance data used in the NCEP global model, on two fixed outer domains centered on St. Croix (with grid spacings of 40.5 and 13.5 km). A vortex-following nest of 4.5 km grid spacing was added for forecasts. We focus on the ensemble forecasts initialized at 00Z 12 Sept 2010. Of the 30 members in the ensemble, 13 reached tropical storm strength during the 120-h integration.

Large ensemble spread in the intensity forecasts reveals both the practical and intrinsic limits of predictability of this storm, before and during its genesis. Preliminary composite analysis of developing versus non-developing members shows that the middle-level moisture and low-level vorticity are two of the primary contributors for the tropical cyclone formation. However, the relationship between genesis and these two contributing factors is by no means linear. Furthermore, the initial moisture or low-level vorticity in some developing storms can be lower than those in some non-developing storms. Also, the initial differences between some developing and non-developing storms may be approaching the measurement accuracy of dropsondes, which are currently one of the most reliable observing platforms.

We also performed a series of sensitivity experiments to examine the impact of the diurnal cycle on the genesis of Karl. Preliminary analysis shows that the diurnal cycle may have significantly affected the timing of genesis in some ensemble members. The diurnal cycle also affects the storm intensity during genesis.