Shorter Time Scale Definitions for the Rapid Intensification of Tropical Cyclones

The rapid intensification (RI) of Atlantic Basin hurricanes is studied using the National Hurricane Center (NHC) Best Track Hurricane Database (HURDAT) and the Statistical Hurricane Intensity Prediction Scheme (SHIPS) output for 1989-2011. Previously, tropical cyclones (TCs) were defined as undergoing RI when maximum surface wind speeds (V_max) increased by 30 knots or minimum sea-level pressures (MSLP) decreased by 42 hPa over 24 h, with those thresholds representing the 95th percentile of all V_max increases and 75th percentile of maximum MSLP drop for a TC over 24 h, respectively. New definitions of RI are developed here by determining these 95th and 75th percentiles of V_max increases and MSLP decreases for 6, 12, and 24 h intervals. The corresponding thresholds for MSLP drops were 1.3 (2.2) hPa h^-1, 1.1 (1.7) hPa h^-1, and 0.9 (1.2) hPa h^-1 over 6,12, and 24 h for 95th (75th) percentiles, and 0.85 (1.3) m s^-1 h^-1, 0.64 (0.86) m s^-1h^-1, and 0.64 (0.75) m s^-1 h^-1 for 95th (75th) percentile of V_max increases over 6, 12, and 24 h. A greater fraction of TCs underwent RI at some point in their life cycle according to 6/12 h intervals, than for 24 h intervals. The timing of RI was also dependent on averaging interval. Statistically significant differences existed for distributions of environmental parameters for TCs that did and did not undergo RI, but not for TCs undergoing RI defined using different time intervals. Thus, larger intensification rates over shorter times are not attributed to environmental parameters.