Potential Impacts of Tropical Cyclone Inner-Core Moisture Initializations on the Predictability of the Onset of Rapid Intensification

Predictions of significant changes in tropical cyclone (TC) intensity, such as rapid intensification (RI), have emerged as a more challenging topic than forecasting TC tracks, since intensification of TCs involves multi-scale physical processes with significant contributions from convective-scale phenomena. Intensifying TCs are known to experience a precession process before the onset of RI, but the vortex tilt magnitude, the duration of the precession process and whether or not the vortex is able to complete the precession process are dominated by the chaotic nature of moist convection. In this study, mechanisms that determine the variability of these precession processes are explored through sets of sensitivity and ensemble forecast experiments.

We have used the Pennsylvania State University’s experimental real-time ensemble Kalman filter analysis of Hurricane Harvey (2017) that assimilated GOES-16 all-sky radiances in convection-permitting Weather Research and Forecasting model (WRF-ARW) simulations. This analysis resulted in a highly accurate forecast of intensity and track, and realistically represented the storm’s rapid intensification. Starting with an analysis based on 18 hours of cycling data assimilation, we have conducted sensitivity experiments in which we reduced the initial atmospheric water vapor amount by 5 to 20 %. Even initialized with the same wind field, and with fully developed convective updrafts and organization, the vortex tilt magnitude and the duration of precession are significantly modified by the inner-core moist processes. We explore the characteristics of the vortex structures that underwent short/long/uncompleted precession process with small/large tilt magnitude, as well as how moisture convection contributed to accomplishing these vortex structures. The results have implications for the design of future observation networks tasked with providing constraint on predictions of rapidly intensifying tropical cyclones.