Applying a Novel Ice Crystal Trajectory Growth Model (ICTG) to Study Hurricane Eyewall Microphysics

Understanding tropical cyclone (TC) ice and mixed-phase microphysics remains a challenge due to lack of remotely sensed and in-situ aircraft observations and accurate simulations of ice microphysics. This study employs a novel Ice Crystal Trajectory Growth (ICTG) model to effectively trace individual ice crystals along their paths within TCs. The ICTG model offers a departure from conventional microphysical modeling by accommodating particles' free evolution in a Lagrangian framework, in contrast to typical Eulerian microphysics schemes. This innovation allows for the ability to analyze the interconnection between the ice crystal growth and its advection through a specific environment and provides more in-depth ice crystal characteristics (e.g., mass, shape, density, and growth type). Since the ICTG tracks particle evolution, the model can address questions regarding ice particle diversity that cannot be answered with traditional schemes.

This research investigates the cold-phase microphysics of TC eyewalls. A simulation of Hurricane Harvey (2017) during its rapid intensification (RI) phase was run using the Penn State WRF-EnKF system with data assimilated from all-sky infrared and microwave radiances from GOES and NASA GPM constellation satellites, and other conventional observations. The ICTG model is used to trace ice crystal growth and movement within the storm at different stages during the RI. A notable advantage of the input for the ICTG lies in its frequent temporal and spatial particle growth data across scales. This is enabled by the 1 km horizontal resolution and frequent 5-minute temporal resolution of the Harvey simulation.

This investigation reveals that the updraft in the downshear region of the eyewall significantly contributes to the lofting of ice, generating stratiform precipitation downwind. Ice crystals originating at 8 km altitude that were lofted upwards by the downshear-left updraft saw greater mass growth at their new altitude, than particles that remained near 8 km altitude despite similar residence times. These findings underscore the potential of the ICTG model to enhance our understanding of TC ice microphysics and its implications for intensity and structural changes of TCs.