Using the Ice Crystal Trajectory Growth (ICTG) Model in Tropical Cyclones: Unique and Extreme Ice Crystal Pathways in Hurricane Harvey

Weather forecast models have improved remarkably within the past several decades, but there are still several shortcomings that limit forecast accuracy, including cloud and precipitation microphysics. In particular, accurately modeling ice microphysics remains challenging because there are many complex sub-grid scale processes involved. These processes are so numerous that their representation in models needs to include large and often faulty assumptions, which can lead to significant errors in forecast models.

In an effort to reduce these errors and improve our scientific understanding, a new Lagrangian model was developed in a recent paper by Laurencin et al. (2022). The Ice Crystal Trajectory Growth (ICTG) model uses a Lagrangian framework to follow individual ice crystals and their growth processes through a storm simulated separately in an Eulerian model. This Lagrangian approach provides a more detailed representation of the wide variety of ice crystals and their properties. In this project, we use the ICTG model to study ice microphysics in a tropical cyclone (TC) eyewall. TC ice microphysics can have a significant impact on the storm’s evolution. Here, we use a simulation of Hurricane Harvey (2017) that was run using the Weather Research and Forecasting (WRF) model initialized using the Pennsylvania State University Ensemble Kalman Filter (PSU-EnKF) system where all-sky infrared and microwave satellite observations are included in the data assimilation.

Ongoing work related to this model is looking at dominant patterns for ice crystals. The research here focuses on unique particles including those that make 1-2 full revolutions around the TC and those that become trapped in mesoscale circulations, both radially and azimuthally. The research looks at the extreme values of ice crystal properties (particle aspect ratio, particle mass, particle riming mass, etc.) which allows us to better understand the full range of possibilities for an ice crystal in a TC.

The research analyzes how both the ice crystal environment (ice supersaturation, temperature, etc.) and ice crystal properties evolve over time for individual ice crystals. These new findings and potential improvements to ice microphysics in weather forecast models will aid in atmospheric predictions of intensity and structural changes of TCs.