The role of graupel in facilitating tropical cyclone rapid intensification as seen in idealized simulations

Abstract

Microphysical processes and their impacts on tropical cyclone (TC) intensification are not well understood. Due to limited observations and model resolution, all microphysical processes need to be parameterized within models. One such process, hydrometeor fall speed, can impact the number of hydrometeors available and rate of phase changes, thus influencing the amount of latent heat available for the TC. To address microphysical uncertainties, the representation of graupel within the Morrison scheme in the Weather Research and Forecasting (WRF) model is altered by adjusting the fall speed of the hydrometeor, or by removing it from the scheme. A more favorable large‐scale environment for rapid intensification (RI) and a less‐favorable environment are used to isolate how much the environment plays a role in the effects of microphysics. In the RI environment, the graupel adjustments have the greatest impact on intensification. With less graupel, the storms experienced delayed or limited intensification due to decreased vertical mass flux, diabatic heating, and changes in tilt and asymmetry. These differences are more subtle in the less‐favorable environment, resulting in minimal intensification differences. The results demonstrate the importance of graupel in promoting RI.