The Impact of Cloud Microphysics on the Genesis of Hurricane Julia (2010)

Tropical cyclogenesis (TCG) continues to be one of the least understood processes in tropical meteorology today. A growing number of studies have begun to examine the meso-β-scale structures and multi-scale processes leading to TCG. In this regard, our recent modeling work has shown the importance of upper-tropospheric warming and convective bursts in the genesis of Hurricane Julia (2010) within an African Easterly wave, postulating that these features are closely related to the depositional growth of frozen particles aloft. Thus, one of the purposes of this study is to investigate the importance of cloud microphysics processes in TCG through a series of high-resolution Weather Research and Forecast (WRF) model sensitivity simulations, in which the Thompson cloud-microphysics scheme is modified. The following three sensitivity experiments are conducted while holding all the other model configurations identical to a control simulation: i) removal of deposition/sublimation processes; ii) removal of homogenous freezing; and iii) removal of all latent heating from the microphysics scheme.

Results show that removing depositional growth yields a weaker and slower-developing storm with much fewer convective bursts and significantly less upper-level warming when compared to the control simulation. Instead, warming near the storm center occurs in close proximity to the freezing level, which is consistent with the latent heat of fusion. In contrast, removal of homogenous freezing produces a stronger storm as compared to the control, with more extensive upper-tropospheric warming. This finding demonstrates that enhanced depositional growth of ice is occurring due to the lack of homogeneous freezing. Since depositional growth releases more heat into the environment, this results in more prominent upper-level warming, and thus, greater magnitude surface pressure falls. Meanwhile, the depositional growth increases buoyancy aloft to accelerate updrafts and shift the peak level to higher levels, leading to the development of more convective bursts. It is also found that convective development and its placement within the African Easterly wave varies significantly between experiments, which in turn impacts the development of a storm-scale outflow and the accumulation of upper-tropospheric warming near the storm center. It is concluded that depositional growth plays an important role in generating convective bursts warming in the upper troposphere, thereby accelerating TCG.