Investigation of the Dynamics of Extreme Rainfall in Landfalling Tropical Cyclones

Tropical cyclones are prime examples of how multiple hazards associated with distinctive, dangerous weather phenomena (e.g., flash flooding, storm surge, and tornadoes) can often occur in the same place at the same time. Previous research and real-world events (e.g., Hurricane Harvey in 2017) have identified how concurrent, collocated flooding and tornado threats (referred to hereafter as TORFF events) in tropical cyclones present particular forecasting and communication challenge for civil authorities. This is especially true given the opposite recommended life-saving action for each threat.

Recent research by the authors has identified the ability for near-surface rotation in strong low-level vertical wind-shear to enhance rainfall rates through dynamical pressure effects from the rotation. Specifically, the rotationally induced pressure perturbations serve to enhance the low-level updrafts and lift thermodynamically stable parcels that still contain moisture and instability, which in turn enhances the rain rates over non-rotating storms and promotes tornado formation. This provides a dynamical explanation for the prevalence of TORFF events established previously.

Considering that past studies of tropical cyclones have shown the simultaneous occurrence of intense low-level shear, tornadoes, and extreme rain rates, the authors hypothesize that similar rotational enhancement mechanisms, as described above, exist in landfalling tropical cyclone rain bands. This research presents results of the rainfall associated with Hurricanes Harvey (2017) and Florence (2018) with special attention given to the mechanisms supporting intense short-term rainfall production and tornadoes. The operational multi-radar multi-sensor (MRMS) derived rotation tracks and gauge corrected QPE along with a local dual-polarization radar analysis is used to examine the observed frequency of extreme rain-rate and rotation collocation in these two storms, examine the vertical strength and depth of the mesoscale rotation, and characterize the radar identified microphysical characteristics of the rainfall. The observational analysis is supplemented by a high-resolution model simulation of Hurricane Harvey, which allowed a more detailed examination of the influence of the rotation on the dynamics and microphysics of the extreme rainfall producing systems. The results of this research are presented along with a discussion of their possible implications on the forecasting process for tropical cyclone based TORFF events.