Using Observations to Examine Relationships Between Extreme Rainfall Rates and Storm-Scale Rotation in Landfalling Tropical Cyclones

Landfalling tropical cyclones (TCs) can deliver multiple hazards to coastal communities, including tornadoes, flash flooding, storm surge, and violent winds. These hazards often take place in the same locations simultaneously, which can complicate communication by meteorologists, recovery efforts by first responders, and plans of action for sheltering residents. This research seeks to better understand two of these concurrent and collocated hazards (flash flooding and tornadoes) through a close examination of heavy precipitation and storm-scale rotation. For this analysis, Tropical Storm Imelda, which impacted the Texas and southwest Louisiana coasts in September 2019, is assessed. First, a synoptic and mesoscale analysis of the system using operational data is included to demonstrate how Imelda delivered excessive rainfall and high precipitation rates for several days across the region. Then, a closer look at observations is taken to further assess the short-term heavy precipitation rates in the system. A high-density network of roughly 100 gauges in east Texas that provides rainfall data every five minutes allowed for a somewhat rare opportunity to assess exceptional rainfall rates on very fine spatial and temporal scales. Several locations reported one-hour precipitation totals well over 100 mm, which contributed to storm totals in excess of 1000 mm at some locations. Radar observations and mesoscale analyses reveal several periods of extreme rain rates, including one with rainbands soon after Imelda's landfall, and another the next day associated with training convection to the south of the remnant TC. This observational analysis will also be compared to previous work that has used numerical simulations to show that cells with low-level rotation tended to increase the strength of low-level updrafts and enhance rainfall production. Given these findings and additional previous research, it is hypothesized that the operational data and numerical simulation will show that the short-term heavy rain-producing convection in Imelda will tend to have collocated storm-scale rotation that enhances the rain rates. By diagnosing the processes associated with extreme rainfall in landfalling TCs, these findings have the potential to be translated into improvements in short-term flash flood forecasting.