Changes in the Spatial Patterns of Precipitation Bands in Tropical Cyclones During Landfall along the Gulf of Mexico and Atlantic Coasts of the U.S.

Evolutionary periods of intensity change and precipitation distribution in tropical cyclones (TCs) are sometimes misrepresented in numerical weather prediction models due to the rapid nature of TC development and the importance of mesoscale and convective processes. As a result, rainfall totals can be difficult to forecast over short temporal and spatial scales and errors are common within and across forecast models. For example, Hurricane Irma (2017) underwent rapid changes in its approach toward the Florida coast and areas of heavy precipitation were difficult to forecast. The complexity of TC dynamics in the approach to landfall makes our understanding of precipitation structure and its accompanying changes a challenging sub-discipline of tropical meteorology. This study aims to improve our understanding of the evolution of TC precipitation in previous landfalling storms and aid forecasters in the prediction of rainfall totals.

In this study, we quantify structural changes in the spatial distribution of rainbands in 63 landfalling TCs along the Gulf of Mexico and Atlantic coasts of the United States between 1998 and 2014. We utilize the Tropical Rainfall Measuring Mission (TRMM) 3B42 product, which includes 3-hour instantaneous rain-rate measurements at a 0.25° × 0.25° resolution estimated from multiple satellites. Three variables are used to assess spatial changes in TC precipitation bands during the period around landfall; 1) area, 2) radial dispersion, and 3) azimuthal coverage. Calculations are made using two distinct precipitation thresholds, 0.254 mm hr^-1 and 5 mm hr^-1, in order to capture and compare changes in trace and moderate to heavy inner-core precipitation, respectively. Temporally, storms are evaluated 72, 48, and 24 hours prior to landfall; at landfall; and 24 and 48 hours post-landfall for both precipitation thresholds. We also evaluate if there is a correlation between TC precipitation change and landfall location. Precipitation area and radial dispersion are projected to increase on approach to landfall in conjunction with decreasing azimuthal coverage. We also expect there to be a geographical component to the changes we observe in the distribution of rainbands at the upper and lower precipitation thresholds as these storms interact with dry air and baroclinic zones in the mid-latitudes. By better understanding changes in precipitation bands around landfall, operational meteorologists will be better equipped to aid in public preparedness and provide improved rainfall forecasts to emergency management personnel.