Evolving Geometries in the Moisture Budgets and Precipitation Structures of US Gulf Coast Landfalling Hurricanes

According to a 2014 publication by former National Hurricane Center director Dr. Edward Rappaport, water-related deaths from inland flooding and storm surge are the leading causes of hurricane-related fatalities in the United States. Large tropical cyclones (TCs), such as Hurricane Katrina (2005), are particularly harmful because they not only impact a broader land area with destructive winds and flooding rains, but also because a more expansive wind field results in a larger storm surge. Observations reveal that TCs of a given intensity vary significantly in size, and there is mounting modeling and observational evidence that supports large-scale moisture as one controlling factor. Thus, the influence of environmental moisture on hurricane morphology is a critical research question, especially during the period around landfall, which has been less studied. In this analysis, we quantify the spatial distribution of moisture in Hurricane Katrina by formulating three shape metrics that encompass characteristic geometries of TCs moving into the mid-latitudes: asymmetry (A), fragmentation (F), and dispersiveness (D). Using satellite-derived rainfall and reanalysis-derived moisture convergence, the geometric patterns are measured every 3 hours, and a moving Mann Whitney U test is applied to determine significant break points in the evolution of moisture. As Hurricane Katrina moves into the central Gulf of Mexico, these shape statistics reveal organization into a highly symmetric, central, and circular structure that persists from 1800 UTC Aug 27 to 1200 UTC Aug 28. Subsequently, significant (p < 0.01) modifications to the moisture budget and precipitation fields are observed beginning at 1200 UTC Aug 28, or 6 hours before the timing of peak intensity (24 hours before landfall). As Katrina approaches the Gulf coastline, a dry slot emerges on the northwest periphery and wraps cyclonically inward, impinging toward the inner core and reducing moisture convergence and precipitation in this region. When we compare these results with other major hurricanes in the 2004-2012 period (n = 11 storms), a distinct pattern emerges with a significant (p < 0.05) number of storms displaying a redistribution to the moisture budget during a window beginning 12-24 hours before landfall. As the TCs approach land and move inland, moisture convergence and precipitation become more dispersed from the TC center but the overall structure maintains a cohesive, although asymmetric, pattern. These results suggest that midlatitude air masses influence TC structure prior to landfall while the inner core is still over warm water. Drier air and vertical wind shear are hypothesized as the main contributors to this reorganization of convection. Based on these findings, TC structural evolution needs to be closely monitored in conjunction with large-scale moisture availability in order to more accurately forecast the scale and spatial distribution of TC wind and water impacts.