Mesoscale Distribution of Intense Rainfall in Eastern North Carolina during Hurricane Matthew (2016)

Hurricane Matthew approached the Southeastern United States on 7 October 2016 and its center tracked within range of NEXRAD radar until 9 October, when it moved eastward away from North Carolina. The extensive NEXRAD dataset captured many stages of the lifecycle of Matthew as it tracked northward along the coast. Herein, we focus on the characteristics of rainfall in the final stage of its life cycle, as it underwent extratropical transition along the coasts of North and South Carolina on 8 and 9 October. A storm total of over 100 mm of rainfall occurred over a wide region of eastern North and South Carolina, and of a particular interest, a 50–100 km wide corridor of extremely large rainfall amounts in excess of 300 mm was observed from northeastern South Carolina extending through inland eastern North Carolina and ending in southeastern Virginia.

Consistent with many previously documented cases of tropical cyclones approaching the southeastern US, synoptic-scale forcing was favorable for ascent over the eastern half of North Carolina. Investigating finer scale features reveals that the corridor of extreme precipitation was focused along a baroclinic zone that developed near the coast of North Carolina on 8 October at the boundary of warm, moist oceanic air advected inland by the cyclone and cooler near-surface air present inland. At the time, a surface high pressure over New England would have potentially induced cold air damming over interior parts of North and South Carolina, which might have helped to intensify the magnitude of the temperature gradient along the coastal front. The relative roles of the cold air damming and the circulation of the cyclone superimposed onto a meridional temperature gradient in developing the coastal front is explored using regional model simulations in which the Appalachian Mountains are removed or the 2D temperature forcing fields are homogenized.

Convective echoes (as classified by Powell et al. [2016]) in Matthew took on different characteristics based on whether they resided over the ocean (in the warm sector) or over land (along the baroclinic zone). Ocean echoes had a bimodal distribution in echo top height; one mode occurred near 4 km, and the other at about 10 km. Convective echoes over land were almost exclusively in the deep mode. On average, radar reflectivity for oceanic convection was lower than that of terrestrial convection at similar altitudes. Convective echoes were tracked inland from the ocean, and their dual-polarimetric characteristics were captured as they approached the baroclinic zone. In the 90 minutes before convective echoes make landfall, only small changes in reflectivity are observed. However, distinct changes in ZDR were observed. 90 minutes before landfall, ZDR in convective echoes exceeded 1 dB up to 6 km. The depth of the high ZDR region steadily decreased as echoes neared land. In contrast, land based echoes exhibited more vertically uniform ZDR of 0.4–0.6 dB below the 0ºC level. A possible explanation is that more numerous smaller drops occur over land where echoes encounter aerosol concentrations that are larger than those present over the ocean.