The South Carolina Flood of October 2015: Moisture Transport Analysis and the Role of Hurricane Joaquin

Abstract

AbstractRecord-setting rainfall occurred over the state of South Carolina in early October 2015, with maximum accumulations exceeding 500 mm. During the heavy rainfall, Hurricane Joaquin was located offshore to the southeast of the flooding event. Prior research, storm summaries, satellite imagery, and media accounts suggest that Joaquin played a major role in the flooding, mostly through the provision of additional water vapor. Here, numerical simulations are utilized to elucidate Joaquin?s role in the flooding and to diagnose moisture transport mechanisms. The South Carolina precipitation event and the track of Hurricane Joaquin are reasonably represented by two control simulations, a 36-km simulation without nesting and another with 12- and 4-km nests added; the latter improves upon a negative intensity bias for Joaquin. A band of intense moisture transport into the flooding region is associated with a narrow, diabatically produced cyclonic lower-tropospheric potential vorticity (PV) maximum. Simulations in which Joaquin is removed exhibit a similar moisture transport mechanism and also produce a band of heavy precipitation, though the axis of heaviest precipitation shifts northward into North Carolina, and there is a modest reduction (~7%) in area-averaged rainfall. Removing Joaquin produces negligible changes in regional total water vapor content but diminished upper-tropospheric diabatic outflow. The diminished outflow allows greater eastward progression of an upper-level trough, consistent with the northward precipitation shift and with weaker forcing for ascent. Changes in the upper jet associated with Joaquin appear to exert a greater influence on the flooding event than Joaquin?s contribution to water vapor content.