Tropical Storm Erin (2007): A polarimetric radar analysis of eyewall and rainband convection during overland re-intensification

Tropical Storm Erin made landfall on the Texas coastline as a weak (18 m/s maximum sustained wind) tropical storm on 16 August 2007. After landfall, it continued tracking northwestward into west Texas as a tropical depression before degenerating further and tracking back northeast towards Oklahoma. While passing over central Oklahoma on the evening of 18-19 August 2007, Erin's remnants unexpectedly re-intensified and developed an eye-like feature that was clearly discernable in WSR-88D radar imagery. During this brief re-intensification period, the center of the low (as well as the newly-formed eye) traversed a dense region of surface and remote sensing observation networks that provided abundant data of high spatial and temporal resolution. In this study, we examine data from the polarimetric WSR-88D KOUN radar in an attempt to compare the structure of the convection to that of tropical storms that have been observed over open water, as well as to better understand the possible role of the eyewall convection in the re-intensification process. The polarimetric structure of its most prominent rainband will also be discussed.

The re-intensification of Erin was atypical since it occurred well inland, about 800 km from the location of its landfall, and attained stronger sustained winds (25 m/s with isolated gusts as high as 37 m/s) and a lower central pressure than while over water. In total, Erin produced widespread rainfall of 75-175 mm over central Oklahoma, with some reports reaching as high as 250 mm. Although the system produced strong surface winds and heavy rainfall, the National Hurricane Center did not classify it as a tropical storm due to its transient lifespan. Initial observations of the radar data, however, reveal several similarities to those documented in tropical cyclones over open water. For example, the eyewall convection of Erin during the period of re-intensification is noted to slope outward with height. Furthermore, a region of intense convection that formed during the re-intensification process appears to be similar to the “convective bursts” that have been related to tropical storm intensification over open water. A closer examination of one particular “convective burst”, which appeared to go through a growth and decay process as it rotated around the eyewall, is presented in order to examine the structure of the deep convective updrafts (as indicated by columns of high ZDR) and downdrafts. Electrical characteristics of the system will also be examined using data from the Oklahoma Lightning Mapping Array.