| Despite 13 ms-1 ambient wind shear, Tropical Storm (TS) Gabrielle underwent 22 hPa pressure falls within 3 hours prior to 0900 UTC 14 September. The rapid intensification of Gabrielle was atypical due to an upper-level trough approaching from the northwest. Additionally, a convective cell developed in the downshear left quadrant of the center, moved cyclonically and inward to the 17 km radius. During this time, The Tropical Rainfall Measurement Mission (TRMM) had one of its most intense 85 GHz scattering measurements ever (Molinari and Vollaro, 2010).;This study utilizes WRF version 3.2 ARW core for various model simulations. Each model simulation consisted of 4 nested domains, with the largest and coarsest at 27 km resolution and the finest domain at 1 km resolution. Although the biggest intensity change within the model occurred during a one hour period beginning three hours prior to that of the observed, the overall structure and evolution of TS Gabrielle is quite well forecast. The location of modeled convection is within the downshear left quadrant, much like that of the observed with similar cellular convection structure. Modeled convection was much more intense than that of the observed however with much larger values of reflectivity.;During the hour of most rapid fall in surface pressure, a small low appeared adjacent to intense convection and large vorticity on the downshear side of a broad weak vortex. Over the following hour, this low rotated cyclonically and became more symmetric, with subsidence developing at the center and maximum convection shifting outward to the 20 km radius. Thereafter, vertical motion and vorticity near the storm center were banded rather than cellular. The primary development from a broad disorganized tropical storm with embedded convection to a well-defined small vortex thus occurred within a single hour. The evolving vortex contained dramatic asymmetries in CAPE, with high values downshear right prior to the convective outbreak being consumed by the convection, and near zero values of CAPE to the northwest of the center. In those aspects it resembled a subtropical storm. |
