Vertical Wind Shear Influences on Tropical Cyclone Formation and Intensification during TCM-92 and TCM-93

Abstract

Vertical wind shears between 200 and 850 mb are calculated from operational analyses and special interactive analyses for Tropical Storm Steve during the Tropical Cyclone Motion (TCM-93) field experiment and for Typhoon Omar at the end of the TCM-92 experiment. The operational Fleet Numerical Meteorology and Oceanography Center (FNMOC) analyses have strong 200-mb winds crossing over the intensifying storms, which leads to vertical wind shears exceeding the 12.5 m s?1 threshold value believed to prevent tropical cyclone intensification. Interactive analyses are produced with the multiquadric interpolation technique that blends composited cloud-drift winds and aircraft reports between 1800 and 0000 UTC and between 0600 and 1200 UTC, sets of synthetic observations to represent missing or mislocated tropical circulations, and the FNMOC analyses that are used as a first-guess field. These interactive analyses indicate that the high winds at 200 mb associated with low-latitude circulations such as monsoon depressions or other tropical cyclones that appear to be impinging on Steve and Omar are actually deflected around the convective outflows. Vertical wind shears calculated from the interactive analyses are well below the threshold vertical wind shear value, which is consistent with the observed intensification of Steve and Omar. In seven of the nine Steve analyses, the insertion of the composite observations alone resulted in deflected flow around convective outflow, so that the reduced shears are not an artifact of synthetic observation insertions. It is hypothesized that the large vertical wind shears associated with the low-latitude circulations may actually be concentrated in a shallow layer that may be opposed and deflected by a similar shallow layer of convective outflow above developing and intensifying tropical cyclones. In that case, an understanding of the role of vertical wind shear and prediction of tropical cyclone intensification may require special analyses of the type developed following the TCM-93 experiment.