VERTICAL WIND SHEAR INFLUENCES ON TROPICAL CYCLONE FORMATION AND INTENSIFICATION DURING TCM-92 AND TCM-93

Vertical wind shears between 200 and 850 mb are calculated from operat ional analyses and special interactive analyses for Tropical Storm Ste ve during the Tropical Cyclone Motion (TCM-93) field experiment and fo r Typhoon Omar at the end of the TCM-92 experiment. The operational Fl eet Numerical Meteorology and Oceanography Center (FNMOC) analyses hav e strong 200-mb winds crossing over the intensifying storms, which lea ds to vertical wind shears exceeding the 12.5 m s(-1) threshold value believed to prevent tropical cyclone intensification. Interactive anal yses are produced with the multiquadric interpolation technique that b lends composited cloud-drift winds and aircraft reports between 1800 a nd 0000 UTC and between 0600 and 1200 UTC, sets of synthetic observati ons 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-l atitude circulations such as monsoon depressions or other tropical cyc lones that appear to be impinging on Steve and Omar are actually defle cted around the convective outflows. Vertical wind shears calculated f rom the interactive analyses are well below the threshold vertical win d shear value, which is consistent with the observed intensification o f Steve and Omar. In seven of the nine Steve analyses, the insertion o f the composite observations alone resulted in deflected flow around c onvective outflow, so that the reduced shears are not an artifact of s ynthetic observation insertions. It is hypothesized that the large ver tical wind shears associated with the low-latitude circulations may ac tually be concentrated in a shallow layer that may be opposed and defl ected by a similar shallow layer of convective outflow above developin g and intensifying tropical cyclones. In that case, an understanding o f the role of vertical wind shear and prediction of tropical cyclone i ntensification may require special analyses of the type developed foll owing the TCM-93 experiment.