Tropical Cyclone Formation in Environments with Cool SST and High Wind Shear over the Northeastern Atlantic Ocean*
Weather and forecasting
This work analyzes the environment conditions in which tropical cyclones have formed during the months of October, November, and December in the northeastern Atlantic Ocean. The study period begins in 1975, the year of publication for the Hebert-Poteat and Dvorak techniques for satellite classification of tropical and subtropical systems, respectively. The northeastern Atlantic Ocean is defined as the portion of the Atlantic basin north of 20°N and east of 60°W. Purely subtropical storms were
... pical storms were excluded from the study to focus on the conditions for tropical cyclone formation. Genesis was defined as the time in the official HURDAT record when the cyclone 1) had been classified as tropical, and 2) had maximum sustained winds of at least tropical storm force (greater than or equal to 34 kt). NCEP/NCAR Reanalysis data were used for atmospheric variables, and Reynolds SST and NOAA ERSST data were used for sea surface temperatures. Dynamic and thermodynamic parameters were analyzed to determine the environmental conditions during the period prior to genesis. Wind shear (magnitude and spatial orientation), vertical temperature profiles and stability indices were computed on 6-h intervals for the thirty hours prior to genesis. Synoptic geopotential height patterns were analyzed on 12-h intervals for the thirty-six hours prior to genesis. iii Seventeen of the twenty tropical cyclones in the study set had identifiable nontropical precursors. These seventeen were subdivided into non-frontal baroclinic (NFB), frontal-weak (FW), and frontal-strong (FS) types based on the relative strength of the low-and mid-level vorticity maxima thirty hours prior to genesis. The three tropical cyclones with identifiable tropical precursors were observed to have the weakest 500 hPa vorticity maxima, consistent with the tropical wave structure. Sixteen of the seventeen storms from non-tropical precursors developed over sea surface temperatures less than 26.5°C. Local environmental wind shear varied widely among the twenty storms. Large ranges in magnitude and disparate spatial patterns were observed even among types. Wind shear was therefore determined to have secondary importance in the genesis environment. Geopotential height fields also had few discernable patterns within types. Thermodynamic variables showed distinct differences between NFT storms and the nontropical types. Vertical temperature profiles for NFT systems were consistently warmer at all levels than the non-tropical systems. Thus storms originating from tropical precursors occurred in environments with larger convective available potential energy and higher equilibrium levels relative to those for non-tropical precursors. Static stabilities were lower for the NFT type relative to the baroclinic types when using 300 hPa as the upper level. This combination of cool sea surface temperature, moderate instability, and low equilibrium levels in the pre-genesis environments suggests that tropical cyclones from baroclinic precursors in this region are shallower than typical tropical cyclones, which would reduce the effects of high environmental shear. iv Dedication To my mother, Dr. Jacqueline Kern, who thought it was fantastic that her tenyear-old daughter wanted to go to graduate school in atmospheric science: I would not have accomplished it without your support.