25th Conference on Hurricanes and Tropical Meteorology


Environmental Vertical Wind Shear and Tropical Cyclone Intensity Change Utilizing Enhanced Satellite Derived Wind Information

Gregory M. Gallina, Univ. of Wisconsin/CIMSS, Madison, WI; and C. S. Velden

The qualitative effects of vertical wind shear on a tropical cyclone (TC) structure and intensity change are well known within the tropical community. However, specific quantitative relationships remain elusive, partly due to the lack of consistent observations of the wind field near TCs over the data sparse oceanic regions.

The University of Wisconsin Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS) has developed high density wind observations using multi-spectral geostationary satellite imagery. Utilizing the wind information provides added information on vertical wind structure in the TC environment. CIMSS currently produces a wind shear analysis, which includes the assimilation of the satellite-derived winds. A 3-year archive of high-resolution analyses has been compiled to determine quantitative thresholds of shear versus time derivatives of TC intensity tendencies as a function of TC strength and potential intensity. These analyses were used to examine similarities and differences in wind shear roles in the North Atlantic and Western North Pacific basins.

As expected, results have indicated a strong statistical relationship between vertical shear and subsequent TC intensity change. The quantitative results correspond nicely with the widely accepted qualitative relationship, where higher (lower) vertical wind shear in the TC environment leads to weakening (strengthening) of the TC. The same statistical relationship is also found for storms of different strength and thermodynamical potentials. These results were determined by stratifying the database by best-track pressure information and Thermodynamical Potential using Maximum Potential Intensity (MPI). Storms that are strong or close to their thermodynamical maximum tend to weaken but they weaken more (less) rapidly in a high (low) shear environment. Similarly, weak or storms that are far from their thermodynamical maximum strengthen at different rates dependent on shear magnitudes.

Further statistical tests show discrete time-lags between the onset of shear and TC intensity change, as suggested by recent idealized modeling studies. The time lag is longer (~ 24h) for more intense (well-developed) storms experiencing lower magnitudes of shear (7-8 m/s), while the lag is only 12-18h for weaker storms in higher shear environments.

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Session 3C, Tropical Cyclone Intensity Change II (Parallel with Sessions 3A, 3B, & 3D)
Monday, 29 April 2002, 4:00 PM-5:30 PM

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