Vortex Structural Evolution during Tropical Cyclone Intensification in Moderate Vertical Shear

It has long been known that tropical cyclone (TC) structure and intensity change are highly sensitive to environmental vertical wind shear. A low value of vertical wind shear (i.e., < 5 m s^-1 between 850 and 200 hPa) is considered a favorable environment for intensification, while high vertical wind shear (>10 m s^-1) is generally detrimental to intensification. However, there is a range of moderate shear values, between 5-10 m s^-1, where the response of the TC is uncertain, and it is in this range of shear values where significant forecast uncertainty lies. Whether or not a TC intensifies in this shear environment is dependent on characteristics of the environment as well as the TC vortex.

One aspect of the vortex structure that has received considerable attention recently is precipitation structure, and how that structure varies in a shear-relative framework. Recent aircraft, satellite, and modeling-based studies have identified different modes of precipitation (e.g., deep vs. shallow convection, stratiform precipitation) and linked these modes to the potential for TC intensification. In general, intensification has been associated with a greater azimuthal coverage of precipitation, with the distribution of precipitation (and deep convection in particular) on the upshear side of a TC being identified as a critical determinant of TC intensification. Relationships between the azimuthal distribution of precipitation and TC intensification have invoked efficiency concepts; namely, a greater azimuthal symmetry of precipitation provides a larger projection of diabatic heating onto wavenumber-0, a configuration more efficient for vortex spin-up.

This study will present results from recent and ongoing case studies and composite analyses of airborne Doppler radar and dropsonde data to identify shear-relative vortex structures and how they relate to TC intensification. Attention will be focused on the upshear precipitation distribution and structure, with processes that encourage (or discourage) an enhancement of upshear precipitation being a primary emphasis. Such processes include moistening in the midlevels, surface enthalpy fluxes, variations in the structure of precipitation upshear, and a reduction in vortex tilt. These results suggest a complex interplay between precipitation and vortex structure and TC intensification, with processes that provide a more favorable local environment for the maintenance of convection upshear being an important player.