The distribution of helicity and intense convection in tropical cyclones

Tropical cyclones embedded in environments of moderate to strong vertical wind shear (5-15 m s-1) have been shown to maintain their intensity, or even strengthen, for 24-48 hours after the onset of shear. The distribution of precipitation in these storms is highly asymmetric with intense eyewall and inner rainband convection preferentially occurring downshear and left of the ambient vertical wind shear vector. Recent observational studies employing dropsonde data have revealed that helicity (the scalar product of the three-dimensional velocity and vorticity vectors), convective available potential energy (CAPE) and radial inflow are also maximized in the downshear semicircle, with values of helicity often exceeding those associated with continental severe weather outbreaks. It was hypothesized that deep, intense convective cells, so called vortical hot towers (VHTs), that repeatedly, and preferentially, form in the downshear left quadrant, help the tropical cyclone vortex maintain its intensity in the presence of strong vertical shear.

Two limitations of the observational studies noted above are the absence of dropsonde data within the vital inner core region and the scarcity of three-dimensional kinematic data with fine enough temporal resolution to evaluate the proposed hypotheses regarding VHTs and intensity change. To address these issues, we employ model output from NCAR's Advanced Hurricane WRF (AHW) model at 1.33 km resolution to fully document the azimuthal and radial distributions and evolutions of helicity, CAPE, and intense convection in tropical cyclones embedded in environments with a variety of magnitudes and directions of vertical wind shear. By examining a large number (~10) of simulated storms, it is anticipated a greater understanding of the dynamics of tropical cyclone vortices in shear will be gained.