Thermodynamic and Kinematic Influences on Precipitation Symmetry in Sheared Tropical Cyclones: Bertha and Cristobal (2014)

Two cases of intensifying sheared tropical cyclones, Bertha and Cristobal (2014), are examined using airborne observations.  Both TCs exhibited a distinct azimuthal wavenumber-1 precipitation asymmetry, with the maximum occurring in the downshear to downshear-left region, consistent with many prior studies.  Both TCs also featured convergence and precipitation occurring within the high inertial stability region inside the radius of maximum wind, a configuration known to be associated with TC intensification.  However, during their respective intensification periods, Bertha’s convection was more symmetrically distributed with relatively modest vertical motions, while Cristobal’s convection was more azimuthally confined with more vigorous vertical motions.

It is hypothesized that in Bertha, enhanced surface winds and latent heat fluxes left of shear helped counteract convective downdrafts that transported low entropy air into the boundary layer, contributing to a more symmetric precipitation distribution.  In contrast, the more-vigorous downdrafts associated with the intense downshear convection in Cristobal decreased the boundary layer entropy left of shear to a larger degree relative to the azimuthal mean.  This boundary layer entropy decrease combined with subsidence-induced capping in the upshear quadrants helped prevent convective initiation until air reached the downshear region, where the boundary layer entropy had recovered sufficiently via surface enthalpy fluxes.  It is speculated that although intense downshear convection in sheared tropical cyclones can be beneficial through vortex stretching and diabatic heating, it can also act as a limit on TC intensification by cooling the boundary layer over the upshear half of the TC via downdrafts, contributing to the sheared TC’s inability to achieve a more symmetric convective distribution that is associated with most rapid intensification events.