Dropsonde composites of asymmetric hurricane boundary layer structure in relation to environmental vertical wind shear

This talk presents the asymmetric structure of the hurricane boundary layer in relation to the environmental vertical wind shear in the inner core region. GPS dropsonde data deployed by research aircraft in 19 hurricanes are analyzed in a composite framework. Kinematic structure analyses based on Doppler radar data from 96 eyewall penetration legs are compared with the dropsonde composites. Shear-relative quadrant-mean composite analyses indicate that both the kinematic and thermodynamic boundary layer height scales tend to decrease with decreasing radius, consistent with previous axisymmetric analyses. There is still a clear separation between the kinematic and thermodynamic boundary layer heights. Both the thermodynamic mixed layer and the height of maximum tangential wind speed are within the inflow layer. The inflow layer depth is found to be deeper in quadrants down shear, with the downshear right (DR) quadrant being the deepest. The mixed layer depth and height of maximum tangential wind speed are alike at the eyewall, but are deeper outside in quadrants left of the shear. The results suggest also that air parcels acquire equivalent potential temperature (θe) from surface fluxes as they rotate through the upshear right (UR) quadrant from the upshear left (UL) quadrant. Convection is triggered in the DR quadrant in the presence of asymmetric mesoscale lifting coincident with a maximum in θe. Energy is then released by latent heating in the downshear left (DL) quadrant. Convective downdrafts bring down cool and dry air to the surface and lower θe again in the DL and UL quadrants. This cycling process may be directly tied to shear-induced asymmetry of convection in hurricanes.