Hydrometeor Size Sorting in the Asymmetric Eyewall of Hurricane Matthew (2016)

Polarimetric radar observations of Hurricane Matthew's asymmetric eyewall were captured by WSR-88D radars from 1500 UTC on 7 October 2016 to 0000 UTC on 8 October 2016. Raindrop size sorting was observed within the eyewall, marked by a differential reflectivity (Z(DR)) enhancement region situated upwind of a specific differential phase (K-DP) enhancement region, both overlapping the maximum reflectivity. This signature indicated that the largest raindrops fell out of the eyewall updrafts faster than the smaller, abundant drops that were advected further downstream by the primary circulation. Airborne Doppler radar observations revealed an updraft structure in an azimuthal location consistent with the size-sorting signature and previous observational studies of eyewall kinematic asymmetries. Given that a tropical cyclone's environment or internal dynamics can modulate the eyewall's kinematic and microphysical structure, we used a simple size-sorting model that only includes sedimentation and advection of raindrops by the axisymmetric tangential wind to examine how an eyewall size-sorting signature responds to artificial changes in the tangential wind speed and initial raindrop size distributions (DSDs). The axisymmetric tangential wind was retrieved from WSR-88D radar observations using the Ground-Based Velocity Track Display technique. The simple model was capable of producing an eyewall size-sorting signature with an azimuthal separation between the simulated Z(DR) and K-DP enhancements in general agreement with the observed separation (similar to 20 degrees) at low levels. Sensitivity tests showed that the azimuthal separation between the Z(DR) and K-DP enhancements responded to changes in the tangential wind speed, but not to changes in the initial DSDs aloft.