Radial and Azimuthal Variations in Convective Burst Structure in Tropical Cyclones from Airborne Doppler Observations

The structure and azimuthal variation of convective bursts (CBs; vertical columns with an updraft >= 5.5 ms-1) in tropical cyclones (TCs) are examined using airborne Doppler observations collected from NOAA P-3 aircraft missions. In general, the upward forcing of the bursts causes subsidence at most levels radially inward and outward of the burst center. Enhanced reflectivity at all levels is located below and radially inward of the intense updrafts. The updrafts cause a positive vertical vorticity perturbation radially inward and negative perturbation radially outward indicating that tilting is the main generation mechanism. At the top of CBs there is enhanced divergence due to the weakening of the updrafts.

CB structure and its variation as a function of shear-relative quadrant are then considered for intensifying (IN; intensity increase of at least 20 kt (24 h)-1) and steady-state (SS; intensity remaining between ±10 kt (24 h)-1)TCs. Previous studies using satellite observations show the azimuthal distribution of high altitude reflectivity to be different for IN and SS storms, with the largest differences found in the upshear quadrants. Analysis of CB three dimensional structure shows higher maximum reflectivity values and stronger updrafts occurring at higher altitudes for IN storms in all shear-relative quadrants, with the greatest differences being upshear. The distribution of peak updraft strength for IN storms is more axisymmetric than SS storms due to enhanced weakening of the CBs upshear. Therefore, the presence of CBs upshear can be an added tool for forecasters to determine if a TC will undergo RI.