Vertical velocity and microphysical distributions related to the rapid intensification of Hurricane Dennis (2005)

While airborne radars and satellites continue to provide snapshots of isolated, intense vertical velocities (i.e., convective bursts) within the inner cores of tropical cyclones, statistical understanding of their evolutions with tropical cyclone intensity remains limited. This research uses 10-min output from a 1-km Weather Research and Forecasting simulation of Hurricane Dennis (2005) to characterize statistical distributions of convective bursts, outlying downdrafts, and microphysical cloud properties that evolve with an extreme subset of intensity change – rapid intensification (RI). Two interpretations of RI, which are based on 24-h changes in either a) maximum 10-m wind speed or b) minimum sea-level pressure, yield > 24-h differences in the onset of simulated RI. It is shown that reducing the interval over which thresholds of these variables are examined yields better agreement and focuses the onset of RI on unique inner-core structural morphology and intensification rate parallel to observations. During the few hours preceding this onset of RI, asymmetric convection transforms into an eyewall, in which maximum 10-m wind speed subsequently increases 31 kts in only 6 h.

Contoured frequency by altitude/time diagrams are used to characterize the vertical distributions and evolution of simulated vertical velocity prior to and during RI and refine interpretation of the vertical structure of convective bursts and associated reflectivity. By analyzing distributions of vertical velocity at discrete height levels, relationships between convective bursts and RI are not subject to column-confined averages and arbitrary thresholds commonly used to identify the former. These diagrams show unique changes in the breadth of vertical velocity distributions, and thus the structures of convective bursts, that evolve with RI. Although upper-tropospheric vertical velocity distributions continually broaden (e.g., 99.9th percentile vertical velocity continually increases) during the ~24-h period prior to the onset of RI, lower-to-mid-level distributions exhibit this trend only after the onset of and during RI. While upper-tropospheric vertical velocity distributions transition from broadening to narrowing at the onset of RI, the distribution tails continue to converge toward the vortex center. Decrease in the 99.9th percentile vertical velocity, and thus reduced frequency of convective bursts, during RI appears to be related to enhanced precipitation loading. It is shown that the magnitude, vertical extent, duration, and inner-core proximity of convective bursts and associated microphysical cloud properties exhibit unique trends with some interpretations of RI.