Using Trajectories to Study the Thermodynamic Characteristics of Extreme Updrafts Simulated in Hurricane Wilma (2005)

Hurricane Wilma (2005) underwent a record-breaking 18-hour rapid intensification (RI) on 18-19 Oct, featuring a minimum central pressure (P_MIN) deepening rate of 83 hPa (12 h)^-1 which lead to a peak P_MIN and maximum surface wind of 882 hPa and 82 m s^-1, respectively. At peak intensity, Wilma (2005) was the strongest hurricane ever recorded in the Atlantic basin. A 1-km Weather Research and Forecasting (WRF) model prediction of Wilma captures convective bursts (CBs), with vertical velocity peaking above 15 m s^-1 in the upper troposphere, that rotate cyclonically through the eyewall prior to and during RI. Similar features have been documented in observational studies of other tropical cyclones (TCs), and it is possible that they facilitate TC intensification through their compensating subsidence into the eye and associated central pressure falls. The thermodynamics and three-dimensional structure of TC convective bursts remain poorly understood. While wind-induced heat fluxes above warm sea surface temperatures (SSTs) provide an ample source of high-equivalent potential temperature air to a TC boundary layer, excessive hydrometeor loading, warming from latent heating, and the entrainment of surrounding dry air could all, at least in theory, render the eyewall a hostile environment for maintaining buoyant updrafts. The complex three-dimensional structure and relatively rapid temporal evolution of Wilma’s CBs, relative to vortex-scale processes, motivates a Lagrangian approach to studying their thermodynamics. When parcel trajectories are computed from numerical model output, computational stability constraints typically require that the integration be performed over a time step significantly smaller than the data output interval. Traditionally, linear time interpolation is used to estimate the three-dimensional wind field at the trajectory computation times. However, large errors may be incurred by this method when estimating nonlinearly evolving flow fields if the model output time interval is too coarse. Here, we develop a new trajectory model tailored for TC applications that accounts for the advection of the vertical velocity field in the time interpolations. After performing validations of this new technique using analytical tests and model data from two TC cases, we show how it can be used to trace Wilma’s CBs in three dimensions to portions of the boundary layer characterized by higher ocean surface latent and sensible heat fluxes, relative to nearby regions.