Using Airborne Doppler Radar Data to Examine Eyewall Angular Momentum Budgets

A mesoscale budget analysis of absolute angular momentum (AAM) in the hurricane eyewall is presented using airborne dual-Doppler wind data from Hurricane Guillermo (1997). Multiple consecutive passes were made through the storm, allowing observed and budget-estimated changes to be directly assessed. Although the budget-estimated tendency is not in agreement with the observed tendency, the individual budget terms do show consistency with previous numerically based AAM budgets. The accuracy of the Doppler-derived wind field impacts the budget analysis; errors associated with the Doppler data are analyzed in depth. Aspects of the data processing and Doppler synthesis procedure that most directly impact the budget are identified (e.g., interpolation, mass continuity constraint, and sampling strategy). In order to more directly compare the Doppler-based budget with numerically based results, the Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) fifth generation non-hydrostatic Mesoscale Model (MM5) is used to perform an AAM budget analysis of a simulated hurricane. The numerically based budget is used to examine the impact of temporal resolution of the data on the budget, and to provide an additional context for examining the role of eddies in the Doppler-based budget. Consistent with recent mesoscale numerical budget studies, the eddy terms in the Doppler-based and numerically based AAM budgets are important in the eyewall and cannot be neglected. Contributions to the eddy flux convergence, and thus local changes in symmetric AAM, appear to be related to internal processes like mixing between the eye and eyewall, as well as the interaction of eyewall vorticity asymmetry with the environmental flow.