Analysis of gradient wind balance during the rapid intensification of Hurricane Wilma (2005)

Understanding the rapid intensification (RI) of Hurricane Wilma (2005) in terms of the maximum wind has been carried out in a series of papers, this study examines the gradient wind balance and imbalance during RI, based on a 72-h Wilma prediction conducted using the Weather Research Forecast Model (WRF) with the 1-km grid resolution. Results show that the pressure gradient force (PGF) near the eyewall increases rapidly during the RI period. The maximum PGF associated with the gradient wind balance is determined by the strong radial gradient of the thermal field near the eyewall, resulting from the warming in the eye and adiabatic cooling near the eyewall. The maximum PGF occurs inside the radius of the maximum wind, causing a convergence region and intense updrafts, which favors RI. In a balanced symmetric framework, the hydrostatic PGF accounts for a major fraction (70 %–90 %) of the predicted PGF, and the secondary circulation is underestimated by the Sawyer-Eliassen equation within the boundary layer, at upper levels and through the intense eyewall updrafts where gradient wind imbalance occurs. The unbalanced force accelerates the boundary-layer radial inflow that contributes to the inward contraction of the eyewall and then enhances the radially inward advection of high absolute angular momentum. The eyewall convection strengthens where the boundary-layer inflows, convergence, and heat and moisture flux are collocated. With the high inertial stability, a sufficient conversion efficiency from energy to kinematic energy favors the RI of Wilma.