How Does Hurricane Edouard (2014) Evolve toward Symmetry before Rapid Intensification? A High-Resolution Ensemble Study

Abstract

Abstract A 14-member high-resolution ensemble of Edouard (2014), a moderately sheared tropical storm that underwent rapid intensification (RI), is used to determine causes of vortex alignment and precipitation symmetry prior to RI. Half the members intensify similarly to the NHC’s best track, while the other seven ensemble members fail to reproduce intensification. Analyses of initial conditions (vertical wind shear, sea surface temperatures, relative humidity, vortex structure) reveal that lower humidity and weaker, more tilted vortices in nonintensifying members likely increase their susceptibility to boundary layer flushing episodes. As the simulations progress, vortex tilt, inner-core humidity, and azimuthal variations in the structure of precipitation best differentiate the two ensemble subsets. Although all members initially are slowly intensifying asymmetric storms, the RI members are unique in that they have more persistent deep convection downshear, which favors vortex alignment via the stretching term and/or precession. As deep convection transitions to stratiform precipitation and anvil clouds in the upshear quadrants, evaporation and sublimation of condensate advected from the downshear quadrants moisten the middle to upper troposphere. This is hypothesized to promote an increase in precipitation symmetrization, a necessary precursor for RI.