The Sensitivity of Hurricane Irene to Aerosols and Ocean Coupling: Simulations with WRF Spectral Bin Microphysics

Hurricane Irene (2011) moved northward along the eastern coast of the United States and was expected to cause severe wind and flood damage. However, the hurricane weakened much faster than was predicted. Moreover, the minimum pressure in Irene occurred, atypically, about 40 h later than the time of maximum wind speed. Possible reasons for Irene's weakening and the time shift between maximum wind and minimum central pressure were studied in simulations using WRF with spectral bin microphysics (WRF-SBM) with 1-km grid spacing and ocean coupling. Both ocean coupling and aerosol distribution/concentration were found to influence Irene's development. Without ocean coupling or with ocean coupling and uniform aerosol distribution, the simulated maximum wind occurred at about the same time as the minimum pressure. With ocean coupling and nonuniform spatial aerosol distributions caused by aerosols from the Saharan air layer (band) and the continental United States, the maximum wind occurred about 40 h before the simulated minimum pressure, in agreement with observations. Concentrations of aerosols of several hundred per cubic centimeter in the inner core were found to initially cause convection invigoration in the simulated eyewall. In contrast, a weakening effect dominated at the mature and the decaying stages, when aerosols from the band and land intensified convection at the simulated storm's periphery. Simulations made with 3-km instead of 1-km grid spacing suggest that cloud-scale processes interactions are required to correctly simulate the timing differences between maximum wind and minimum pressure.