Simulation of a landfalling hurricane using spectral bin microphysical model: effects of aerosols on hurricane intensity (the HAMP contribution)

The evolution of a super hurricane (Katrina, August 2005) was simulated using the Weather Research and Forecasting Model with explicit (non-parameterized) spectral bin microphysics (SBM). The new computationally efficient spectral bin microphysical scheme (FAST-SBM) implemented in the WRF calculates at each time step and in each grid point size distributions of atmospheric aerosols, water drops, cloud ice (ice crystals and aggregates) and graupel/hail. The hurricane evolution was simulated during 72 hours beginning with its bypassing the Florida coast (27 Aug 2005) to its landfall just east of New Orleans, Louisiana (near the end of 29 August). The WRF/SBM was used to investigate the potential impact of aerosols ingested into Katrina's circulation during its passage through the Gulf of Mexico. It is shown that continental aerosols invigorated convection largely at Katrina's periphery, which led to its weakening prior to landfall: Maximum weakening took place ~24 h before landfall, just after its intensity had reached its maximum. The minimum pressure increased by ~15 mb, and the maximum velocity decreased up to 15 m/s. Thus, the model results indicate the existence of another (in addition to a decrease in the surface fluxes) mechanism of weakening of hurricanes approaching the land. This mechanism is related to the effects of continental aerosols ingested into the hurricane's circulation. It is shown that aerosols substantially affect the spatial distribution of cloudiness and hydrometeor contents. The evolution of lightning structure within the simulated Katrina is calculated and compared with that in Katrina. The physical mechanisms of aerosol-induced TC weakening are discussed.