POTENTIAL INDIRECT EFFECTS OF AEROSOL ON TROPICAL CYCLONE INTENSITY: CONVECTIVE FLUXES AND COLD-POOL ACTIVITY

We investigated the impacts of elevated aerosol concentrations on 2008 Western Pacific typhoon (NURI). Based on satellite and global aerosol model output, pollution levels were estimated. The typhoon was simulated for 96 hours beginning 17 August, 2008. During the final 60 hours CCN concentrations were enhanced as it neared the Philippines and coastal China. Early during the ingestion of enhanced CCN, precipitation was reduced due to suppressed collision and coalescence, and storm winds strengthened. Subsequently, greater amounts of condensate were thrust into supercooled levels where the drops froze resulting in greater latent heating. Convection thereby intensified which resulted in enhanced rainfall and more vigorous convectively-produced downdrafts. As the convection intensified in the outer rainbands the storm drifted over the developing cold-pools, and low level wind speeds diminished. Very high amounts of pollution aerosols resulted in large amounts of condensate being thrust into the storm anvil which weakened convective downdrafts and cold-pools, yet reductions in wind speed (although weaker) occurred compared with the clean control run. While the simulated storm was weaker than the observed storm, making it a borderline tropical storm or typhoon, the simulated response to aerosols was much different than anticipated based on previous simulations of aerosol impacts on tropical cyclones. In particular, the storm strengthened during the first period of aerosol ingestion followed by an extended period of storm weakening. Nonetheless, this study suggests that ingestion of elevated amounts of CCN into a tropical cyclone (TC) can appreciably alter the intensity of the storm. However, the pollution aerosols have very little impact on the storm track.