Impacts of Enhanced Cloud Condensation Nuclei (CCN) on the Organization of Convection for Monsoon Depressions

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This project seeks to understand the effect of aerosol pollution on monsoon depressions. Monsoon depressions are synoptic-scale systems (2,000km to 3,000km) with baroclinic (or tilted) vertical structure which propagate westward, forming in the Bay of Bengal and producing as much as 150 to 200 mm of rainfall during their passage over any location. They occur during the monsoon season, when the vertical shear of the zonal wind is strong enough to inhibit the formation of hurricane-strength tropical cyclones, and they are distinct from weaker but similar systems called "monsoon lows", which have a barotropic (or upright) vertical structure. The Principal Investigator argues that the heavy aerosol pollution found over the Indian sector causes a reduction in the frequency of occurrence of monsoon depressions. This claim is based on several lines of evidence: first, there has been a dramatic reduction in the occurrence of monsoon depressions, from an annual average of 7 to 8 historically to 1, 0, and 2 in the years 2009, 2010, and 2011. Second, aerosol pollution has increased dramatically over the Indian sector in recent years. Third, global forecast models, which presumably do not account for the change in aerosol pollution, have overpredicted monsoon depressions in the recent years with few depressions. Fourth, preliminary experiments with the Weather Research and Forecasting (WRF) model show that the strength of monsoon depressions, and the intensity and organization of precipitation within them, can be dramatically reduced by increasing the aerosol burden from relatively pristine (5x10^8 particles per cubic meter) to heavily polluted (5x10^10 per cubic meter) values. The research is based on the hypothesis that aerosols suppress monsoon depressions because they slow the formation of rainfall, as more aerosols lead to more cloud condensation nuclei (CCN), which produce a greater number of smaller cloud droplets, which take longer to coalesce into raindrops. Monsoon depressions form through a mutual interaction between the baroclinic/barotropic instability of the large-scale flow and the latent heat of condensation accompanying precipitation within the depression. This mutual interaction can be suppressed by the excess aerosols, which increase time required for the development of precipitation by roughly a factor of two, and thus produce a mismatch between the timescales of the hydrodynamic instability and the organization of precipitation and accompanying latent heat release. Work performed under this award tests the hypothesized suppression of monsoon depressions by aerosol pollution using more sophisticated and realistic simulations, and also performs analysis of aerosol forcing, based on satellite data, during periods when monsoon depressions form and when they are absent.The suppression of monsoon depressions addressed in this research is of both scientific and societal interest, given the large population that is affected by them. If successful, the results of this work could be directly applied to operational forecasting systems used to predict these weather systems. In addition, the project provides support and training for a postdoc and a graduate student, thereby providing for the next generation of scientists in this research area.