7.2 Aerosol indirect effects on cold pools and the feedbacks to subsequent convective development

Tuesday, 18 August 2009: 10:45 AM
The Canyons (Sheraton Salt Lake City Hotel)
Susan C. van den Heever, Colorado State University, Fort Collins, CO

Enhanced aerosol concentrations are often associated with a decrease in the surface precipitation through their suppression of the warm rain process. Such changes to the surface precipitation are likely to have an impact on the characteristics of the evaporatively-generated cold pool produced by the storm, thereby affecting subsequent convective development and the resultant surface precipitation. The response of the cold pool to enhanced aerosol concentrations may therefore at times offset the effects of aerosol indirect forcing on precipitation through the cold pool's influence on the organization of subsequent convection. The relationship between aerosol indirect effects on precipitation characteristics and the associated dynamic forcing of the cold pool has been investigated through the use of several cloud-resolving simulations of convective storms over Florida.

Toward the end of NASA's CRYSTAL-FACE field campaign conducted over Florida during July 2002, high concentrations of Saharan dust, which can serve as cloud condensation nuclei and ice nuclei, were observed over the peninsula of Florida. Cloud-resolving model simulations have been conducted using the Regional Atmospheric Modeling System (RAMS) to investigate the impacts of varying aerosol concentrations on the characteristics of the cold pools produced by the convection developing over the Peninsula. Aerosol concentrations are prognosed within RAMS making it an extremely useful model with which to conduct these simulations. The model was initialized using 40km Eta data. Four nested grids were employed with grid spacing of 500m on the finest grid. A two-moment bulk microphysics parameterization scheme with cloud, rain and five ice species was used, and LEAF-2 was utilized to represent the surface processes. The aerosol field was initialized with vertical profiles of both clean and high aerosol concentrations observed during the CRYSTAL-FACE field campaign. The simulations were run for a 12 hour time period. The results of the clean and high aerosol cases were compared.

The simulations show that while enhanced aerosol concentrations do cause a decrease in the surface precipitation throughout much of the life cycle of the storms, there are significant time periods when the precipitation produced by the high aerosol case is greater than that of the clean case. It is during these times that the more intense cold pools and associated gust fronts of the storms in the clean case have outrun their associated updrafts, resulting in a weakening of the storm systems and a concomitant decrease in surface precipitation. In the high aerosol case the updrafts tend to remain co-located with their gust fronts resulting in relatively steady storm systems over a period of several hours. These steady storm systems in the high aerosol case produce greater amounts of surface precipitation than in the clean case until such time as storms re-develop in the clean case. The details of the aerosol indirect effects on the microphysical and precipitation characteristics of these convective storms, their resultant impacts on the cold pool characteristics and dynamical forcing of convection, and the subsequent influence on surface precipitation will be presented.

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