Modeling aerosol impacts on convective storms in different environments

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Sunday, 17 January 2010
Exhibit Hall B2 (GWCC)
Rachel L. Storer, Colorado State University, Fort Collins, CO; and S. C. van den Heever and G. Stephens

Aerosols are known to have both direct and indirect effects on clouds through their role as cloud condensation nuclei. This study examines the effects of differing aerosol concentrations on convective storms developing under different environments. The Regional Atmospheric Modeling System (RAMS) was used to achieve the goals of this study. An initial model sounding was chosen and consistently modified to obtain a variety of CAPE values. Additionally, the model was initiated with varying concentrations of aerosol available to act as cloud condensation nuclei; these concentrations were chosen to represent scenarios ranging from clean to polluted. Model convection was triggered with a warm temperature perturbation and the simulations were run out for multiple hours of model time. The simulations showed a splitting storm with an isolated, long-lived right mover and a left mover that was short-lived and resulted in widespread secondary convection.

Each model run produced long-lived convective storms that had similar storm development, yet differed slightly based on the initial conditions. Runs with higher initial CAPE values produced the strongest storms overall, with stronger updrafts and larger amounts of accumulated surface precipitation. Simulations initiated with larger concentrations of aerosols formed similar storm structures, but showed some distinctive changes due to aerosol indirect effects. These storms showed an increase in cloud droplet number concentration and a decrease in mean cloud droplet diameter, which led to a suppression of precipitation and an increase in cloud water.

While effects on the microphysics of the storm showed clear trends, dynamic effects were harder to deduce. Slight trends were noted in variables such as cloud fraction and updraft speed, however much stronger trends in these variables occurred due to changes in initial CAPE. The strongest dynamic feedbacks existed in the strength and extent of surface cold pools, which led to a higher sensitivity in the secondary convection to changes in aerosol concentration. For the most part, changes in the cold pool due to aerosol indirect effects were on the same order as, or even larger than those due to changes in CAPE. The details of the microphysical and dynamic effects that were seen in the model simulations will be presented.