Impacts of Forest Fire Smoke Aerosols on Deep Convective Clouds in WRF-CHEM Simulations

Although impacts of aerosols on deep convective clouds (DCCs) have been intensely discussed in recent years, a number of questions on this relationship still remain unanswered; for instance, whether amounts of precipitation from DCCs increase or decrease with aerosol concentrations, whether convection strengthens or weakens, and how radiative properties of anvil clouds could change with aerosol concentrations. The difficulty partly lies in their short lifetime, relatively unpredictable locations and timing of occurrences, and vigorous vertical motion, since these factors all make it challenging for us to make measurements inside DCCs. However, clarifying the effects of varying aerosol concentrations on DCCs is a crucial step toward obtaining more accurate weather forecasts and future climate projections.

This study uses the Weather Research and Forecasting model coupled with Chemistry (WRF-CHEM) as a cloud-resolving model to investigate the impacts of forest fire smoke on microphysical, dynamical, and radiative properties of DCCs. Our case study focuses on a DCC observed over northeast Colorado during the Deep Convective Clouds and Chemistry (DC3) field campaign on June 22nd, 2012. This case was quite unique as smoke from the High Park Fire was directly injected into a DCC. According to our observational data analysis, this forest fire indeed raised the aerosol concentration over the area, and some of the aerosols could act as cloud condensation nuclei. Comparing the WRF-CHEM simulations with and without forest fire smoke input, we found that the inclusion of the fire smoke changes the simulated microphysics of the DCC; especially graupel mixing ratio is quite different in the two runs. These microphysical differences lead to differences in dynamics and radiative properties of the cloud as well. We address physical mechanisms behind the differences between the two runs, which all stem from the differences in aerosol concentrations in the atmosphere.