Model evaluation of aerosol wet scavenging in deep convective clouds based on observations collected during the DC3 campaign

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Description

Deep convective storms greatly influence the vertical distribution of aerosols by transporting aerosols from the boundary layer to the upper troposphere and by removing aerosols through wet scavenging processes (i.e. in-cloud [or “rain-out”] and below-cloud [or “wash-out”] scavenging). Model representation of wet scavenging is a major uncertainty in simulating the vertical distribution of aerosols due partly to the lack of constraints by observations. The effect of wet scavenging on ambient aerosols in deep mid-latitude continental convective clouds is studied for a severe storm case in the vicinity of the ARM Southern Great Plains site on May 29, 2012 during the Deep Convective Clouds and Chemistry Project (DC3) field campaign. The convective transport to the upper troposphere is characterized using aircraft measurements of CO and acetone (propanone). A mass budget analysis of the changes in aerosol (i.e., sulfate [SO42-], nitrate [NO3-], ammonium [NH4+], chloride [CL-], organic aerosols [OA], and black carbon [BC]) total mass and number concentrations, chemical composition, and size distributions in the boundary layer and upper troposphere after the active convection is used to infer wet scavenging below cloud and in the anvil. The chemistry version of Weather Research Forecasting model (WRF-Chem) simulates the storm initiation timing and structure reasonably well when compared against radar observations from the NSSL national 3-D reflectivity Mosaic data. Simulated vertical profiles of humidity and temperature also closely agree with radiosonde measurements before and during the storm. Aerosol budget analysis results based on the simulation and observations indicate underprediction of wet-scavenging effect with the default model configuration. A new treatment of ice-borne aerosol is coupled to the Morrison microphysical scheme in WRF-Chem to improve the representation of aerosol wet scavenging. In addition to evaluating the new treatment, the sensitivity of aerosol wet scavenging to different microphysical schemes will also be investigated. The implication of the results from this study for improving model representation of wet-scavenging processes in deep convective clouds will be discussed.