Examining feedbacks of aerosols to urban climate with a model that treats 3-D clouds with aerosol inclusions

[1] Anthropogenic aerosol particles alter clouds, radiation, and precipitation, thereby affecting weather, climate, and air pollution. To examine and understand such feedbacks, a module that simulates the evolution, movement, and microphysics of three-dimensional size-resolved mixed-phase clouds and precipitation and their multicomponent aerosol inclusions was developed and implemented into the GATOR-GCMOM global-through-urban air pollution-weather-climate model. A unique feature of the module is that aerosol particles and their chemical components are tracked in time and space within size-resolved liquid, ice, and graupel and interstitially within clouds. Modeled parameters were evaluated against in situ data, compared with MODIS climatologies, and nested with emission data down to 5 km resolution to study aerosol-cloud feedbacks in Los Angeles. Although updrafts are not resolved during deep convection at this resolution, most convection is shallow in Los Angeles. This resolution is also near the lower limit for which a hydrostatic solution to vertical momentum (used here) is similar to a nonhydrostatic solution. Aerosols in Los Angeles were found to increase cloud optical depth, cloud liquid water, cloud fraction, net downward thermal-infrared radiation, soil moisture, the relative humidity, and (slightly) middle-boundary layer air temperatures. Aerosols also decreased precipitation, surface solar, and near-surface temperatures. Both boundary layer warming due to black carbon and surface cooling due to all aerosol components increased stability, inhibiting cloud top growth under some conditions. Aerosols may feed back to themselves by increasing cloud liquid, gas dissolution, and aerosol volume upon evaporation. They may also decrease visibility by increasing the relative humidity and decrease photolysis below them by enhancing cloud thickness.