The Impact of Aerosols on Fog Formation and Energy Budget in the California Central Valley

A special fog phenomenon that occurs in the California Central Valley, called Tule Fog, is created when warm air rises and cold mountain air descends into the valley during the night. The cooler temperatures and reduced sunlight of the winter months make Tule Fog vanish slowly, and the fog can persist for days and cause safety issues. Aerosol particles emitted in California air basins can serve as cloud condensation nuclei (CCN) and their number concentration, size, and composition can alter fog formation and the radiative budget. The primary goal of this research is implementing a CCN source-oriented module in the Weather Research and Forecasting model, including the chemistry component (WRF-Chem) to investigate the effects of aerosols from various sources on fog formations and their optical properties. In this research, two fog events will be studied to tackle the objective: one on February 21, 2007 and the other on January 17, 2011. A new module for particulate matter (PM) operations that are Source-Oriented has been developed within the WRF-Chem model (SO-WRF-CHEM6D) by Prof. Kleeman's group at UC Davis. The SO-WRF-CHEM6D model tracks 6-dimensional chemical variables (X, Z, Y, Size Bins, Source Types, Species). Particle radius and number concentration are calculated explicitly for each source and bin. The initial tests use 25 chemical species from 5 emissions sources (dust, gasoline, diesel, meat cooking, and others) and 8 size bins, spanning range from 0.01 to 10 microns. We are now developing a new source-oriented CCN (activated PM) variable in SO-WRF-CHEM6D. The new model also includes the cloud feedbacks (e.g. wet scavenging) on aerosol distribution and number concentration. To understand the influence of aerosol particles serving as CCN in clean and polluted environments on the fog formation and radiation budget, five experiments will be conducted for each event. The SO-WRF-CHEM6D model will be integrated for 7-10 days, depending on the duration of the event. In all experiments, the two-moment Purdue Lin microphysics scheme will be used. Aerosol direct and indirect effects will be studied in these numerical experiments. Comparison among these experiments will help us understand the impact of aerosol-cloud-radiation effects on the energy and moisture budget, boundary layer instability, fog lifetime, fog thickness, etc.