A numerical study of smoke transport and dispersion associated with the October 2007 Southern California wildfires

In October 2007, severe wildfires broke out across southern California, resulting in the substantial emission of trace gases and particles to the atmosphere. To address the regional impacts of smoke from these fires, we performed numerical simulations of the meteorological conditions, using the Weather Research and Forecasting (WRF) model, and the smoke transport and dispersion, using a Lagrangian particle dispersion model called FLEXPART. The transport and diffusion of atmospheric constituents over the western U.S. is often constrained by the mountainous terrain, which has a profound effect on regional meteorology particularly in the lower troposphere where fire emissions are released and most of the transport and diffusion occurs.

The meteorological fields simulated by the WRF model were evaluated by comparing them with observed weather data from a network of surface weather stations and six upper air rawinsonde stations in California and Nevada. The simulated smoke distribution was compared with MODIS Hazard Mapping System (HMS) satellite imagery. A series of sensitivity experiments wherein the depth over which particles in the FLEXPART model are released was used to explore the sensitivity of the resultant particle distribution to different injection heights. These sensitivity experiments highlight the importance of including accurate information about plume rise and equilibrium height in numerical simulations of smoke dispersion, as well as their importance to anticipating the regional impact of smoke from wildland fires in the southwestern United States.