The impact of land surface modeling on photochemical air quality simulations using the MM5/CMAQ system in the Portland/Vancouver airshed

Elevated ozone levels occurred periodically in the summer time within the Portland/Vancouver airshed. The region is influenced by complex terrain effects and unique meteorological conditions associated with a heterogeneous urban area located along the foothills of the Cascade Mountains and intersected by the Columbia River. These features require special care to obtain meteorological simulations suitable for air quality modeling. In addition to the effects of this type of complex terrain, land-surface characteristics such as soil moisture and vegetation cover can have a significant impact on PBL parameters which are crucial for air quality modeling. To investigate the sensitivity of air quality simulations to the input meteorological fields, the MM5/SMOKE/CMAQ system was employed using two different land surface models and results were compared to observed meteorological and ozone parameters for two ozone episodes in the region (during July, 1997 and July, 1998).

Results obtained using the Noah Land Surface model in MM5 were compared with those from the 5-layer soil model. The impact of these different land surface models on the air quality simulations was evaluated. The results show that there are significant differences in surface temperatures and PBL heights using two models. LSM predicted generally lower surface temperatures as well as PBL heights in western Washington and Oregon and significantly improved simulations of surface ozone levels.