To understand the water quality and the ecological health of coastal waters near major cities, the effects of the precipitation and runoff in urban areas need to be addressed. This is particularly true for the city of Los Angeles because the waters that flow through the storm drains in the Los Angeles urban area are not treated before they reach Santa Monica Bay and the Pacific Ocean. The research that is reported in this paper is part of a project to combine several models to address this problem, including a meso-scale atmospheric model, air and water chemistry models, and an urban runoff model. The research is currently focusing on the transport and fate of nitrates in the Los Angeles urban environment, since nitrates discharged to the ocean can upset the ecosystem balance. The nitrogen cycle will be followed starting with nitrate precursors produced by auto emissions and industrial processes, which are dispersed and undergo chemical transformation. Nitrogen will also be followed through the deposition and entrainment of nitrates into surface water runoff during rain events, where nitrates are carried into sewers and natural waterways, and eventually to the ocean.
The emphasis of this paper is on simulating the weather and precipitation associated with a winter storm event in the Los Angeles area. The precipitation event of 4-6 December 1987, which follows an air pollution event on 3 December where measured nitrate concentrations were high, will be simulated. The complex interactions of the meso-scale forcings in this urban area with the synoptic-scale weather will be presented. In particular, the roles of the land-ocean interface and the complex topography surrounding the city in determining the precipitation quantities and distribution will be discussed.