Observational requirements for describing boundary layer characteristics over a complex-terrain coastal region

Measurements from field observations are frequently used by mesocale models in a four-dimensional data assimilation (FDDA) mode to reduce model forecast errors and produce an adequate description of meteorological fields for air pollution applications. A large number of measurements, however, are difficult and expensive to make in areas of complex terrain. The question of what observations are necessary to achieve the optimal modeling results is of practical as well as scientific interest. This paper explores the answer to this question by using a mesoscale model and data from the Southern California Ozone Study (SCOS'97) conducted in and around the Los Angles Basin area from June through October in 1997. The SCOS'97 data set, including data from 26 wind profiler/RASSes, 12 rawinsondes, 6 aircraft and over 200 surface stations, is the most complete meteorological data set ever assembled in a complex terrain coastal region. We begin by identifying important flow patterns and features of boundary layer evolution from analyzing the surface and upper air data. A set of numerical experiments are carried out using RAMS to see how these circulation patterns and boundary-layer features are captured by the model when the model is used in 1) a pure forecast mode, 2) a FDDA mode with all the available data, and 3) a FDDA mode with sub sets of the available data (e.g., profiler winds only, or RASS temperatures only, or a combination of profiler/RASSes at various different locations). These numerical experiments help identify critical site locations and measurements that can significantly improve model's ability to describe important flow and boundary layer characteristics, as well as sites or measurements that are less critical or provide redundant information. This information can be useful for more efficient planning of future field experiments in regions of complex terrain.