Advances in computers have provided the means for generating fine resolution mesoscale numerical weather predictions (NWPs). Each computer advance brings demands for forecasts on ever smaller scales, especially by such disciplines as air pollution modeling and fire weather forecasting. Weather forecasts and observations on very small scales are essential for driving the models used in these important decision-making processes. Even with the improvements in mesoscale NWPs, the horizontal scales desired by these communities are still too small to be treated by current computer technology in a timely and practical fashion. Even if the computer resources were adequate, mesoscale model parameterizations are not necessarily appropriate for these small scales, thereby potentially introducing significant model error in mesoscale NWPs.

One possible solution to the problems outlined above is to use a mesoscale model to predict on the scales for which it is both appropriate and practical, and supplement those forecasts with a diagnostic model to address the smaller scale topographic effects. This approach has already been implemented on an experimental basis, using the forecasts from a mesoscale model run at a moderate horizontal resolution (12 km) as input into the Winds on Critical Streamline Surfaces diagnostic model (WOCSS, see Ludwig et al., 1991 for a description). The WOCSS approach is used to adjust the mesoscale forecast winds to fine resolution orography. This system has proven quite practical in preliminary tests. This study extends recent work by Mohammed (2000), who used the composite mesoscale model-WOCSS system to obtain fine scale (~3 km) results. He showed that the combination provided better results in the coastal region of the California Bight than could be obtained by running the mesoscale model itself for a similarly fine grid. We will expand Mohammed's tests to case studies of the central California coastal zone and examine potential shortcomings and biases of the mesoscale model-WOCSS system. We will also compare the quality of the diagnosed winds to a baseline fine-scale mesoscale model forecast using a consistent resolution of the complex coastal orography. The Navy's Coupled Ocean/Atmosphere Prediction System (COAMPS) and the National Center for Atmospheric Research/Penn State's Mesoscale Model Version 5 (MM5) are the mesoscale models which will be used to drive the WOCSS diagnostic model in this study, although the approach should work with most mesoscale models.

REFERENCES

Ludwig, F. L., J. M. Livingston, and R. M. Endlich, 1991: Use of mass conservation and critical dividing streamline concepts for efficient objective analysis of winds in complex terrain. J. Appl. Meteor., 30, 1490-1499.

Mohammed, R. M., 2000: Forecasting mesoscale winds on complex terrain using a simple diagnostic model. Master's Thesis, Naval Postgraduate School, Monterey, California, 130 pp.