Enabling high-resolution simulations of atmospheric flow over complex terrain in the WRF model

High-resolution or microscale simulations of atmospheric flow are critical for many applications such as atmospheric transport and dispersion and wind energy forecasting, among others. Achieving accurate high-resolution simulations of atmospheric flows requires a multi-scale approach in which a wide range of atmospheric processes and scales of motion are represented. In other words, information from regional weather patterns at the mesoscale must be included in high-resolution simulations at the microscale. In the Weather Research and Forecasting (WRF) model, grid nesting provides the framework for communicating between scales. However, grid nesting alone is often insufficient. For example, when grid nesting is applied in regions of complex terrain, resolved terrain slopes can become large, causing numerical errors arising from grid skewness of the terrain-following coordinates.

The immersed boundary method, a method which eliminates conforming grids and the errors associated with terrain-following coordinates, has been implemented into the WRF model (Lundquist et al. 2010,2012). This implementation, WRF-IBM, has been validated for idealized cases and real urban cases with excellent results; however, to date WRF-IBM has been applied with idealized lateral boundary conditions and thus is not multi-scale. Furthermore WRF-IBM uses a no-slip boundary condition, while use of a wall model is typical in atmospheric modeling.

In this work, we detail a multi-year effort to develop WRF-IBM for real, multi-scale simulations, including full atmospheric physics. Results from three aspects of this project are presented: initializing IBM domains using real meteorological and surface data, developing a nest interface between domains using terrain-following and IBM coordinates, and modifying the IBM boundary condition to include a wall model. Companion abstracts by Wiersema et al. and Bao et al. give further details on the last two developments. The case studies of Askervein hill and the recent MATERHORN field experiment in Utah are used to validate our model developments.