Wind Resource Assessment over the Columbia River Gorge using a Three-Dimensional Planetary Boundary Layer Parameterization

Abstract:

Complex topography presents a grand challenge for numerical weather prediction (NWP) in mesoscale simulations. To generate accurate wind forecasts over complex topography, high-resolution simulations are needed to resolve terrain effects. Currently, however, NWP models typically utilize one-dimensional (1D) planetary boundary layer (PBL) parameterizations that assume horizontal homogeneity and account for only vertical turbulent mixing. This approach is justified when the horizontal grid cell size is relatively large (approximately 10 km and greater); however, the assumption of horizontal homogeneity breaks down at finer resolutions. In such instances – particularly when the grid cell spacing approaches the so-called “grey zone” (approximately 100 m to 1 km) – horizontal gradients of turbulent stresses become non-negligible and should be considered. These three-dimensional (3D) effects are especially important in atmospheric flows over complex terrain.

In light of this modeling challenge, we have developed a 3D PBL parameterization for high-resolution mesoscale simulations using the Weather Research and Forecasting (WRF) model. The new PBL scheme, which is based on the algebraic model developed by Mellor and Yamada (1982), accounts for the 3D effects of turbulence by calculating explicitly the momentum, heat, and moisture flux divergence in addition to the turbulent kinetic energy. Here, we use the 3D PBL parameterization to simulate real-world atmospheric conditions that were studied during the Weather Forecast and Improvement 2 field experiment in the complex terrain of the Columbia River Gorge. Specifically, we use observations to illustrate how the representation of topographically induced phenomena differ between the 1D and 3D PBL solutions.