Improvements to Low-Level Wind Forecasts in Complex Terrain from WFIP2

The Second Wind Forecast Improvement Project (WFIP2) is a multiagency venture aimed at improving low-level wind forecasts in regions of complex terrain for the benefit of wind-energy applications. To achieve this objective, major modifications were developed for the Mellor–Yamada–Nakanishi–Niino (MYNN) turbulence parameterization, a component of the RAP/HRRR physics suite in the WRF–ARW modeling framework. These modifications were intended to address frequent RAP/HRRR forecast-failure modes in the Columbia Basin; namely, high wind-speed biases associated with the premature erosion of low-level cold pools during the cold season, and low wind-speed biases within diurnally varying, thermal-trough-induced gap flows during the warm season. Other aspects of the WRF–ARW framework were also investigated, including horizontal (and 3D) mixing, and refinements to numerical methods.

Drawing upon in-situ measurements collected during the WFIP2 field campaign, the forecast improvements made during WFIP2 will be demonstrated. Results from single-run case studies and extended retrospective test periods will be presented, using forecasts from the RAP, HRRR, and 750-m nest, as well as forecasts from "cold start" WRF–ARW configurations. The path to operational implementation of RAP/HRRR model improvements will also be discussed. Finally, we will reflect on the unsolved modeling challenges of WFIP2 to motivate future RAP/HRRR physics development.