Improved Understanding and Modeling of Key Atmospheric Phenomena during WFIP2: Cold Pools, Gap Flows, and Mountain Waves

The Columbia River Basin east of the Cascades comprises much of eastern Washington and Oregon, and it was the focal area of the second Wind Forecast Improvement Project (WFIP2). The basin is surrounded by high terrain on all sides, including the Cascade Mountain range to the west, resulting in flows that are channeled, piled up in natural reservoirs, or forced up over topography or more stable air below. The results are cold pools, gap flows, mountain waves, mountain wakes, and other types of terrain-influenced circulations occurring at many different scales. All of these vary in intensity and behavior depending on the atmospheric conditions, and all present unique challenges to short-term forecasting for wind energy in the region. The following three primary weather phenomena were identified as the focus for WFIP2 observational and model improvement efforts:

• Cold pools. These routinely develop within the Columbia Basin east of the Cascades in the cold season. Depending upon the synoptic situation, cold pools can be associated with stagnant flow or light to moderate easterly flow. The mix-out of these cold pools under transition to westerly flow is one of the greatest challenges of wind energy forecasting in this region, as the mix-out causes large increases or ramps in wind power production.
• Gap flows. These intense flows can occur any time of year, can be either westerly or easterly, and result from different synoptic weather patterns. WFIP2 focused on warm-season, westerly gap flows forced by inland daytime heating but that are also influenced by synoptic-scale transient weather patterns. These strong gap flows drive reliably large amounts of wind power production, but their onset and decay are not well-predicted.
• Mountain waves and wakes. Under conditions of strong, deep westerly flow, primarily in the cold and transition seasons, flow over the Cascade crest as well as local topographic features in the Columbia Basin can produce a variety of gravity wave and topographic wake responses. In such situations, quasi-stationary mountain waves often appear in satellite imagery. Subtle shifts in the position of these waves strongly affect production at individual wind farms. Also, large wakes in the lee of Mounts Adams and Hood can produce meandering low-wind bands that extend hundreds of km downwind of these volcanoes.

Observations and model results from WFIP2 will be used to describe each of these three phenomena, including what we have learned about their structure and evolution, model parameterization changes that have improved our ability to forecast them, and modeling challenges that remain.