Observations of Subgrid-Scale Surface Flux Variability in WFIP 2

The second Wind Forecast Improvement Project (WFIP 2), described in general elsewhere, is an extensive observational and modeling study designed to improve our ability to forecast winds, and the associated wind power production, at approximately 100 m above the ground. WFIP 2 is centered in the Columbia Gorge of Oregon and Washington in an area moderately complex terrain. Instruments associated with WFIP 2 have been deployed from the immediate coast in the west to Walla Walla, Washington in the east and from Prineville, Oregon in the south to Yakima Washington in the north. The field phase of WFIP 2 officially began on October 1, 2015, and the intention of the study is to investigate the spectrum of physical process that affect wind forecasts over multiple scales and over a complete annual cycle.

One of the challenges of applying mesoscale models in complex terrain is that many of the assumptions underlying subgrid-scale parameterizations may be questionable, especially as horizontal resolutions become finer in an effort to account for the terrain. In particular, these subgrid-scale parameterizations, for both the surface layer and the planetary boundary layer in general, generally assume that effects of horizontal gradients of fluxes and mean values and negligible compared with vertical gradients. One of the objectives of WFIP 2 is to evaluate subgrid-scale variability of mean values and fluxes in the surface layer and to assess the impact that this may have on the parameterizations.

To carry out this assessment, an area representative of a grid cell of a mesoscale model, was instrumented with flux instrumentation at 10 locations and surface mean meteorological stations at an additional two. Referred to within WFIP 2 as the “physics site”, these twelve stations are located in an area of moderately complex terrain roughly represented by an isosceles triangle with its apex oriented toward the east and having a height (E-W direction) of approximately 3 km and a base (N-S direction) of approximately 2.5 km. Flux measurements at from nine of the ten sites are being made at 10 and 20 m above the surface. At the apex of the triangle, flux measurements are being made at three heights on an 80-m tower. The mean meteorological measurements at the other two stations are being made at a height of 3.5 m. Together, these measurements provide an opportunity of assess the horizontal variability of means and fluxes on the subgrid scale and to relate the spatial and temporal variability of these quantities to the profile of winds and fluxes from the surface to 80 m. The data set will also provide an opportunity to evaluate LES models as a tool for improving the parameterizations. This presentation will provide a description of the physics site, the problems to be addressed using the data, and a preliminary assessment of spatial and temporal variability observed at the physics site.