Energetica India Magazine December 2020

WIND POWER any larger-scale affects occurring that violate that assumption of uniformity, their influence will not be captured. As it turns out, there are many such effects, and they can impose signifi - cant gradients or other patterns of wind speed across the site. Fortunately, all of them are represented by mesoscale weather prediction models. What are these effects? • Orographically altered flows that arise from mountains, val - leys, gaps, etc. that lie outside of the immediate project vicinity. • Realistic representation of the effects of variation in atmo- spheric stability, from unstable to strongly stable (e.g., +10°C per 100 meters vertical stratification). • Sea-breeze patterns arising from coastal features outside the project vicinity. • Storm tracks, monsoon circulations, thunderstorms, weath- er fronts, or other recurring large-scale weather phenomenon. Note that many of these phenomena are dependent on accu- rate representation of water phase changes, clouds, precip- itation, atmospheric radiation, and planetary boundary layer processes, which only mesoscale models include. • Vertically propagating orographic gravity waves. This high- ly underappreciated but very important topographic flow phe - nomenon leads to deceleration of flow windward of a ridge and acceleration leeward of it, even for very subtle rises in terrain. Only mesoscale models have the flow dynamics capable of supporting this phenomenon. • Transience. Linear/CFD are typically run multiple times for different combinations of steady, uniform flow conditions. As such, the simulated flow field contains no realistic wind fluctu - ations, which are the norm rather than the exception at many wind sites, and can affect the spatial wind resource. Finally, it is important to state that on-site measurements are still considered “ground truth” for wind resource at the project site, and the description above should not be taken to mean that mesoscale models preclude the need for on-site measure- ments. Various techniques have been developed to spatially bias-correct mesoscale model output with on-site observa- tions, retaining the best of both worlds: the ground truth of the observations and the weather-driven spatial wind patterns pro- duced by the mesoscale model. Model resolution Mesoscale models can be run at a wide range of grid resolu- tions, including microscale (≤ 400 m), and now even delving into the “large eddy simulation” (LES) range (grid spacing ≤ 100 m). A practical approach is to use sufficiently fine resolu - tion to capture phenomena at the scale of typical turbine spac- ing (a few hundred meters), and then downscale the result to the turbine pad (10s of m grid spacing) with a simplified model. Such downscaling can produce accurate wind maps as long as the “leap” from the mesoscale model grid (e.g. 200 m) to the downscaled resolution (e.g., 30 m) is not too large (preferably < a factor of 10). Below are shown raw model (non-downscaled) mean 100-m wind speed maps over an area approximately 20 km x 20 km (Fig. 1), with model resolutions of 600 m (left) and 200 m (right) grid spacing. Below those maps are the same output down- scaled to 30 m using a 1-arcsecond digital elevation model (Fig. 2). The raw model output clearly shows the effect of the higher resolution from the mesoscale model (Fig. 1 right com- pared to left). The extra detail in the downscaled model wind field is also clearly evident (Fig. 2 compared to Fig. 1). The two downscaled maps look similar to each other, yet differences can still be seen. The map downscaled from 200 m is like- ly more accurate, because a smaller downscaling “leap” was taken (factor of 6.7, compared to 20). Mesoscale Model error analysis ArcVera compiled a mesoscale modeling and met tower data- set, to explore the relationships of mesoscale modeling 39 energetica INDIA- December_2020 Figure 1. Low-resolution (600 m) vs. High-resolution (200 m) Wind Maps, with no downscaling Figure 2. Low-resolution (600 m) vs. High-resolution (200 m) Wind Maps, with downscaling to 30 m.