Operational limits of WAsP

Bowen and Gylling Mortensen (2004) explore the limits of WAsP and develop practical recommendations to stay inside the operational envelope. However, the recommendations are useful reminders for any micro-scale modeller:

1. The same weather regime

WAsP assumes a unique speed-up ratio between the reference and predicted sites for each wind direction sector. Therefore, a high level of cross-correlation in wind speed between the two locations is an essential condition for accurate predictions. A high correlation level ensures that both sites lie within the same weather regime and that large-scale terrain effects such as channelling is small.

2. Near-Neutral conditions

WAsP assumes that the speed-up ratio is independent of wind speed and that climatic conditions are neutrally stable. 10-minute average speed-up ratios can depend on wind speed and can vary significantly between summer and winter. However, speed-up values at high wind speeds tend to a unique value close to the long-term average for that sector.

3. Not too steep

Proper use of linear models like WAsP is confined to terrain with sufficiently gentle slopes to ensure predominantly attached flows. CFD RANS models depend upon parametrizations derived from horizontally-homogeneous boundary-layers and can struggle to simulate flow separation and recirculation. The ability to predict whether the flow will separate is an essential step in estimating any micro-scale model’s performance. The onset of flow separation depends on the hill’s slope, the surface roughness length, and the hill’s shape. Developing a simple flow separation indicator Bowen and Gylling Mortensen (2004) suggest using a single critical terrain slope of 0.3 as a conservative value for all hills. The ruggedness index (RIX) is defined as the percentage fraction of the terrain along the prevailing wind direction, which is over this critical slope.

Bowen, A. J, and N Gylling Mortensen. 2004. WAsP Prediction Errors Due to Site Orography. Risø-R 995(EN). Denmark, Roskilde: Risø National Laboratory. https://orbit.dtu.dk/en/publications/wasp-prediction-errors-due-to-site-orography.