Characterizing the Great Plains Low-Level Jet Wind Resource using Doppler Lidar
Abstract
A major North American wind resource during the warm season is the nocturnal low-level jet (LLJ) of the Great Plains. A dynamic acceleration of flow above the surface in response to cooling of the earth surface in the evening, this LLJ occupies an expansive geographical region thousands of kilometers across, and thus represents a vast resource. LLJ properties there have been observed to be relatively homogeneous in the horizontal, but the jet speed varies from night to night. Thus, accurate LLJ forecasts are essential to anticipate the amount of power that will be generated and transferred to the power grid. Current numerical weather prediction (NWP) models do not predict LLJ speeds and heights to sufficient accuracy for this application. The height of the LLJ maximum in this region generally occurs at 100-200 m above ground, traditionally a difficult atmospheric layer to obtain measurements - but most important for wind energy as the layer occupied by the turbine blades. Here we use the high-resolution Doppler lidar, an ideal instrument for this difficult layer, to obtain properties of wind and turbulence at high spatial and temporal resolution, to document the evolution of the LLJ through entire nights. These observations show that the strongest evening accelerations occur 100 m or more above ground and that near-surface measurements do not reflect the intensity of this increase in wind speeds aloft. The shear in the layer below the jet maximum tends to be nearly constant with height and strong (~0.1 s-1), so that for this type of flow, power-law-exponent extrapolations significantly underestimate both shear and extrapolated wind speeds aloft. Routine turbulence in the layer below the LLJ varies directly with LLJ speed but occasional bursts or events of strong turbulence or periodic-wave activity can adversely affect or even disable turbine hardware. Critical areas where better understanding is urgently needed for wind energy applications include understanding and predictability of LLJ dynamics, improvement of the representation of stable boundary layer and stable mixing processes in NWP models, and understanding of turbulence characteristics under stable flow conditions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.A11H..03B
- Keywords:
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- 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 3307 Boundary layer processes;
- 3322 Land/atmosphere interactions (1218;
- 1631;
- 1843);
- 3329 Mesoscale meteorology;
- 3379 Turbulence (4490)