The Low Level Jet and Wind Energy

The Low Level Jet and Wind Energy

The Great Plains of the U.S. are home to hundreds of wind farms generating tens of thousands of MW of power for the electrical grid. It is not a coincidence that the same geography also features a ubiquitous low level jet (LLJ). This phenomenon occurs regularly throughout the year in the southern Great Plains and elsewhere where wind farms tend to be located. Southerly LLJs tend to be strongest (wind speeds can reach in excess of 25 m s-1) but northerly LLJs do occur (Song et al. 2005). The height of the LLJ typically varies between about 50 m and 400 m, and therefore frequently extends into the rotor plane of most utility-scale turbines (i.e. 40 - 140 m) The LLJ facilitates the high capacity factors that make the Great Plains (and other locales) favorable for wind energy production; however, a special concern is the large vertical shears (upwards of 15 m s-1 per 100 m) that can occur across the turbine rotor plane. Different classification schemes for LLJs have evolved over time (e.g. Bonner 1968) but two types commonly occur: (1) the nocturnal LLJ, caused by radiative cooling after sunset, and (2) a pre-frontal LLJ caused by the increasing pressure gradient ahead of the frontal system.

Critical observational and forecasting issues concerning LLJs are 1) their variation in height, 2) the strength of the vertical wind speed gradient, 3) their formation and persistence, 4) spatial characteristics such as width and depth, and 5) intermittent turbulence leading to propagation of strong winds towards the surface.

The Wind Forecasting Improvement Project (WFIP) was a multiyear Department of Energy (DOE)/National Oceanographic and Atmospheric Administration (NOAA) sponsored study whose main purpose was to demonstrate the scientific and economic benefits of additional atmospheric observations and model enhancements on wind energy production forecasts. A key component of the WFIP field deployment was the combination of surface and remote sensing platforms (including four wind profilers, seven SoDARS, and a 200-m instrumented tower in the Southern Study Region) as integrated observation sites (IOSs), to ensure the complete capture of the wind profile from the surface through and beyond the top of the atmospheric boundary layer (typically 1 – 3 km in this region). The one-year continuous observation period produced a unique data set that has provided valuable insight into the temporal and spatial extent of the LLJ over the U.S. Southern Plains. Here, we present observational corroboration and forecast depiction of the regularity and predictability of the LLJ (concomitant with peaks in wind power production), and evidence of its morphology as a characteristic undulating sheet extending horizontally hundreds of km yet exhibiting a thickness of only 100 - 200 m.

References

Bonner, W. D., 1968:Climatology of the low level jet. Mon. Wea. Rev., 96, 833-850.

Song, Jie, Ke Liao, Richard L. Coulter, Barry M. Lesht, 2005: Climatology of the Low-Level Jet at the Southern Great Plains Atmospheric Boundary Layer Experiments Site. J. Appl. Meteor., 44, 1593–1606.