A Case Study of the Great Plains Low-Level Jet: July 17-18, 2010 (Photo Credit: Sharon Sullivan, Weather Research and Forecasting Model - WRF)
Evolution of the low-level jet over the sloped Great Plains terrain at f14 (7 pm CDT July 17), f19 (midnight), and f24 (5 am CDT on July 18th).
A short case study is presented for July 17- July 18, 2010 to determine if there are characteristics present that define a classic summertime low-level jet. The nocturnal Great Plains Low-Level Jet (LLJ) is a fast-moving ribbon of air in the lowest level of the atmosphere (about half a mile above the surface). Wind speeds may increase to 40-60 miles per hour after midnight. The Great Plains Low-Level Jet is commonly found from Texas northward to Nebraska and centered geographically over the Oklahoma/Kansas border. Climatological analysis indicates that the low-level jet is most common in June and July, with a peak around mid-July.
There has been much debate over the past 50 years as to the mechanisms that initiate the low-level jet, but two main theories seem to emerge: Holton’s and Blackadar’s theory. Blackadar’s theory suggests that the low-level jet occurs as a result of the clockwise rotation of the nocturnal wind. Holton considered the diurnal variations of the heating and cooling of the sloping Kansas terrain to allow a pressure gradient to develop. During the daytime, a strong temperature gradient sets up from east to west along a constant height surface, where the strongest heating occurs above the highest slope of the terrain. At night, the pressure gradient reverses and the boundary layer begins to decouple from the surface layer. Both methods are acting in the LLJ, but the Blackadar method seems to dominate more.
A cross-section completed using the Weather Research and Forecasting (WRF) model, a mesoscale numerical weather prediction system, shows a weak low-level jet with a weakly-defined jet core. The wind speeds increase significantly over the region in the early morning hours, as one would expect to see with the presence of an LLJ. The jet starts to appear at f14 (7 p.m.) and reaches its maximum at f21 with wind speeds in excess of 45 mph. The jet core appears over Tescott, Kansas in this case at a height of ~3000 feet above the surface. The jet became weaker at f24 (5 a.m.) and can be seen propagating to the east. A few scattered showers developed overnight due to enhancement by the low-level jet with the tendency for the storms to move south of the low-level jet axis with time.
To learn more about the Great Plains Low-Level Jet and other North American phenomena, please click here!
© 2019 Meteorologist Sharon Sullivan