On the Anomalous Counterclockwise Turning of the Surface Wind in the Plains of the U. S. During the Warm Season

Under synoptically quiescent conditions the low-level shear changes in the Plains of the U. S. during the warm season, as the surface wind undergoes a diurnal oscillation. This oscillation typically elongates the hodograph during the early evening hours, enhancing the threat of severe convective storms. The oscillation is a response to the Coriolis force (which rotates the wind in a clockwise direction), upslope and downslope pressure-gradient forces in response to the diurnal heating and cooling cycle near the ground, and to diurnal changes in vertical mixing in a sheared environment. From analyses of long-term averaged winds from the Oklahoma and West Texas mesonets during the first half of the warm season there is evidence that, unlike the clockwise turning with time common just above the ground seen in many numerical studies and analytical models, the wind at anemometer level actually turns in a counterclockwise direction, “counter” to conventional wisdom. Such counterclockwise rotation has been noted in some sea breezes. The counterclockwise turning is found even when clouds reduce solar insolation significantly, but the amplitude is much reduced. We also investigate the causes of the anomalous turning of the wind using ensemble forecasts from WRF during part of the warm season in 2016. The data are averaged by time of day to eliminate synoptic-scale variability and the forces are analyzed both at the lowest grid level, where the anomalous counterclockwise turning is reproduced, and at 100 m above the surface, where a clockwise turning is reproduced.