Diurnal Variations in the 2-Dimensional nature of wind shear in the planetary boundary layer

Vertical profiles of hourly-averaged wind speeds and directions remotely sensed at Schenectady, NY using the NOAA profiler during the month of July 2002 are analyzed. A technique for defining the 2-dimensional nature of wind shear is presented where shear of the wind with height is decomposed into components that are parallel to and perpendicular to the average wind vector in a predefined layer. In the lowest kilometer, wind shear perpendicular to the mean wind in that layer reaches its greatest magnitude of slightly greater than -5 m/s/km at about 3AM local time, and is lowest near sunset (-1 m/s/km). Generally we see a “veering” of wind with height (wind direction turning clockwise with increasing height above the surface) in the lowest kilometer, although about 25% of the time, winds “back” with height (counterclockwise rotation with height). On average, we see about –3 m/s/km wind shear perpendicular to the mean wind in the lowest km, although the 90th and 10th percentile shears are –8 m/s/km and +2 m/s/km. Even during daytime periods when the lowest kilometer is clearly within the “well mixed” planetary boundary layer, we do not observe “well mixed” wind speeds and directions. In fact, we see evidence for something like an “inverse” of a nocturnal jet during early morning time periods where winds in the layer 500-1000m above the surface are reduced to will below a frictionally-balanced pressure gradient and Coreolis wind, and wind speeds are frequently higher near the surface than in the upper boundary layer during this brief time period due to an oscillatory dynamic response of the air in the upper PBL due to the sudden imposition of frictional drag by the growing boundary layer. Shear magnitudes are considerably greater than calculated thermal wind or Ekmann shear would dictate, suggesting that shear in the PBL is not in a balanced dynamic state with respect to the forces acting on air in the lowest kilometer.