The effect of baroclinicity on the wind in the planetary boundary layer

The influence of baroclinicity on wind within the planetary boundary layer was investigated using two years of wind lidar measurements collected at a suburban site in northern Germany (Hamburg) and a rural-coastal site in western Denmark (Hovsore). Measurements were made up to a height of 950 m. The surface geostrophic wind, the surface gradient wind and the gradient wind were estimated using the pressure and geopotential fields from a mesoscale model. At both sites the atmospheric flow was typically baroclinic. The distribution of the geostrophic wind shear was approximately Gaussian with a mean close to zero and a standard deviation of approximately 3 m s(-1) km(-1). The geostrophic wind shear had a strong seasonal dependence because of temperature differences between land and sea. The mean wind profile in Hamburg, observed during an intensive campaign using radio sounding and during the whole year using the wind lidar, was influenced by baroclinicity. For easterly winds at Hovsore, the estimated gradient wind decreased rapidly with height, resulting in a mean low-level jet. The turning of the wind in the boundary layer, the boundary-layer height and the empirical constants in the geostrophic drag law were found to be dependent on baroclinicity for neutral conditions.