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

Abstract

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 (Høvsøre). 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−1km−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 Høvsøre, 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.

Ten Years of Boundary-Layer and Wind-Power Meteorology at Høvsøre, Denmark

“…These 'large-scale' winds provided further understanding of the observed vertical wind shear and wind turning at Høvsøre for a 1-yr lidar campaign; particularly close to the PBL height, baroclinity was found to significantly influence the wind at Høvsøre (Peña et al 2014a;Floors et al 2014). Floors et al (2013) used two roughness descriptions to simulate the flow at Høvsøre with the WRF model: the standard one based on MODIS land-cover data 2 and the other based on observations at Høvsøre (z o was computed similarly as shown in Sect.…”

“…As horizontal temperature gradients also influence atmospheric flow, Floors et al (2014) investigated the effect of baroclinity on the vertical wind shear and veer at Høvsøre by analyzing large-scale winds derived from NWP model simulations and lidar wind-speed and direction observations up to 1000 m. They found that the largest thermal wind-velocity vectors at a height of 966 m point northwards and southwards in July and December, respectively, i.e. perpendicular to the sea-land temperature contrast at Høvsøre.…”

“…Baroclinity has been found to play a significant role in the LLJ formation at Høvsøre; a decrease of the geostrophic wind speed with height causes a wind-speed maximum at low levels (Peña et al 2014a;Floors et al 2014). For a one-year lidar campaign, the easterly flow was on average highly baroclinic (due to a large temperature contrast between land and sea) with a geostrophic wind shear of −3 m s −1 km −1 and a jet at 600 m. WRF model simulations, made with two different PBL schemes at two different vertical resolutions, tended to underestimate the LLJ magnitude at Høvsøre compared with lidar observations up to 650 m for a four-week period in autumn 2010 ).…”

Wind turning in the atmospheric boundary layer over land

“…Mendenhall, ; Hoxit, ; Floors et al . ), but the effects of other variables appear to be less investigated. According to surface Rossby number ( Ro ) similarity theory, the near‐neutral wind‐turning angles decrease with Ro (Blackadar, ; Kung, ; Hess and Garratt, ).…”

“…; Floors et al . ). However, using those methods allows for only single‐point studies and we want to complement those studies with a widespread and comprehensive climatology over large regions.…”