Wind-turning over the atmospheric boundary layer in observations and models

As an attempt to find a way of evaluating the surface drag in global models, we have derived a climatology of the boundary-layer wind-turning angle over land (Lindvall and Svensson, 2019). It is based on radiosonde observations from 800 stations in the Integrated Global Radiosonde Archive (IGRA). The climatology and how the wind turning depend on a suite of parameters is analyzed. Results from previous studies indicating the importance of the planetary boundary layer (PBL) stratification for the angle of wind turning are confirmed. A clear increase in the wind-turning angle with wind speed, particularly for stratified conditions, is also evident. According to Rossby number similarity theory, the crossisobaric angle for a neutral and barotropic boundary layer decreases with the surface Rossby number, Ro. The IGRA observations indicate that this dependence on Ro might partly be linked to the dependence of the stratification on the wind speed, a dependence that seems to prevail even for the high wind speeds, a criterium that traditionally is used to approximate a neutral PBL. The vertical distribution of the turning of the wind is analyzed using the high resolution Stratospheric Processes And their Role in Climate (SPARC) data. For unstable cases, there is a maximum in the directional wind shear around the PBL top, whereas for the most stable class of cases there is a maximum near the surface. The midlatitude cross-isobaric mass transport is estimated using the IGRA data. The wind-turning angles from reanalysis fields and climate models are also presented, they generally underestimate the turning angle.