Influences of Land Surface Fluxes on Wind Turning in the Planetary Boundary Layer

The turning of wind with height in the planetary boundary layer (PBL) plays an important role in shaping convective storm environments and may be influenced by land surface characteristics. A 25-year (1999 to 2024) climatology of wind turning in the PBL and PBL depth is presented here, using twice-daily radiosonde observations from the Integrated Global Radiosonde Archive (IGRA). We analyzed 72 sounding sites in the contiguous United States (US) that are still operational as of 2024, evaluating the annual variation of PBL depth and wind turning angle, and the geographical distribution of wind turning angle. The monthly median wind turning angle in the evening (00 UTC) displays a seasonal cycle (range: [9°, 20°]) with a minimum in mid-summer (9°), while morning (12 UTC) values remain relatively constant throughout the year (range: [19°, 22°]). Geographically, the climatological median wind turning angle is largest in the central and northern US, highlighting a probable dependence of wind turning angle on Coriolis parameter. In contrast, the climatological median wind turning angle is slightly reduced along coastlines and is lowest in areas dominated by topography (e.g. the Rocky Mountains or the Appalachian Mountains), which may be a result of increased baroclinicity and roughness in those regions. Our results are mostly supportive of the results of a similar study, wherein the authors found a similar geographic distribution of wind turning angle in the US. However, we found a more continuous distribution of turning angle in the south-central US. Along with analyzing the temporal and geographic variation of wind turning angle, additional analyses that utilize surface flux measurements are planned to further understand how land surface fluxes (i.e. momentum and heat fluxes) influence wind turning angle, and the broader impact of land surface fluxes on atmospheric flow relevant to different convective environments.