Analysis of Model Results for the Turning of the Wind and Related Momentum Fluxes in the Stable Boundary Layer

Abstract

The turning of wind with height and the related cross-isobaric (ageostrophic) flow in the thermally stable stratified boundary layer is analysed from a variety of model results acquired in the first Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS1). From the governing equations in this particular simple case it becomes clear that the cross-isobaric flow is solely determined by the surface turbulent stress in the direction of the geostrophic wind for the quasi-steady state conditions under consideration. Most models indeed seem to approach this relationship but for very different absolute values. Because turbulence closures used in operational models typically tend to give too deep a boundary layer, the integrated total cross-isobaric mass flux is up to three times that given by research numerical models and large-eddy simulation. In addition, the angle between the surface and the geostrophic wind is typically too low, which has important implications for the representation of the larger-scale flow. It appears that some models provide inconsistent results for the surface angle and the momentum flux profile, and when the results from these models are removed from the analysis, the remaining ten models do show a unique relationship between the boundary-layer depth and the surface angle, consistent with the theory given. The present results also imply that it is beneficial to locate the first model level rather close to the surface for a proper representation of the turning of wind with height in the stable boundary layer.

The impact of changes in parameterizations of surface drag and vertical diffusion on the large-scale circulation in the Community Atmosphere Model (CAM5)

“…In CTRL the subgridscale orographic stress appears to give an improvement in the Northern Hemisphere throughout the atmospheric column, but it does not have an effect on the bias in the Southern Hemisphere, which is as large as in NoTMS. Enhancing the diffusion, which is common practice in many climate models and leads to an increase in the surface drag and an increase in cross-isobaric flow (Svensson and Holtslag 2009) should give weaker zonal winds. However, having the longer tail stability functions in the PBL Longtail does not alleviate the biases (Fig.…”

“…Synoptic systems and the large-scale circulation are affected by boundary-layer wind turning, which gives the ageostrophic flow in the boundary layer (Adamson et al 2006;Beare 2007;Svensson and Holtslag 2009). The strength of the ageostrophic flow can be estimated following Svensson and Holtslag (2009).…”

“…The strength of the ageostrophic flow can be estimated following Svensson and Holtslag (2009). The planetary boundarylayer momentum equations for the mean flow are derived from the Navier-Stokes equations by neglecting molecular viscosity and horizontal turbulent momentum flux divergence and introducing Reynolds averaging to separate the mean and the turbulent part of the flow (e.g.…”

“…With a higher diffusivity, the lifetime of synoptic cyclones appears to decrease. The increase in surface stress together with deeper boundary layers increases the vertically integrated cross-isobaric flow into a cyclone, thus affecting its development (Svensson and Holtslag 2009;Adamson et al 2006;Beare 2007).…”

“…It has been documented, for example in GABLS1, that the wind turning in the boundary layer becomes too small, the planetary boundary layer height (PBLH) too high and low level jets tend to be smoothed out and located too far from the surface (Brown et al 2005(Brown et al , 2008Cuxart et al 2006;Svensson and Holtslag 2009). The ECMWF model employs enhanced diffusion and Sandu et al (2013) concluded that while reducing the diffusion improved the simulation of the stable boundary layer, the near-surface temperature and the large-scale circulation were degraded.…”