Evaluation of a TKE diffusion parametrization in IFS Single Column Model

• An implementation of a diffusion parametrization based on Turbulent KineticEnergy (TKE) in a single column model of the weather forecast model Integrated Forecast System (IFS) is evaluated. Simulations are performed for four test cases GABLS1,GABLS3, DICE and GABLS4. The test cases have different idealization and gives together a comprehensive framework for evaluating the performance for a NWP in representing stably stratified atmospheric boundary layers, diurnal cycles and transition between stable andunstable stratification. The sensitivity to the mixing length scale is evaluated by simulations with alternative formulations and by diagnostic evaluation. An estimation of ageostrophicmass flux is made for idealized test cases.The TKE parametrization is less diffusive than the currently used operational parametrization and shows more realistic stable boundary layers. The magnitude of the diffusivity shows to be very sensitive to length scale formulation. The more diffusive formulations yield weaker gradients of temperature and wind at the surface and smaller angle between surface stress and geostrophic wind. The lower limit of the length scale showed to be ofimportance to avoid unrealistic vertical gradients of wind and temperature close to the surface. Moreover, the diagnosed length scales formulations showed to have very simila rbehaviour when turbulence ceases in stable conditions. The ageostrophic mass flux was higher in simulations with stronger diffusion, except that the new TKE scheme in one test case showed similar values as the operational parametrization.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Meteorologi och atmosfärforskning (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Meteorology and Atmospheric Sciences (hsv//eng)

Nyckelord

Boundary-layer meteorology

Integrated Forecasting System

Length scale

Single Column Model

Stable boundary layer

meteorologi

Meteorology

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