Nocturnal Boundary Layer Regimes in a Numerical Weather Prediction Model

Since long, boundary-layer meteorology has distinguished between 'weakly-stable' boundary layers (WSBL) and 'very stable' boundary layers (VSBL). The former occur under strong winds and overcast conditions and are dominated by strong, continuous turbulence. The latter occur for light winds and clear conditions in which case turbulence is weak and patchy. In recent years, the importance of the coupling with the surface and radiation processes has become more and more apparent, especially for increasing stability. The subtle interactions between the various physical processes make modeling of the SBL a real challenge for numerical weather prediction models.

Recent studies have demonstrated that the WSBL and the VSBL represent two qualitatively different regimes. This was also found by Van Hooijdonk et al. (2015) who analyzed 10 years of observations from the Cabauw observatory, The Netherlands.

In the present work we explore to what extent a numerical weather prediction model represents observed characteristics of both SBL regimes. To this end, we make use of a single-column model (SCM) version of the Regional Atmospheric Climate Model 2.0 (RACMO), in operation at KNMI, The Netherlands. The SCM consists of a modified version of the IFS CY31r1 model from ECMWF. A major difference with the IFS code is that the first order (turbulence) closure scheme is replaced by a TKE scheme with more realistic mixing characteristics. Our analysis is based on 10 years of daily simulations for Cabauw. Large-scale forcings were taken from operational RACMO forecasts. As in Van Hooijdonk et al. (2015) we construct ensemble averages for various classes of mechanical forcing. The results are compared with observations.

Reference Van Hooijdonk, I.G.S, J.M.M. Donda, H.J.H. Clercx, F.C. Bosveld, and B.J.H. van de Wiel, 2015: Shear Capacity as Prognostic for Nocturnal Boundary Layer Regimes, J. Atmos. Sci, 72, 1518-1532.