The effect of boundary-layer parametrization schemes on the structure and dynamics of an idealized cold front simulated with WRF

Previous studies have shown that the planetary boundary layer (PBL) strongly modifies the structure and dynamics of synoptic-scale fronts. Within the boundary layer, frontal zones are a balance between large-scale, frontogenetical processes which intensify the front, and turbulent, frontolytical processes which weaken the front. Thus, the boundary-layer structure of fronts simulated by numerical weather prediction models is, to a large extent, determined by how the boundary-layer mixing is parametrized, yet few studies have compared the effect that different PBL schemes have on the structure of simulated fronts. We use the Weather Research and Forecasting (WRF) model to simulate an idealised weather system and focus our attention on the boundary-layer structure of the cold front which develops. The simulation is repeated with five different treatments of the PBL, including a no-boundary-layer experiment. Using different PBL schemes can cause a difference of up to 25% in the intensity of the cross-front temperature gradient at a height of 100m, and also modify the rate at which the front intensifies. The temperature and momentum tendencies due to the different PBL schemes are compared to allow the effect of each PBL scheme on the frontal structure to be quantified. In addition, to determine whether parametrized turbulent mixing is dependent on horizontal resolution, the experiments were repeated at four horizontal grid spacings: 100km, 20km, 4km and 1.33km. Results show that momentum and temperature tendencies decrease with increasing grid spacing as expected, but that when high resolution tendencies are averaged to coarser resolutions, notable differences in the amount of mixing remain. In addition, the relative magnitude of the PBL tendencies compared to the adiabatic tendencies increases as the grid spacing decreases, indicating that PBL schemes become more influential as the resolution increases.