Analyzing the atmospheric scales involved in sea-breeze formation and frontal characteristics

We investigate sea-breeze (SB) frontal passages troughout a 10-year period. Spanning the whole period, numerical simulations from the Weather Research and Forecasting (WRF) model are compared with a comprehensive observational database from the Cabauw Experimental Site (Ruisdael Project). On the one hand, a fine horizontal resolution of 2 km is employed in the numerical simulations, and the observational vertical levels within the first 200 m above the surface are replicated. On the other hand, an algorithm based on objective and strict filters is applied to both observations and simulations to select the SB events. This methodology allows to investigate the atmospheric scales influencing the SB formation and their interaction with local turbulence in a robust and objective way.

By carrying out a filter-by-filter comparison, we find that the simulated large-scale conditions show a good rate of coincidence with the observations (69%). Small biases in the large scale wind direction, however, induce important deviations in the surface-wind evolution. Regarding the mesoscale forcings, the land-sea temperature gradient is overestimated in average up to 4 K, producing stronger SB fronts in WRF. The analysis of the SB frontal characteristics and impacts is carried out by classifying the events into three boundary-layer regimes (convective, transition and stable) based on the value of the sensible-heat flux at the moment of the SB onset. The stronger SB in the model leads to enhanced turbulence particularly in the convective and transition regimes: the friction velocity, for instance, is overstated by around 50% at the SB onset. In addition, the arrival of the SB front enhances the stable stratification and gives rise to faster afternoon and evening transitions compared with situations solely driven by local atmospheric turbulence.

The obtained results can be considered a benchmark of the aspects to be improved in order to produce finer SB forecasts and more adequate representations of the associated physical processes, particularly during the afternoon and evening transition of the ABL.