Analyzing the synoptic-, meso- and local-scale involved in sea breeze formation and frontal characteristics

Abstract

Sea breeze (SB) frontal passages, the relevant factors influencing their formation and their interaction with local turbulence, are analyzed. To proceed, numerical simulations from the Weather Research and Forecasting (WRF) model are compared with a comprehensive observational database from the Cabauw Experimental Site for Atmospheric Research site, spanning a 10-year period (January 2001 to December 2010). The fine horizontal resolution of 2 km and the replication of the observational vertical levels allow for a more precise comparison. An algorithm based on objective and strict criteria was applied to both observations and simulations to select the SB events. By carrying out a filter-by-filter comparison, we find that the simulated large-scale conditions show a good rate of coincidence with the reanalysis (69%). Small biases in the large-scale wind direction, however, induce important deviations in the surface-wind evolution. Regarding 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 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 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.