Aircraft observations of sea-breeze frontal structure

Detailed aircraft observations of sea-breeze frontal structure and dynamics are presented for two cases of well defined sea-breeze fronts near the east coast of England. In the first case the sea-breeze was advancing into a well mixed convective boundary layer with strong turbulence and an offshore breeze of around 3 m s(-1). In the second case the sea-breeze was penetrating into a convective boundary layer characterized by weaker turbulence and an offshore wind speed of 2-3 m s(-1). Indeed, during the course of the measurements a stable internal boundary layer was forming in the early evening. Cross-sections of the frontal structure are derived from aircraft traverses at a range of heights along a fixed line normal to the coast. A typical head-like structure is observed in both cases, with a region of strong mixing immediately seawards of the head. An approximately exponential fall-off in dissipation rate with distance seaward of the leading edge of the front is found. Heat and momentum fluxes are used to derive turbulent kinetic energy (TKE) budgets for the mixing region and indicate that turbulence in this mixing region is maintained by TKE generated by strong shear at the top of the cold-air inflow. In the first case the shear production of TKE is almost twice that of the second case, suggesting that the presence of ambient turbulence in the convective boundary layer has a significant effect upon the frontal dynamics. In the second case, as the turbulence decayed, a curious wave-like structure appeared behind the leading edge of the front. The waves have wavelengths of 1-3 km and it is unlikely that they were caused by Kelvin-Helmholtz instability. It is suggested that the waves might be a solitary wave-train emerging as the sea-breeze interacts with a low-level stable layer forming in the early evening.