Proper Orthogonal Decomposition in the analysis of a laboratory simulation of land-a and sea- breeze regimes

A land- and sea-breeze (LSB) circulation under a calm stably stratified environment was simulated in the laboratory using a temperature-controlled water tank. The floor of the tank was divided into two sections representing land and sea. Two heat exchangers, each of them connected to a thermostat, simulated the diurnal thermal cycle typically experienced by the surface in coastal zones. A third heat exchanger positioned at the top of the tank provided a stable thermal stratification. Particletracking velocimetry was applied to evaluate the two-dimensional velocity field in the vertical centreline section of the tank orthogonal to the coastline, while a rack of thermocouples measured the vertical temperature profile near the coastline and further inland. It is shown that the overall flow consists of a closed circulation caused by the periodic change of the horizontal temperature difference between land and sea surfaces. Furthermore, the formations of cellular convection during the first phase of warming of the land-side as well as the genesis of the sea-breeze front were detected and analysed. Application of the proper orthogonal decomposition (POD) technique allowed the vortical large-scale structures in the flow to be determined. The results suggest that the energy contained in the first POD eigenmodes rapidly converged with the first mode, associated with the overall LSB circulation, being dominant with 73% of the energy. The other less energetic modes were mainly associated with the cellular convection.