A Theoretical Study of the Sea and Land Breezes of Circular Islands

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Abstract
The axisymmetric sea and land breezes of circular islands are studied. First we show, from the equation of turbulent energy, that the marked horizontal convergence of the sea breeze intensifies the turbulence in the flow and that this conclusion affects the equations that model the turbulence. Next, the equations of motion are integrated numerically for two circular islands: a “large” island of radius 51.25 km and a “small” island of radius 26.25 km. The horizontal grid is 2.5 km and the vertical 100 m beginning from the top of a postulated constant-flux layer 25 m thick. Large island. The sea-breeze front (SBF) is much better developed than in the case of a straight coast. In the first hours of the afternoon the low-level winds ahead of the front are nearly opposite in direction to those of the sea breeze behind the front. There is a strong horizontal convergence in advance of, and divergence behind, the front up to an altitude of 400–500 m; the reverse distribution is the case aloft, where the winds ahead of the surface position of the front depart from the picture of a countercirculation and blow in the same direction as the sea breeze near the surface. The maximum computed upward velocity occurs about the front and reaches 50 cm sec−1. The most surprising feature is the formation in the afternoon of an “eye” of downward velocities around the island center ahead of the front. By late afternoon, however, the island center becomes the center of upward motion. All the above events are connected with an instability developing about the SBF, including the evolution of a pressure low about the surface position of the front. This low deepens as the front moves inland. The land breezes are horizontally divergent with the largest speeds found over the sea. Small island. The SBF reaches the center about 1300 local time so that little development can take place along the front. However, the upward velocities reach 150 cm sec−1 over the center. In conclusion, it is pointed out that the most interesting features of the sea breeze occur well inland, whereas those of the land breeze occur well offshore.

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