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The role of sound waves in sea‐breeze initiation

The dynamics of the pressure distribution associated with the sea‐breeze circulation is investigated. Analysis of hourly observations of surface pressure reveals that, on a typical sea‐breeze day, large surface‐pressure falls are recorded very far inland. Using a linear numerical model we show that this can only be understood when sound waves are taken into account. Sound waves are excited when air in the boundary layer over land expands due to diabatic heating. the numerical study reveals that vertically travelling sound waves induce a pressure increase through the entire atmosphere above the heated layer within a few minutes, while horizontally travelling sound waves induce a surface‐pressure decrease over land and a surface‐pressure increase over sea, with the resulting horizontal surface‐pressure gradient initiating the sea breeze. Because the signal travels inland at the speed of sound (about 300 m s‐1), points at a distance of more than 1000 km from the coast experience a surface‐pressure decrease within an hour after the initiation of diabatic heating over land. the pressure decrease near the earth's surface over land, as well as the pressure increase aloft, is in accord with the observations at mountain stations and the analyses of the observed daytime surface‐pressure changes in summer over the continent and adjacent seas. the implications of these findings for hydrostatic modelling of sea breezes are investigated. It appears that after one hour the hydrostatic sea breeze is 10% stronger than the non‐hydrostatic sea breeze, while the hydrostatic return current is about 20% weaker.