ON GRAVITY WAVES IN THE ATMOSPHERE

Abstract

Seven times in a year's continuous observations, marked oscillations with periods from 5 to 15 minutes were simultaneously recorded on a barograph and a damped anemometer located at La Jolla, California. The oscillations often followed a reversal of the land- and sea-breeze regime, and they were sometimes preceded by a pressure pulse. Perturbations of pressure (p) and wind speed (v) attain double amplitudes up to several millibars and several meters per second, respectively, with maximum pressure occurring at the time of maximum ?orbital? wind. This suggests propagating gravity waves in the atmosphere. Their velocity (C) can be inferred from the La Jolla records according to the impedance relationship, p = ?vC; the computed arrival time at Point Loma, 11 miles to the south, agrees with the recorded arrival. Phase velocities are of the order of 10 m/s and greatly exceed ambient winds. Wavelengths range from 4 to 10 kilometers. A slight effect on sea level is apparent. Under steady meteorological conditions, there is good coherence for at least four wavelengths in the direction of propagation, but less coherence at right angles to this direction. The wave crests appear to be oriented normal to the wind shear between the upper and lower winds. The observed wave velocity is of the order given by the shallow ?water? theory, i.e., (gh ? ln ?)½, where h is the elevation of the inversion layer, and ? ln ? is the logarithmic change in potential temperature across this layer. The observed period is not inconsistent with the period 2π/s of the fundamental mode of the least dispersive (longest) ?trapped? waves, where s2 = g d (ln ?)/dz is a measure of the stability above the inversion layer.