Detailed Acoustic Structure above the Tropopause

Willis L. Webb U.S. Army Signal Missile Support Agency, White Sands Missile Range, N. Mex.

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Abstract

The general features of atmospheric acoustic structure in the lower mesosphere have been known for several decades. Initial studies were accomplished by surface observations of anomalous propagation of pressure perturbations generated by explosions. Recently the expulsion of explosive grenades from rockets has served to reduce the spatial observation parameter from many miles to the order of 10,000 ft. The material considered here is based on a resolution of the order of 1000 ft over most of the region of interest. Data are presented on speed of sound structure evaluated from wind and temperature measurements obtained from meteorological rocket system observations. The environment is assumed to be a perfect gas of molecular composition the same as dry air at the earth's surface. The seasonal course of the general mesospheric sonic gradient is discussed, illustrating a midwinter maximum of some 6 × 10−3 per second and a mid-July minimum of 1 × 10−3 per second for a sound front approaching from the west in the horizontal planes in mid-latitudes. The same data in the sub-polar case presents a reduced maximum of some 4 × 10−3 per second and a shift in the date of minimum value to mid-June. Strength and frequency of occurrence of detailed features are presented, and extreme values of sonic gradient are presented as a function of height interval. The acoustic variability is considered from the latitude point of view, and the temperature and wind contributions to the sonic gradient are discussed.

Abstract

The general features of atmospheric acoustic structure in the lower mesosphere have been known for several decades. Initial studies were accomplished by surface observations of anomalous propagation of pressure perturbations generated by explosions. Recently the expulsion of explosive grenades from rockets has served to reduce the spatial observation parameter from many miles to the order of 10,000 ft. The material considered here is based on a resolution of the order of 1000 ft over most of the region of interest. Data are presented on speed of sound structure evaluated from wind and temperature measurements obtained from meteorological rocket system observations. The environment is assumed to be a perfect gas of molecular composition the same as dry air at the earth's surface. The seasonal course of the general mesospheric sonic gradient is discussed, illustrating a midwinter maximum of some 6 × 10−3 per second and a mid-July minimum of 1 × 10−3 per second for a sound front approaching from the west in the horizontal planes in mid-latitudes. The same data in the sub-polar case presents a reduced maximum of some 4 × 10−3 per second and a shift in the date of minimum value to mid-June. Strength and frequency of occurrence of detailed features are presented, and extreme values of sonic gradient are presented as a function of height interval. The acoustic variability is considered from the latitude point of view, and the temperature and wind contributions to the sonic gradient are discussed.

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