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J. T. Priestley and R. J. Hill

Abstract

Three different instrument systems are compared in their ability to either directly or indirectly measure humidity, temperature, and refractive-index fluctuations. Each system consists of a basic instrument—a Lyman-α hygrometer, an infrared absorption hygrometer or a radio refractometer—configured with its own fine-wire resistance thermometer. All measurements were obtained at a height of 5.2 m in the atmospheric surface layer. We present time series from these instruments, power spectra of humidity, temperature, and radio refractive index, as well as temperature-humidity cospectra, phase spectra, and coherence spectra. The temperature and humidity are either very well correlated or anticorrelated. The temperature-humidity cospectra have the inertial subrange power law up to wavenumbers where instrumental effects interfere. The refractive-index structure parameters calculated from the humidity and temperature fluctuations measured by the Lyman-α and its fine wire agree substantially with the structure parameters determined from the refractometer. The degradation of cospectra caused by sensor separation, the space averaging by the infrared absorption hygrometer, and the flushing-distance problem of the refractometer are illustrated.

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R. B. Fritz, R. J. Hill, J. T. Priestley, and W. P. Schoenfeld

Abstract

Rain rate during light precipitation in winter was measured with high temporal resolution optical systems at a site in Illinois. In addition to quasi-periodic variations, a clearly sinusoidal oscillation in rain rate was found imbedded in the general precipitation. The phase shift in the occurrence of the oscillation at two sensors, with the simultaneous recording of sinusoidal fluctuations of the attenuation of a millimeter wave signal, allows simulation of this particular rain pattern by a simple model. The basic mechanism that can produce a rain event with such a sinusoidal pattern is not clearly understood.

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