Measuring High-Frequency Humidity, Temperature and Radio Refractive Index in the Surface Layer

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.