Calibration of a Lyman-α Sensor to Measure In-Cloud Temperature and Clear-Air Dewpoint Temperature

Abstract

A number of previous studies have described how airborne temperature-sensor wetting can lead to erroneous temperature measurements under various conditions. During the 1985 Joint Hawaii Warm Rain Project (JHWRP) a large number of trade-wind cumuli were sampled by the University of Wyoming King Air. It was found that both the reverse flow and the Rosemount sensors showed signs of wetting during the project. Therefore, a post-field calibration of the Lyman-α hygrometer was performed to derive the temperature in the cloudy air. The University of Wyoming Lyman-α sensor is a simple system with a high-pressure emitter tube. Both water vapor and oxygen contribute to the total absorption at the altitudes where the sensor is used. Thus, a highly simplified two-gas model is proposed for its function, and the instrument is calibrated by comparison with the dewpoint temperatures in clear-air soundings. An accuracy of ±0.5°C is estimated for the calibration. Absorption by water vapor and oxygen constitute nearly 70% and 30% of the total absorption, respectively. Even for moderate liquid water contents (≈1 g m?3), the difference between the temperatures derived from the Lyman-α and from immersion sensors approaches theoretical predictions for fully wetted sensors. The present study shows a somewhat higher degree of wetting of the reverse-flow sensor than recently published results using an Ophir radiometer.