Jb. Jensen et Gb. Raga, CALIBRATION OF A LYMAN-ALPHA SENSOR TO MEASURE IN-CLOUD TEMPERATURE AND CLEAN-AIR DEWPOINT TEMPERATURE, Journal of atmospheric and oceanic technology, 10(1), 1993, pp. 15-26
A number of previous studies have described how airborne temperature-s
ensor wetting can lead to erroneous temperature measurements under var
ious conditions. During the 1985 Joint Hawaii Warm Rain Project (JHWRP
) a large number of trade-wind cumuli were sampled by the University o
f Wyoming King Air. It was found that both the reverse flow and the Ro
semount sensors showed signs of wetting during the project. Therefore,
a post-field calibration of the Lyman-alpha hygrometer was performed
to derive the temperature in the cloudy air. The University of Wyoming
Lyman-alpha sensor is a simple system with a high-pressure emitter tu
be. 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 calibrat
ed by comparison with the dewpoint temperatures in clear-air soundings
. An accuracy of +/-0.5-degrees-C is estimated for the calibration. Ab
sorption by water vapor and oxygen constitute nearly 70% and 30% of th
e total absorption, respectively. Even for moderate liquid water conte
nts (almost-equal-to 1 g m-3), the difference between the temperatures
derived from the Lyman-alpha and from immersion sensors approaches th
eoretical predictions for fully wetted sensors. The present study show
s a somewhat higher degree of wetting of the reverse-flow sensor than
recently published results using an Ophir radiometer.