Characterization and correction of relative humidity measurements from vaisala RS80-A radiosondes at cold temperatures

Radiosonde relative humidity (RH) measurements are known to be unreliable a t cold temperatures. This study characterizes radiosonde RH measurements fr om Vaisala RS80-A thin-film capacitive sensors in the temperature range 0 d egrees to -70 degreesC. Sources of measurement error are identified, and tw o approaches for correcting the errors are presented. The corrections given in this paper apply only to the Vaisala RS80-A sensor, although the RS80-H sensor is briefly discussed for comparison. A temperature-dependent correction factor is derived from statistical analy sis of simultaneous RH measurements from RS80-A radiosondes and the NOAA cr yogenic frostpoint hygrometer. The mean RS80-A measurement error is shown t o be a dry bias that increases with decreasing temperature, and the multipl icative correction factor is about 1.3 at -35 degreesC, 1.6 at -50 degreesC , 2.0 at -60 degreesC, and 2.4 at -70 degreesC. The fractional uncertainty in the mean of corrected measurements, when large datasets are considered s tatistically, increases from 0.06 at 0 degreesC to 0.11 at -70 degreesC. Th e fractional uncertainty for correcting an individual sounding is about +/- 0.2, which is larger because this statistical approach considers only the m ean value of measurement errors that are not purely temperature dependent. The correction must not be used outside the temperature range 0 degrees to -70 degreesC, because it is a meaningless extrapolation of a polynomial cur ve fit. Laboratory measurements of sensor response conducted at Vaisala are used to characterize some of the individual sources of RS80-A measurement error. A correction factor is derived for the dominant RS80-A measurement error at cold temperatures: an inaccurate approximation for the sensor's temperature dependence in the data processing algorithm. The correction factor for tem perature-dependence error is about 1.1 at -35 degreesC, 1.4 at -50 degreesC , 1.8 at -60 degreesC, and 2.5 at -70 degreesC. Dependences and typical mag nitudes are given for measurement errors that result from the temperature d ependence of the sensor's time constant, and from several smaller bias erro rs and random uncertainties.