Performance of the Vaisala RS80A/H and RS90 Humicap Sensors and the Meteolabor “Snow White” Chilled-Mirror Hygrometer in Paramaribo, Suriname

摘要

In climate research there is a strong need for accurate observations of water vapor in the upper atmosphere.Radiosoundings provide relative humidity profiles but the accuracy of many routine instruments isnotoriously inadequate in the cold upper troposphere. In this study results from a soundings programexecuted in Paramaribo, Suriname (5.8°N, 55.2°W), are presented. The aim of this program was to comparethe performance of different humidity sensors in the upper troposphere in the Tropics and to test differentbias corrections suggested in the literature. The payload of each sounding consisted of a chilled-mirror“Snow White” sensor from Meteolabor AG, which was used as a reference, and two additional sensors fromVaisala, that is, either the RS80A, the RS80H, or the RS90. In total 37 separate soundings were made.For the RS80A a clear, dry bias of between 4% and 8% RH is found in the lower tropospherecompared to the Snow White observation, confirming the findings in previous studies. A mean dry bias wasfound in the upper troposphere, which could be effectively corrected. The RS80H sensor shows a significantwet bias of 2%–5% in RH in the middle and upper troposphere, which has not been reported before.Comparing observations with RS80H sensors of different ages gives no indication of sensor aging or sensorcontamination. It is therefore concluded that the plastic cover introduced by Vaisala to avoid sensorcontamination is effective. Finally, the RS90 sensor yields a small but significant wet bias of 2%–3% below7-km altitude.The time-lag error correction from Miloshevich et al. was applied to the Vaisala data, which resulted inan increased variability in the relative humidity profile above 9- (RS80A), 8- (RS80H), and 11-km (RS90)altitude, respectively, which is in better agreement with the Snow White data.The averaged Snow White profile is compared with the average profiles of relative humidity from theEuropean Centre for Medium-Range Weather Forecasts (ECMWF). No significant bias is found in eitherthe analyses or the forecasts. The correlation coefficient for the Snow White and ECMWF data between 200and 800 hPa was 0.66 for the 36-h forecast and 0.77 for the analysis.