Dry biases of humidity measurements from the Vaisala RS80-A and Meisei RS2-91 radiosondes and from ground-based GPS

During the Tsukuba GPS (Global Positioning System) Dense Network Campaign Observations in the autumn of 2000, we performed 3 hourly upper air soundings using two types of radiosondes. One was the Meisei RS2-91, operational sonde of the Japan Meteorological Agency (JMA) made by Meisei Electric Co. Ltd. with an independent thin-film capacitive sensor, and the other was Vaisala RS80-15G with A-type Humicap humidity sensor (RS80-A hereafter). It was found that the Precipitable Water Vapor (PWV) measured by Vaisala RS80-A were clearly smaller (dry bias) by 3-4 mm (about 4-6%) than those by JMA RS2-91 and those analyzed from GPS observations. The dry bias error of Vaisala RS80-A was also confirmed with dual sonde balloon flights equipped with both Vaisala RS80-A and JMA RS2-91 instruments, and near simultaneous flights of two types of radiosondes. It was found that the dry bias error was reduced by the new tight protective cap for the humidity sensor, but substantial amount of dry bias error still remained. We further compared humidity measurements by the two radiosondes to a chilled-mirror dew-point hygrometer using a two-pressure type humidity calibration chamber. The experiment was conducted under room temperature conditions. It showed that relative humidity as measured by the Vaisala RS80-A was about 5 to 15% smaller than that of the dew-point hygrometer. The bias was larger in high humidity conditions. Although the dry bias of Vaisala RS80-A radiosondes has been pointed out in several other studies, the dry bias revealed in this study is much larger than previously reported, indicating that a problem still exists in RS80-A humidity measurements taken in moist air conditions such as Japan. The RS2-91 showed only small dry bias relative to the dew-point hygrometer (less than 4%). However, the Aerological Observatory of the JMA reported that they improved the RS2-91 humidity sensor since 1999 (Shibue et al. 2000). RS2-91 sondes manufactured before and in 1999 had a dry bias with the same order of magnitude as the Vaisala RS80-A. There have been several reports (e.g., Ohtani et al. 2000) that the PWV derived by GEONET (Japanese nationwide GPS array) and other campaign GPS observations agreed well with those by JMA or Vaisala radiosondes. The implication of the dry bias errors of the radiosonde to the previous GPS studies is that the past GPS analysis in Japan produced a drier atmosphere than actual conditions. It is recommended that the past GPS data, including the GEONET data, be reanalyzed using the most advanced GPS analysis methods.