This report presents the results of an investigation to improve data quality of water vapor measurements in Paramaribo (Surinam) during February/March 2004. Two types of water vapor sensors are used: a capacitive polymer sensor applied in the Vaisala RS80 and RS90 radiosondes and the Snow White (SW) chilled mirror hygrometer produced by Meteolabor
AG. Two types of RS80 radiosondes are used: one using an A-Humicap polymer humidity sensor and one using a more sensitive H-Humicap sensor. The RS90 radiosonde uses two alternately heated H-Humicap sensors.
The RS80 radiosondes have been subject to a 100% relative humidity (RH) ground check before launch, using a specially designed calibration box. For the RS80-18LE and RS80-15GE (both A-Humicap) no significant bias in the RH data is found. One of the tests pointed at an increase of the bias with the age of the radiosonde. The RS80-15GHE (H-Humicap) radiosondes tested in Paramaribo, have an average wet bias of 4,1 ‡ 1,4 %, mainly caused by a calibration error.
Apart from a possible bias, all RS80 RH data agree well with the SW data in the lower troposphere (0-10 km). The SW data can be used as a reference up to the tropopause. If the RS80-15GHE RH data are corrected for their individual wet bias, better agreement with SW is reached. In the middle and upper troposphere R$80 RH data lose detail and are not reliable anymore.
Below -30 °C, the capacitance of the RS80 polymer humidity sensors becomes strongly temperature dependent, causing a dry bias. The R$80 (A- and H-Humicap) RH profiles are corrected for this behavior with formula's found in literature, showing better agreement with the SW profiles.
Finally, a comparison is made between ECMWF model output and the SW, RS80 and RS90 sounding data. Considering the large scale of the model output (1-degree longitude and I-degree latitude), ECMWF profiles agree well with the sounding data in Paramaribo.
Pier Dolmans. Assessment of data quality of water vapor measurements in Paramaribo : January-March 2004
KNMI number: TR-265, Year: 2004, Pages: 46