Accuracy assessment of water vapour measurements from in-situ and remote sensing techniques during the DEMEVAP 2011 campaign at OHP

Abstract

Abstract. The Development of Methodologies for Water Vapour Measurement (DEMEVAP) project aims at assessing and improving humidity sounding techniques and establishing a reference system based on the combination of Raman lidars, ground-based sensors and GPS. Such a system may be used for climate monitoring, radiosonde bias detection and correction, satellite measurement calibration/validation, and mm-level geodetic positioning with Global Navigation Satellite Systems. A field experiment was conducted in September–October 2011 at Observatoire de Haute Provence. Two Raman lidars, a stellar spectrometer (SOPHIE), a differential absorption spectrometer (SAOZ), a sun photometer (AERONET), 5 GPS receivers and 4 types of radiosondes (Vaisala RS92, MODEM M2K2-DC and M10, and Meteolabor Snow-White) participated in the campaign. A total of 26 balloons with multiple radiosondes were flown during 16 clear nights. This paper presents preliminary findings from the analysis of all these datasets. Several classical Raman lidar calibration methods are evaluated which use either Vaisala RS92 measurements, point capacitive humidity measurements, or GPS integrated water vapour (IWV) measurements. A novel method proposed by Bosser et al. (2010) is also tested. It consists in calibrating the lidar measurements during the GPS data processing. The methods achieve a repeatability of 4–5%. A drift in the IGN-LATMOS Raman lidar calibration of 15% over the 45 days of the experiment is evidenced but not yet explained. When this drift is removed, the precision of the calibration factors improves to 2–3%. However, the variations in the absolute calibration factor between methods and types of reference data remain at the level of 7%. The intercomparison of radiosonde measurements shows good agreement between RS92 and Snow-White measurements up to 12 km. An overall dry bias is found in the measurements from both MODEM radiosondes. Investigation of situations with low RH values (<10%) in the lower and middle troposphere reveals, on occasion, a lower RH detection limit in the Snow-White measurements compared to RS92 due to a saturation of the Peltier device. However, on other occasions, a dry bias is found in RS92, instead. Raman lidar water vapour measurements were useful to distinguish between which of the radiosondes was biased. On average, both RS92 and Snow-White measurements show a slight moist bias at night-time compared to GPS IWV, while the MODEM measurements show a large dry bias. The spectrometer IWV measurements contained a large bias that is currently under investigation. The sun photometer (daytime) and calibrated Raman lidar (night-time) IWV measurements showed excellent agreement with the GPS IWV measurements.