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

[摘要] The Development of Methodologies for Water Vapour Measurement (DEMEVAP)project aims at assessing and improving humidity sounding techniques andestablishing a reference system based on the combination of Raman lidars,ground-based sensors and GPS. Such a system may be used for climatemonitoring, radiosonde bias detection and correction, satellite measurementcalibration/validation, and mm-level geodetic positioning with GlobalNavigation Satellite Systems. A field experiment was conducted inSeptember–October 2011 at Observatoire de Haute-Provence (OHP). Two Ramanlidars (IGN mobile lidar and OHP NDACC lidar), a stellar spectrometer(SOPHIE), a differential absorption spectrometer (SAOZ), a sun photometer(AERONET), 5 GPS receivers and 4 types of radiosondes (Vaisala RS92, MODEMM2K2-DC and M10, and Meteolabor Snow White) participated in the campaign. Atotal of 26 balloons with multiple radiosondes were flown during 16 clearnights. This paper presents preliminary findings from the analysis of allthese data sets. Several classical Raman lidar calibration methods areevaluated which use either Vaisala RS92 measurements, point capacitivehumidity measurements, or GPS integrated water vapour (IWV) measurements. Anovel method proposed by Bosser et al. (2010) is also tested. It consists incalibrating the lidar measurements during the GPS data processing. Themethods achieve a repeatability of 4–5%. Changes in the calibration factorof IGN Raman lidar are evidenced which are attributed to frequent opticalre-alignments. When modelling and correcting the changes as a linearfunction of time, the precision of the calibration factors improves to 2–3%.However, the variations in the calibration factor, and hence theabsolute accuracy, between methods and types of reference data remain at thelevel of 7%. The intercomparison of radiosonde measurements shows goodagreement between RS92 and Snow White measurements up to 12 km. An overalldry bias is found in the measurements from both MODEM radiosondes.Investigation of situations with low RH values (< 10%RH) in thelower and middle troposphere reveals, on occasion, a lower RH detectionlimit in the Snow White measurements compared to RS92 due to a saturation ofthe Peltier device. However, on other occasions, a dry bias is found inRS92, instead. On average, both RS92 and Snow White measurements show aslight moist bias at night-time compared to GPS IWV, while the MODEMmeasurements show a large dry bias. The IWV measurements from SOPHIE(night-time) and SAOZ (daytime) spectrometers, AERONET photometer (daytime)and calibrated Raman lidar (night-time) showed excellent agreement with theGPS IWV measurements.

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