A multi-site intercomparison of integrated water vapour observations for climate change analysis

摘要

Water vapour plays a dominant role in the climatechange debate. However, observing water vapour over a climatological timeperiod in a consistent and homogeneous manner is challenging. On one hand,networks of ground-based instruments able to retrieve homogeneous integratedwater vapour (IWV) data sets are being set up. Typical examples are GlobalNavigation Satellite System (GNSS) observation networks such as theInternational GNSS Service (IGS), with continuous GPS (Global PositioningSystem) observations spanning over the last 15+ years, and the AErosolRObotic NETwork (AERONET), providing long-term observations performed withstandardized and well-calibrated sun photometers. On the other hand,satellite-based measurements of IWV already have a time span of over 10 years(e.g. AIRS) or are being merged to create long-term time series (e.g. GOME,SCIAMACHY, and GOME-2).This study performs an intercomparison of IWV measurements from satellitedevices (in the visible, GOME/SCIAMACHY/GOME-2, and in the thermal infrared,AIRS), in situ measurements (radiosondes) and ground-based instruments (GPS,sun photometer), to assess their use in water vapour trends analysis. To thisend, we selected 28 sites world-wide for which GPS observations can directlybe compared with coincident satellite IWV observations, together with sunphotometer and/or radiosonde measurements. The mean biases of the differenttechniques compared to the GPS estimates vary only between −0.3 to 0.5 mmof IWV. Nevertheless these small biases are accompanied by large standarddeviations (SD), especially for the satellite instruments. In particular, weanalysed the impact of clouds on the IWV agreement. The influence of specificissues for each instrument on the intercomparison is also investigated (e.g.the distance between the satellite ground pixel centre and the co-locatedground-based station, the satellite scan angle, daytime/nighttimedifferences). Furthermore, we checked if the properties of the IWV scatterplots between these different instruments are dependent on the geographyand/or altitude of the station. For all considered instruments, the onlydependency clearly detected is with latitude: the SD of the IWV observationswith respect to the GPS IWV retrievals decreases with increasing latitude anddecreasing mean IWV.