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The remote sensing of total column water vapour (TCWV) from polar orbiting,sun-synchronous satellite spectrometers such as the Medium Resolution ImagingSpectrometer (MERIS) on board of ENVISAT and the Moderate ImagingSpectroradiometer (MODIS) on board of Aqua and Terra enables observations ona high spatial resolution and a high accuracy over land surfaces. Theobservations serve studies about small-scale variations of water vapour aswell as the detection of local and global trends. However, depending on theswath width of the sensor, the temporal sampling is low and the observationsof TCWV are limited to cloud-free land scenes.  This study quantifies the representativeness of a single TCWV observation atthe time of the satellite overpass under cloud-free conditions byinvestigating the diurnal cycle of TCWV using 9 years of a 2-hourly TCWV dataset from global GNSS (Global Navigation Satellite Systems) stations. It turnsout that the TCWV observed at 10:30 local time (LT) is generally lower thanthe daily mean TCWV by 0.65 mm (4 %) on average for cloud-free cases.Averaging over all GNSS stations, the monthly mean TCWV at 10:30 LT,constrained to cases that are cloud-free, is 5 mm (25 %) lower thanthe monthly mean TCWV at 10:30 LT of all cases. Additionally, the diurnalvariability of TCWV is assessed. For the majority of GNSS stations, theamplitude of the averaged diurnal cycle ranges between 1 and 5 % of thedaily mean with a minimum between 06:00 and 10:00 LT and maximum between16:00 and 20:00 LT. However, a high variability of TCWV on an individual dayis detected. On average, the TCWV standard deviation is about 15 %regarding the daily mean

Representativeness of total column water vapour retrievals from instruments on polar orbiting satellites