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The estimation of the precipitable water vapour content (W) with high\udtemporal and spatial resolution is of great interest to both meteorological\udand climatological studies. Several methodologies based on remote sensing\udtechniques have been recently developed in order to obtain accurate and\udfrequent measurements of this atmospheric parameter. Among them, the relative\udlow cost and easy deployment of sun–sky radiometers, or sun photometers,\udoperating in several international networks, allowed the development of\udautomatic estimations of W from these instruments with high temporal\udresolution. However, the great problem of this methodology is the estimation\udof the sun-photometric calibration parameters. The objective of this paper is\udto validate a new methodology based on the hypothesis that the calibration\udparameters characterizing the atmospheric transmittance at 940 nm are\uddependent on vertical profiles of temperature, air pressure and moisture\udtypical of each measurement site. To obtain the calibration parameters some\udsimultaneously seasonal measurements of W, from independent sources, taken\udover a large range of solar zenith angle and covering a wide range of W,\udare needed. In this work yearly GNSS/GPS datasets were used for obtaining a\udtable of photometric calibration constants and the methodology was applied\udand validated in three European ESR-SKYNET network sites, characterized by\uddifferent atmospheric and climatic conditions: Rome, Valencia and Aosta.\udResults were validated against the GNSS/GPS and AErosol RObotic NETwork (AERONET) W estimations. In both the validations the agreement was very high, with a percentage RMSD of about 6, 13 and 8 % in the case of\udGPS intercomparison at Rome, Aosta and Valencia, respectively, and of 8 %\udin the case of AERONET comparison in Valencia.\ud \udAnalysing the results by W classes, the present methodology was found to\udclearly improve W estimation at low W content when compared against AERONET in terms of % bias, bringing the agreement with the GPS (considered the reference one) from a % bias of 5.76 to 0.52

Precipitable water vapour content from ESR/SKYNET sun–sky radiometers: validation against GNSS/GPS and AERONET over three different sites in Europe