Comparison of CIMEL sun-photometer and ground-based GNSS integrated water vapor over south-western European sites

Abstract

This work analyzes the integrated water vapor (IWV) measured at six Aerosol Robotic Network (AERONET) stations with nearby global navigation satellite system (GNSS) in the Iberian Peninsula for the period 2007–2018. It is shown that both instruments have a high correlation (R2 > 0.91), with small mbe below 1.5 mm and standard deviation (SD) below 2 mm. However, some dependences have been observed when MBE and SD are represented in bins of three variables: IWV, solar zenith angle (SZA), and aerosol optical depth (AOD). The greater or lesser amount of water vapor in the atmosphere seemed to be the more influential variable, increasing dry bias and SD with increasing IWV. Moreover, high SZA values were related to SD increases. A clear seasonal cycle for Cimel–GNSS differences was observed which was mainly related to IWV seasonal cycle. Additionally, AOD did not show a remarkable influence on Cimel–GNSS differences. Finally, the monthly differences are also analyzed with metadata information about Cimel device ID numbers, showing that, for long-term studies, this information can be very valuable.