Assessment of the total precipitable water from a sun-photometer, microwave radiometer and radiosondes at a continental site in southeastern Europe

Abstract. In this study, we discuss the differences in the total precipitable water (TPW), retrieved from a Cimel sunphotometer operating at a continental site in South-East Europe, between the Version 3 (V3) and Version 2 (V2) of the Aerosol Robotic Network (AERONET) algorithms. In addition, we evaluate the performance of the two algorithms comparing their product with the TPW obtained from a collocated microwave radiometer and nearby radiosondes during the period 2007–2017. The TPW from all three instruments was highly correlated, showing the same annual cycle, with lower values during winter and higher during summer. The Sun-photometer and the microwave radiometer depicts the same daily cycle, with some discrepancies during early morning and late afternoon due to the effect of solar zenith angle on the measurements of the photometer. The TPW from the (V3) of the AERONET algorithm has small differences compared with (V2), mostly related to the use of the new laboratory-based temperature coefficients used in V3. The microwave radiometer shows very good performance compared to the radiosondes, especially during nighttime when the differences between the two instruments are almost negligible. The comparison of the sun-photometer data with high-quality independent measurements from radiosondes and radiometer shows that the absolute differences between V3 and the other two datasets are slightly higher compared with V2. However, V3 has a lower dependence from the TPW and the internal sensor temperature, indicating a better performance of the retrieving algorithm. To our knowledge, this is the first in-depth analysis of the V3 TPW and although the findings presented here are for a specific site, we believe that they are representative of other mid-latitude continental stations.