Abstract
Abstract. In this study, we discuss the differences in the total precipitable water (TPW), retrieved from a Cimel sun photometer operating at a continental site in southeast Europe, between 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 values during summer. The sun photometer and the microwave radiometer depict 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 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 measurements are in good agreement with those obtained by the radiosonde, especially during night-time 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 the 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. The calculated one-sigma uncertainty for V3 as estimated, from the comparison with the radiosondes, is about 10 %, which is in accordance with previous studies for the estimation of uncertainty for V2. This uncertainty is further reduced to about 6 % when AERONET V3 is compared with the collocated microwave radiometer. 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.
Highlights
Water vapour is a crucial atmospheric component of Earth’s climate since it is the most abundant greenhouse gas (IPCC, 2013)
We discuss the differences in the total precipitable water (TPW), retrieved from a Cimel sun photometer operating at a continental site in southeast Europe, between version 3 (V3) and version 2 (V2) of the AErosol RObotic NETwork (AERONET) algorithms
The gaps in Cimel sun photometer time series are due to the calibration of the instrument, which requires the reallocation of the instrument
Summary
Water vapour is a crucial atmospheric component of Earth’s climate since it is the most abundant greenhouse gas (IPCC, 2013). Water vapour plays a prominent role in the hydrological cycle through water evaporation and condensation while providing the energy to drive moist convection and resulting precipitation. The large-scale flow and local circulations contribute to the large variability of the spatial and temporal distribution of water vapour. For weather forecasting, precipitation efficiency is strongly related to the water vapour content, which in turn determines the potential stability of the atmo-. K. Fragkos et al.: Assessment of Cimel V3 TPW spheric column. Accurate estimations of water vapour content are essential for meteorological and climate applications such as radiative transfer modelling Paynter and Ramaswamy, 2012) or weather forecasting Accurate estimations of water vapour content are essential for meteorological and climate applications such as radiative transfer modelling (e.g. Paynter and Ramaswamy, 2012) or weather forecasting (e.g. Liang et al, 2015)
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