Retrieval of characteristic parameters for water vapour transmittance in the development of ground-based sun–sky radiometric measurements of columnar water vapour

Abstract

Abstract. Sun-sky radiometers are instruments created for aerosol study, but they can measure in the water vapour absorption band allowing the estimation of columnar water vapour in clear sky simultaneously with aerosol characteristics, with high temporal resolution. A new methodology is presented for estimating calibration parameters (i.e. characteristic parameters of the atmospheric transmittance and solar calibration constant) directly from the sun-sky radiometer measurements. The methodology is based on the hypothesis that characteristic parameters of the atmospheric transmittance are dependent on vertical profiles of pressure, temperature and moisture occurring at each site of measurement. To obtain the parameters from the proposed methodology some seasonal independent measurements of columnar water vapour taken over a large range of solar zenith angle simultaneously with the sun-sky radiometer measurements, are needed. In this work high time resolution columnar water vapour measurements by GPS were used as independent data set, but also the case when such measurements are not available was considered by developing the surface humidity method (SHM). This methodology makes it possible to retrieve the needed independent data set of columnar water vapour using the standard surface meteorological observations (temperature, pressure and relative humidity) more readily available. The time pattern of columnar water vapour from sun-sky radiometer retrieved using both the methodologies was compared with simultaneous measurements from microwave radiometer, radiosondings and GPS. Water vapour from sun-sky radiometer, obtained using GPS independent measurements, was characterized by an error varying from 1 % up to 5 %, whereas water vapour from SHM showed an error from 1 % up to 11 %, depending on the local columnar water occurring at the site during the year. These errors were estimated by comparing water vapour series from sun-sky radiometer against measurements taken by GPS at a nearby station. The accordance between retrievals from sun-sky radiometer and simultaneous measurements from the other instruments was found always within the error both in the case of SHM and of the GPS independent data set.Water vapour obtained using characteristic parameters of the atmospheric transmittance dependent on water vapour was also compared against GPS retrievals, showing a clear improvement with respect to the case when these parameters are kept fixed.

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

“…Several methodologies have been recently developed in order to obtain accurate estimations: Global Navigation Satellite System (GNSS), including the Global Positioning System (GPS) (Bevis et al, 1992;Guerova et al, 2016), microwave radiometers (Elgered et al, 1982) and sun photometers (Campanelli et al, 2014;Halthore et al, 1997;Alexandrov et al, 2004) are the automatic remote sensing instruments able to provide precise and frequent measurements of W . All of them have some pros and cons.…”

“…Finally, the relatively low cost and easy deployment of sun photometers allowed the establishment of several international networks in the last 30 years worldwide. AErosol RObotic NETwork (AERONET) network (Holben et al, 1998;Smirnov et al, 2004), Global Atmospheric Watching Precision Filter Radiometer (GAW-PFR) network (Wehrli, 2000;Nyeki et al, 2005) and European Skynet Radiometers (ESR-SKYNET) network (Campanelli et al, 2012(Campanelli et al, , 2014 provide operationally free estimation of W or develop algorithms and open-source packages for improving the retrieval of this important atmospheric pa- rameter. However, the great problem of this methodology is the estimation of the sun-photometric calibration parameters.…”

“…However, the great problem of this methodology is the estimation of the sun-photometric calibration parameters. The objective of this paper is to validate a new methodology, originally developed and applied at a single site in Japan (Campanelli et al, 2014), at three European ESR-SKYNET sites characterized by different atmospheric and climatic conditions. The methodology is based on the hypothesis that the calibration parameters characterizing the atmospheric transmittance at 940 nm (the wavelength used by sun photometers for retrieving W ) are dependent on vertical profiles of temperature, air pressure and moisture typical of each measurement site.…”

Water vapour retrieval using the Precision Solar Spectroradiometer

“…Other sun photometers such as precision filter radiometers (PFRs) (Nyeki et al, 2005) have also been used by the Global Atmosphere Watch (GAW) WMO programme to monitor IWV. Furthermore, the SKYNET radiometer network (details at http://atmos2.cr.chiba-u.jp/skynet/) also retrieves IWV using Prede-POM sun photometers at many stations (Campanelli et al, 2012(Campanelli et al, , 2014. Finally, national networks of sun photometers that are installed and operating in some countries also provide integrated water vapour (IWV) retrievals; e.g.…”

“…CIMEL, PREDE-POM, MFR) have also been used to calculate water vapour transmittance and thus estimate IWV. Filter radiometer recordings in the spectral region around water vapour absorption bands in the near-infrared region are used to calculate this quantity (Halthore et al, 1997;Campanelli et al, 2018;Nyeki et al, 2005). The World Meteorological Organization (WMO) recommends the use of spectral windows centred around 719, 817 and 946 nm, though the most frequently used is the 946 nm bandpass, which Ingold et al (2000) showed provides the most robust results.…”