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

摘要

Sun–sky radiometers are instruments created for aerosol study, but they canmeasure in the water vapour absorption band allowing the estimation ofcolumnar water vapour in clear sky simultaneously with aerosolcharacteristics, with high temporal resolution. A new methodology ispresented for estimating calibration parameters (i.e. characteristicparameters of the atmospheric transmittance and solar calibration constant)directly from the sun–sky radiometer measurements. The methodology is basedon the hypothesis that characteristic parameters of the atmospherictransmittance are dependent on vertical profiles of pressure, temperature andmoisture occurring at each site of measurement. To obtain the parameters fromthe proposed methodology some seasonal independent measurements of columnarwater vapour taken over a large range of solar zenith angle simultaneouslywith the sun–sky radiometer measurements, are needed. In this work high timeresolution columnar water vapour measurements by GPS were used as independentdata set, but also the case when such measurements are not available wasconsidered by developing the surface humidity method (SHM). This methodologymakes it possible to retrieve the needed independent data set of columnar water vapourusing the standard surface meteorological observations (temperature, pressureand relative humidity) more readily available. The time pattern of columnarwater vapour from sun–sky radiometer retrieved using both the methodologieswas compared with simultaneous measurements from microwave radiometer,radiosondings and GPS. Water vapour from sun–sky radiometer, obtained usingGPS independent measurements, was characterized by an error varying from1% up to 5%, whereas water vapour from SHM showed an error from1% up to 11%, depending on the local columnar water occurring atthe site during the year. These errors were estimated by comparing watervapour series from sun–sky radiometer against measurements taken by GPS at anearby station. The accordance between retrievals from sun–skyradiometer and simultaneous measurements from the other instruments was foundalways within the error both in the case of SHM and of the GPS independentdata set.Water vapour obtained using characteristic parameters of the atmospherictransmittance dependent on water vapour was also compared against GPSretrievals, showing a clear improvement with respect to the case when theseparameters are kept fixed.