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, cheap and easy to implement, is presented for estimating calibration parameters (i.e. characteristic parameters of the atmospheric transmittance and solar calibration constant) directly from the sun-sky radiometers measurements. To initiate 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 the Surface Humidity Method (SHM) was developed allowing to initiate the procedure with columnar water vapour estimated by standard surface meteorological observation (temperature, pressure and relative humidity). The time pattern of columnar water vapour from sun-sky radiometer was compared with simultaneous measurements from microwave radiometer and radiosondings showing respectively a total correlation of 0.98, 0.96 and a total median difference of 2.24 and −0.65 mm. The accordance with radiosondings was found within the uncertainty of the methodology (varying from 10 to 16%) independently on the amount of atmospheric water vapour.