Summertime columnar content of atmospheric water vapor from ground-based Sun-sky radiometer measurements through a new in situ procedure

A new in situ technique for the retrieval of atmospheric water vapor content (i.e., precipitable water content) from Sun photometric direct solar irradiance measurements, taken at the 940 nm wavelength during clear-sky conditions, is presented. The procedure is applied to summer data recorded in 2007, 2008, and 2009 with a Sun-sky radiometer at the San Pietro Capofiume station in the Po valley, Italy. It is a preliminary development of the retrieval procedure providing the columnar water vapor content from measurements performed with PREDE Sun-sky radiometers. The technique brings improvement and innovation by retrieving the best values of constants (a and b), characterizing atmospheric water vapor transmittance while reducing simulation errors, and potentially contains information on seasonal changes in vertical profiles of temperature, air pressure, and moisture at measurement sites. Initially, the in situ procedure needs at least 1 week of independent measurements of precipitable water content taken over a range of solar zenith angles simultaneously with radiometric measurements, but it was also tested for cases in which independent measurements are not available. In the latter, the procedure was started using monthly precipitable water content estimates derived from surface observations of relative humidity, pressure, and temperature. Time patterns and absolute values of precipitable water content retrieved using the in situ procedure were in good agreement with Moderate Resolution Imaging Spectroradiometer retrievals and radiosonde measurements, with correlation coefficients of 0.8-0.9 and low percentage median differences of 7%-13%.