Column Water Vapor Retrievals from Sky Radiometer (POM-02) 940 nm Data

Abstract

 Sky radiometer data are used to retrieve aerosol optical properties and to estimate the effect of aerosols on solar irradiance at the Earth’s surface. The Prede POM-02 is equipped with a 940-nm channel corresponding to a primary water vapor absorption band in the near infrared region, but 940-nm data have been underutilized. Atmospheric water vapor is an important factor in determining the surface radiation budget. To retrieve the columnar precipitable water vapor amount from sun-sky radiometers, the 940-nm channel was calibrated using the Langley method, which accounts for gas absorption. The relation between column water vapor and atmospheric transmittance at 940 nm was determined using simulation data, and the results were used to retrieve column water vapor. This method was applied to data collected at Tsukuba, Japan, in 2011 and compared with global positioning system receiver (GPS), microwave radiometer, and radiosonde water vapor retrievals. The highest correlations were found between GPS and radiosonde observations. A comparison of POM-02 and GPS results showed a bias error of 0.09 g cm^-2; the root mean square error was 0.179 g cm^-2; and r = 0.996. The transmittance of the 940-nm channel was theoretically determined in this study. Therefore, the accuracy of column water vapor retrieval depends on the accuracy of transmittance calculation model.