Calibration of a Sky radiometer (Prede) using observations obtained from Hanle and Merak high-altitude stations in Ladakh

Abstract

The present work was carried out to check the performance of a Prede Sky radiometer and the stability of the solar calibration constant (F0s) at two high altitude pristine stations, Hanle and Merak in Ladakh region during 2008 to 2012. Two procedures for F0s calculation were applied: one based on the normal Langley plot technique, the other based on a modified method that allows determining F0s from measurements of sky irradiance. The instrument measures sun/sky irradiance at five wavelengths in the visible and near infrared regions, and the data were processed using 4.2 version of the Skyrad.Pack software. The inputs of the software are solid view angles (SVA) at the five wavelengths and sky irradiances at different scattering angles normalized by the direct irradiances. Auxiliary sets of data such as surface albedo, atmospheric pressure, and total column ozone of the station are required to run the software. The software has two processing levels. In the first scheme, Aerosol optical depth (AOD) is retrieved along with single scattering albedo (SSA), phase function, and volume size distribution as the products of radiative transfer calculation. In the second scheme, AOD and the other parameters are calculated from Lambert–Beer law after estimating the F0s from the first scheme. On an average, the deviation of SVA during the last five year period was found to be stable with less than 1.0% from the SVA values set during installation. The estimated F0s from normal and improved Langley methods were quite agreed. The estimated F0s show temporal variation and the uncertainties of estimated F0s in the present study were within 2%. Further, uncertainties of AOD and SSA due to ± 2% variation in the F0s were within 0.02 and 0.2, respectively. The estimated coefficients of variation (CV) of the determined F0s show a dependence on the observed AOD.