Comparison of solar spectrum from measurements and that from radiative transfer model under overcast sky condition

The solar spectrum under overcast sky condition is of importance for determining the optical properties of cloud by using a radiative transfer model. In this study, solar spectrum under overcast sky condition was measured by employing a spectroradiometer (Instrument System, model SP-320) at Nakhon Pathom station (13.82 ˚N, 100.04 ˚E), Thailand. The wavelength range of the measurements is 220-2400 nm, with the resolution of 1 nm in the wavelength range: 220-1700 nm and the resolution of 20 nm in the wavelength range: 1700-2400 nm. The overcast sky condition is determined by using the images of the sky taken by an automatic sky camera. Fifteen datasets of the solar spectrum were compared with those calculated by a radiative transfer model called "LIBRADTRAN". The input data of the radiative transfer model were obtained from both measurements and literature. As part of the input data, aerosol optical properties were obtained from an AERONET sunphotometer, total ozone column from OMI/AURA satellite and cloud base height from a ceilometer. It was found that the solar spectrum from the measurements and that from the calculation are in reasonable agreement, with the discrepancy in terms of root mean square error (RMSE) of 0.043 W·m⁻²·nm⁻¹. This comparison indicates that this radiative transfer model is accurate enough for use in determining the optical properties of clouds.