Incident spectral irradiance in the Arctic Basin during the summer and fall

Abstract

Calibrated values of incident spectral irradiance are reported from the Healy Oden Transarctic Experiment during August and September 2005. Spectra were obtained for a wide range of solar zenith angles, cloud conditions, and surface types to provide basic data for regional shortwave radiative energy balance calculations as well as climate model parameterization and validation. Supervised principal component analysis on the spectral continuum showed that three principal components explain over 99% of the variance resulting from darkening across the solar spectrum with increasing cloud cover by volume scattering, from stronger attenuation in the solar infrared relative to visible wavelengths by H2O, and from Rayleigh scattering. Comparison of the observations with the atmospheric radiation model SBDART showed that good agreement was obtained varying only the cloud optical depth. Applying the model, we showed how the surface albedo affects incident spectral irradiance under clear as well as cloudy skies, and we obtained a quantitative estimate of the visual effects of “water sky” and “ice blink.” We also determined the spectral albedo of the atmosphere for a dense arctic stratus cloud deck decoupled from the influence of the underlying surface. Incident spectral irradiances were integrated numerically and compared with calibrated pyranometer observations. Agreement was within 5% for cases where the cloud transparency and incident irradiance did not fluctuate strongly over the 10‐min pyranometer recording intervals. A new set of values for total albedo for clear versus cloudy conditions at high and low Sun angles is presented for six prominent arctic surface types.