Vol. 18, issue 03, article # 8

Abstract:

Regularities in formation of the spatial-angular structure of clear-sky brightness in atmospheric windows in the visible and near-IR spectral regions at large zenith viewing angles are discussed. It is shown based on computer simulation that the angular dependences of brightness components caused by single and multiple scattering behave similarly, but the latter has much less elongation (smoothes the angular dependence). In the azimuth dependence of the diffuse radiation, the main variability (5-10 times and more) is observed in the forward hemisphere because of the priority effect of the single scattering and the aerosol scattering phase function having the pronounced forward peak. The angular behavior in the backward hemisphere is determined by the multiply scattered component and the molecular scattering. The less asymmetric multiple component of sky brightness has a weak and almost linear dependence on the forward peak of the aerosol scattering phase function. The zenith distributions of the sky brightness have a maximum in the near-horizon zone. The characteristics of this maximum depend on the atmospheric turbidity: with the increase of the optical depth, the brightness maximum shifts toward the horizon. As approaching the horizon, the zenith dependences of brightness converge asymptotically to the value of the source function due to the brightness saturation of the surface haze.