The impact of a non-uniform land surface on the radiation environment over an Arctic fjord - a study with a 3D radiative transfer model for stratus clouds over the Hornsund fjord, Spitsbergen

This paper estimates the influence of land topography and cover on 3D radiativeeffects under overcast skies in the Arctic coastal environment, in particular in theHornsund fjord region, Spitsbergen. The authors focus on the impact of anon-uniform surface on: (1) the spatial distribution of solar fluxesreaching the fjord surface, (2) spectral shortwave cloud radiative forcing atthe fjord surface, (3) the solar flux anomaly at the domain surface resultingfrom the assumption of a uniform surface, i.e. the error due to plane parallelassumptions in climate models, and (4) remote sensing of cloud opticalthickness over the fjord. Their dependence on spectral channel, cloud opticalthickness, cloud type, cloud base height, surface albedo and solar zenithangle is discussed. The analysis is based on Monte Carlo simulations of solarradiation transfer over a heterogeneous surface for selected channels of theMODIS radiometer. The simulations showed a considerable impact of the landsurrounding the fjord on the solar radiation over the fjord. The biggestdifferences between atmospheric transmittances over the fjord surface and over theocean were found for a cloud optical thickness τ = 12, low solar zenith angle θ, high cloud base and snow-covered land. For τ = 12, θ = 53°, cloud base height 1.8 km andwavelength λ = 469 nm, the enhancement in irradiance transmittanceover the fjord was 0.19 for the inner fjords and 0.10 for the whole fjord(λ = 469 nm). The land surrounding the Hornsund fjord also hada considerable impact on the spectral cloud radiative forcing on the fjordsurface and the solar flux anomaly at the domain surface due to the uniformsurface assumption. For the mouth and central part of the fjord the error dueto the use of channel 2 of the MODIS radiometer (λ = 858 nm) forcloud optical thickness retrieval was < 1 in the case of low-level clouds(cloud base height 1 km, nadir radiance, θ = 53°, cloudoptical thickness retrieved solely from MODIS channel 2). However, near theshoreline (up to 2 km from it), especially over the inner fjords, the cloudoptical thickness was then overestimated by > 3 for τ = 5 and by> 5 for τ = 20.