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DOI	10.5194/tc-14-1651-2020
	Snow albedo sensitivity to macroscopic surface roughness using a new ray-tracing model
	Larue F.; Picard G.; Arnaud L.; Ollivier I.; Delcourt C.; Lamare M.; Tuzet F.; Revuelto J.; Dumont M.
发表日期	2020
ISSN	19940416
起始页码	1651
结束页码	1672
卷号	14 期号:5
英文摘要	Most models simulating snow albedo assume a flat and smooth surface, neglecting surface roughness. However, the presence of macroscopic roughness leads to a systematic decrease in albedo due to two effects: (1) photons are trapped in concavities (multiple reflection effect) and (2) when the sun is low, the roughness sides facing the sun experience an overall decrease in the local incidence angle relative to a smooth surface, promoting higher absorption, whilst the other sides have weak contributions because of the increased incidence angle or because they are shadowed (called the effective-angle effect here). This paper aims to quantify the impact of surface roughness on albedo and to assess the respective role of these two effects, with (1) observations over varying amounts of surface roughness and (2) simulations using the new rough surface ray-tracing (RSRT) model, based on a Monte Carlo method for photon transport calculation. The observations include spectral albedo (400-1050 nm) over manually created roughness surfaces with multiple geometrical characteristics. Measurements highlight that even a low fraction of surface roughness features (7 % of the surface) causes an albedo decrease of 0.02 at 1000 nm when the solar zenith angle (θs) is larger than 50ĝˆ . For higher fractions (13 %, 27 % and 63 %), and when the roughness orientation is perpendicular to the sun, the decrease is of 0.03-0.04 at 700 nm and of 0.06-0.10 at 1000 nm. The impact is 20 % lower when roughness orientation is parallel to the sun. The observations are subsequently compared to RSRT simulations. Accounting for surface roughness improves the model observation agreement by a factor of 2 at 700 and 1000 nm (errors of 0.03 and 0.04, respectively) compared to simulations considering a flat smooth surface. The model is used to explore the albedo sensitivity to surface roughness with varying snow properties and illumination conditions. Both multiple reflections and the effective-angle effect have a greater impact with low specific surface area (SSA; <10 m2 kg-1). The effective-angle effect also increases rapidly with θs at large θs. This latter effect is larger when the overall slope of the surface is facing away from the sun and has a roughness orientation perpendicular to the sun. For a snowpack where artificial surface roughness features were created, we showed that a broadband albedo decrease of 0.05 may cause an increase in the net shortwave radiation of 80 % (from 15 to 27 W m-2). This paper highlights the necessity of considering surface roughness in the estimation of the surface energy budget and opens the way for considering natural rough surfaces in snow modelling. © 2020 Institute of Electrical and Electronics Engineers Inc.. All rights reserved.
英文关键词	albedo; energy budget; estimation method; Monte Carlo analysis; numerical model; ray tracing; shortwave radiation; snowpack; surface energy; surface roughness
语种	英语
来源期刊	Cryosphere
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/202135
作者单位	Univ. Grenoble Alpes, CNRS, Institut des Géosciences de l'Environnement (IGE), UMR 5001, Grenoble, 38041, France; Univ. Grenoble Alpes, Université de Toulouse, Météo-France, CNRS, CNRM, Centre d'Etudes de la Neige, Grenoble, 38000, France
推荐引用方式 GB/T 7714	Larue F.,Picard G.,Arnaud L.,et al. Snow albedo sensitivity to macroscopic surface roughness using a new ray-tracing model[J],2020,14(5).
APA	Larue F..,Picard G..,Arnaud L..,Ollivier I..,Delcourt C..,...&Dumont M..(2020).Snow albedo sensitivity to macroscopic surface roughness using a new ray-tracing model.Cryosphere,14(5).
MLA	Larue F.,et al."Snow albedo sensitivity to macroscopic surface roughness using a new ray-tracing model".Cryosphere 14.5(2020).