Snow albedo sensitivity to macroscopic surface roughness using a new ray-tracing model
<p>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...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-05-01
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| Series: | The Cryosphere |
| Online Access: | https://www.the-cryosphere.net/14/1651/2020/tc-14-1651-2020.pdf |
| Summary: | <p>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.</p>
<p>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 (<span class="inline-formula"><i>θ</i><sub>s</sub></span>) is larger than 50<span class="inline-formula"><sup>∘</sup></span>. 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; <span class="inline-formula"><10</span> m<span class="inline-formula"><sup>2</sup></span> kg<span class="inline-formula"><sup>−1</sup></span>). The effective-angle effect also
increases rapidly with <span class="inline-formula"><i>θ</i><sub>s</sub></span> at large <span class="inline-formula"><i>θ</i><sub>s</sub></span>. 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.</p>
<p>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<span class="inline-formula"><sup>−2</sup></span>). 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.</p> |
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| ISSN: | 1994-0416 1994-0424 |