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| Titel |
Black carbon in snow in the upper Himalayan Khumbu Valley, Nepal: observations and modeling of the impact on snow albedo, melting, and radiative forcing |
| VerfasserIn |
H.-W. Jacobi, S. Lim, M. Ménégoz, P. Ginot, P. Laj, P. Bonasoni, P. Stocchi, A. Marinoni, Y. Arnaud |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1994-0416
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| Digitales Dokument |
URL |
| Erschienen |
In: The Cryosphere ; 9, no. 4 ; Nr. 9, no. 4 (2015-08-21), S.1685-1699 |
| Datensatznummer |
250116841
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| Publikation (Nr.) |
 copernicus.org/tc-9-1685-2015.pdf |
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| Zusammenfassung |
| Black carbon (BC) in snow in the Himalayas has recently attracted
considerable interest due to its impact on snow albedo, snow and glacier
melting, regional climate and water resources. A single particle soot
photometer (SP2) instrument was used to measure refractory BC (rBC) in a
series of surface snow samples collected in the upper Khumbu Valley, Nepal
between November 2009 and February 2012. The obtained time series indicates
annual cycles with maximum rBC concentrations before the onset of the
monsoon season and fast decreases during the monsoon period. Detected
concentrations ranged from a few up to 70 ppb with rather large
uncertainties due to the handling of the samples. Detailed modeling of the
snowpack, including the detected range and an estimated upper limit of BC
concentrations, was performed to study the role of BC in the seasonal
snowpack. Simulations were performed for three winter seasons with the
snowpack model Crocus, including a detailed description of the radiative
transfer inside the snowpack. While the standard Crocus model strongly
overestimates the height and the duration of the seasonal snowpack, a better
calculation of the snow albedo with the new radiative transfer scheme
enhanced the representation of the snow. However, the period with snow on
the ground without BC in the snow was still overestimated between 37 and 66
days, which was further diminished by 8 to 15 % and more than 40 % in
the presence of 100 or 300 ppb of BC. Compared to snow without BC, the albedo
is reduced on average by 0.027 and 0.060 in the presence of 100 and 300 ppb
BC. While the impact of increasing BC in the snow on the albedo was largest
for clean snow, the impact on the local radiative forcing is the opposite.
Here, increasing BC caused an even larger impact at higher BC
concentrations. This effect is related to an accelerated melting of the
snowpack caused by a more efficient metamorphism of the snow due to an
increasing size of the snow grains with increasing BC concentrations. The
melting of the winter snowpack was shifted by 3 to 10 and 17 to 27 days
during the three winter seasons in the presence of 100 and 300 ppb BC
compared to clean snow, while the simulated annual local radiative forcing
corresponds to 3 to 4.5 and 10.5 to 13.0 W m−2. An increased
sublimation or evaporation of the snow reduces the simulated radiative
forcing, leading to a net forcing that is lower by 0.5 to 1.5 W m−2,
while the addition of 10 ppm dust causes an increase of the radiative
forcing between 2.5 and 3 W m−2. According to the simulations, 7.5 ppm
of dust has an effect equivalent to 100 ppb of BC concerning the impact on
the melting of the snowpack and the local radiative forcing. |
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