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| Titel |
Sensitivity studies on the impacts of Tibetan Plateau snowpack pollution on the Asian hydrological cycle and monsoon climate |
| VerfasserIn |
Y. Qian, M. G. Flanner, L. R. Leung, W. Wang |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 5 ; Nr. 11, no. 5 (2011-03-02), S.1929-1948 |
| Datensatznummer |
250009439
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| Publikation (Nr.) |
 copernicus.org/acp-11-1929-2011.pdf |
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| Zusammenfassung |
The Tibetan Plateau (TP) has long been identified to be critical in
regulating the Asian monsoon climate and hydrological cycle. In this
modeling study a series of numerical experiments with a global climate model
are designed to simulate radiative effect of black carbon (BC) and dust in
snow, and to assess the relative impacts of anthropogenic CO2 and
carbonaceous particles in the atmosphere and snow on the snowpack over the
TP and subsequent impacts on the Asian monsoon climate and hydrological
cycle. Simulations results show a large BC content in snow over the TP,
especially the southern slope. Because of the high aerosol content in snow
and large incident solar radiation in the low latitude and high elevation,
the TP exhibits the largest surface radiative flux changes induced by
aerosols (e.g. BC, Dust) in snow compared to any other snow-covered regions
in the world.
Simulation results show that the aerosol-induced snow albedo perturbations
generate surface radiative flux changes of 5–25 W m−2 during spring,
with a maximum in April or May. BC-in-snow increases the surface air
temperature by around 1.0 °C averaged over the TP and reduces spring
snowpack over the TP more than pre-industrial to present CO2 increase
and carbonaceous particles in the atmosphere. As a result, runoff increases
during late winter and early spring but decreases during late spring and
early summer (i.e. a trend toward earlier melt dates). The snowmelt
efficacy, defined as the snowpack reduction per unit degree of warming
induced by the forcing agent, is 1–4 times larger for BC-in-snow than
CO2 increase during April–July, indicating that BC-in-snow more
efficiently accelerates snowmelt because the increased net solar radiation
induced by reduced albedo melts the snow more efficiently than snow melt due
to warming in the air.
The TP also influences the South (SAM) and East (EAM) Asian monsoon through
its dynamical and thermal forcing. Simulation results show that during
boreal spring aerosols are transported by southwesterly, causing some
particles to reach higher altitude and deposit to the snowpack over the TP.
While BC and Organic Matter (OM) in the atmosphere directly absorb sunlight
and warm the air, the darkened snow surface polluted by BC absorbs more
solar radiation and increases the skin temperature, which warms the air
above through sensible heat flux. Both effects enhance the upward motion of
air and spur deep convection along the TP during the pre-monsoon season,
resulting in earlier onset of the SAM and increase of moisture, cloudiness
and convective precipitation over northern India. BC-in-snow has a more
significant impact on the EAM in July than CO2 increase and
carbonaceous particles in the atmosphere. Contributed by the significant
increase of both sensible heat flux associated with the warm skin
temperature and latent heat flux associated with increased soil moisture
with long memory, the role of the TP as a heat pump is elevated from spring
through summer as the land-sea thermal contrast increases to strengthen the
EAM. As a result, both southern China and northern China become wetter, but
central China (i.e. Yangtze River Basin) becomes drier – a near-zonal
anomaly pattern that is consistent with the dominant mode of precipitation
variability in East Asia.
The snow impurity effects reported in this study likely represent some upper
limits as snowpack is remarkably overestimated over the TP due to excessive
precipitation. Improving the simulation of precipitation and snowpack will
be important for improved estimates of the effects of snowpack pollution in
future work. |
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