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LASG学术报告(yun qian)

【学术报告】Impacts of Tibetan Plateau snowpack pollution on the Asian
hydrological cycle and monsoon climate
【报告人】     Dr. Yun Qian
【报告人单位】Pacific Northwest National Laboratory, Richland, WA
【报告时间】   2010年11月12日(星期五)10:00
【报告地点】   科研楼303会议室
【报告简介】
Impacts of Tibetan Plateau snowpack pollution on the Asian
hydrological cycle and monsoon climate

青藏高原冰雪污染对亚洲季风气候和水循环的影响

Yun Qian (钱云)  Pacific Northwest National Laboratory, Richland, WA
(美国能源部西北太平洋国家实验室)

Abstract

The Tibetan Plateau (TP), the highest and largest plateau in the
world, has long been identified to be critical in regulating the Asian
monsoon climate and hydrological cycle. The snowpack and glaciers over
the TP provide fresh water to billions of people in Asian countries,
but the TP glaciers have been retreating faster than those anywhere
else in the world. In this study a series of numerical experiments
with a global climate model are designed to simulate radiative forcing
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, with concentration larger than 100 μg/kg. 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 forcing induced by aerosols (e.g. BC, Dust) in snow
compared to other snow-covered regions in the world.

Results show that the aerosol-induced snow albedo perturbations
generate surface radiative forcing 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 1.0oC 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. 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 OM (Organic Matter) 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.

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