Transcript Understanding the sensitivity of Eurasian Arctic runoff

Understanding the sensitivity
of Eurasian Arctic runoff
changes to snow cover-related
surface energy flux variations
Amanda Tan
Hydro Group Seminar
March 19, 2007
Presentation Outline
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Introduction
Motivation
Study Domain
Overview of Data Used
Methodology & Corresponding Results
 Runoff
 SWE Trends, Snow Cover Extent
 Albedo
Discussion & Conclusion
Future Work
Introduction
Streamflow
Precipitation
1940
1960
1980
2000
Trend, mm/year-2
Adam and Lettenmaier (2007)
Motivation
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What is the hydrologic sensitivity of runoff in the
northern high latitudes to precipitation, temperature
and potential evapotranspiration and related energy
budget elements?
To what extent is simulated snow cover and snow
properties consistent with observed in-situ and
remotely-sensed snow products over the Eurasian
Arctic?
To what degree can correspondence between model
and observed land surface conditions and trends be
demonstrated in data-sparse high latitude regions?
How can satellite data best be used to increase
confidence in the use of hydrologic models to
interpret long term change?
Study Domain
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Lena Basin
Why the Lena?
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90% continuous
permafrost
65% of discharge
into the Arctic
Ocean
No large
reservoirs
Study Domain
Continuous , 90-100%
Discontinuous, 50-90%
Sporadic, 10-50%
Isolated, <10%
Seasonally Frozen Ground
Overview of Data Used
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Satellite albedo (1982 – 2000)
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Observed Snow Cover (1972 – 2006)
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25km EASE-Grid Weekly Composites (NSIDC)
Gauged River Discharge (1958 -1999)
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AVHRR 5km EASE-Grid Composites (NSIDC)
R-ArcticNet
UW gridded precipitation data (Adam et al, 2007)
Runoff Trends
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28 stations – assessed
for the period 1958 –
1999
Stewart et al., 2002
Three measures:
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Spring Pulse Onset
Centroid of Timing (CT)
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Fractional Flow
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Gauge Locations for the Lena
Basin
Linear Trends in
Streamflow Timing
Runoff Trends
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1978
1986
1999
Bias between adjusted daily streamflow
and actual daily streamflow for Lena at
Kusur
The adjustment
was made to
obtain daily
streamflow
Overall bias
between +0.1/-2
We undersimulate
actual streamflow
Runoff trends
Timing trends for Lena
at Aldan (1958 – 1999)
Runoff trends
Centroid Timing
Spring Pulse
> 10d earlier
0 – 5 days earlier
5 -10 days later
5 – 10 days earlier
0 – 5 days later
> 10 days later
Runoff trends
0 – 1%/yr
-1%/yr - 0%/yr
Centroid Timing
0 – 1%/yr
-1%/yr - 0%/yr
Spring Pulse
Runoff trends
0 – 2%/yr
-2%/yr - 0%/yr
Trends in May Fractional Flow
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0 – 2%/yr
-2%/yr - 0%/yr
Trends in Aug Fractional Flow
21 gauges with increasing May fractional flow
(average trend +0.61%/yr)
15 gauges with decreasing August fractional flow
Runoff trends
0 – 2%/yr
-2%/yr - 0%/yr
MJJA Fractional Flow (Melt)
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0 – 2%/yr
-2%/yr - 0%/yr
NDJF Fractional Flow (Freeze)
All gauges have decreasing melt season flow
21 gauges with increasing winter season flow
Runoff trends
Aldan
Tabaga
0.35
0.3
0.25
0.25
August Fractional Fow
May Fractional Fow
0.3
0.2
0.15
0.1
0.05
0
1955
Kusur
0.2
0.15
0.1
0.05
1965
1975
1985
0
1955
1995
Year
0.25
1965
1975
1985
1995
Year
0.2
Seasonal Fractional Flow (NDJF)
Seasonal Fractional Flow (MJJA)
0.3
0.15
0.1
0.05
0
1955
0.25
0.2
0.15
0.1
0.05
0
1955
1965
1975
Year
1985
1995
1965
1975
Year
1985
1995
SWE Trends
Temperature Trend
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248 grid cells (10000km2)
Precipitation Trend
Role of Snow as a Modulator
Control
Climate
Modulator
Snow
Precipitation
Air
Temperature
Human
Artificial
Reservoirs
Streamflow
(Q) Effects
Annual Q
Subsurface
Storage
ET
Seasonal Q
Snow Cover Extent
1972 - 1976
1982 - 1986
1972
- 1976
1977
- 1981
1977
- 1981
1982
- 1986
1977 - 1981
1982 - 1986
1987 – 1991
1987 – 1991
Snow Cover Extent
1992 - 1996
1992 - 1996
1997 - 2001
1997 - 2001
Snow Cover Extent
Simulated
Observed
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1972 – 1987: Downward
Trend (+0.41%/year)
1988 – 2006: Upward Trend
(+0.27%/year)
Correction?
Albedo Comparisons
Annual Streamflow for the Lena
8500
7500
5500
4500
3500
Year
Baseline
Run 3
Run4
Observed
2000
1990
1980
1970
1960
1950
2500
1940
Streamflow (10³ m³/s)
6500
Albedo Comparisons (Annual)
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Sensitivity test 1
Snowmelt: May 1 –
Sept 1
Sensitivity test 2
Snowmelt: April 15
– September 15
Initial clear sky
albedo taken as
0.85
Albedo Comparisons (Seasonal)
SON
DJF
MAM
JJA
Discussion & Conclusions
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How are we capturing snow cover?
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Has the land cover over the Lena changed in
recent years?
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Snow cover does not display a large downward
trend, on the contrary, for the period of 1980 -2000,
snow cover over the Lena basin has been slightly
increasing
Does this explain the spatial variation in snow cover
extent?
Fractional streamflows are changing, and SWE is
shown to be decreasing due mostly to temperature
trends
Changes in ground surface albedo and
precipitation apparently do not account for
observed discharge trends, particularly in winter.
Future Work
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Separate the albedo from VIC simulations and
from satellite-based data into seasonal (late
winter and early spring) and compare the models
performance.
Obtain the surface long-wave radiation from
ERA40 reanalysis and compare with values
obtained from the model (model gives shortwave
radiation & net radiation).
Use the longwave radiation for from ERA40,
combine it with shortwave radiation and
assimilate it into the model and see its
performance on runoff.
Compare the precipitation from ERA-40
reanalysis to the forcings used in VIC.
Look at land cover changes.