David Braaten, Prasad Gogineni, Claude Laird, Susanne Buchardt*, and Hilary Barbour
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Transcript of David Braaten, Prasad Gogineni, Claude Laird, Susanne Buchardt*, and Hilary Barbour
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Spatial variability of interior ice-sheet accumulation determined with an FM-CW
radar and connections to the NAO
David Braaten, Prasad Gogineni, Claude Laird, Susanne Buchardt*, and Hilary Barbour
* Centre for Ice and Climate, Univ. Copenhagen
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Introduction• Snow accumulation is important in understanding ice
sheet mass balance and the accumulation/precipitation climatology.
• Detecting near-surface internal layers with radar allow regional scale assessments of snow accumulation on time scales of 1 year or less.
• Radar data permit spatial averaging to overcome local uncertainty caused by wind-generated surface features.
• Regional scale assessments of accumulation on annual time scales can lead to an understanding of links between climate indices and ice-sheet accumulation.
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RadarsInstrument Measurement Freq. BW/
Res. Depth Power Altitude Antenna Installs
AccumRadar -Surface
Internal LayeringIce Thickness
1250 MHz
1500 MHz10 cm
300 m 100 mW Surface12-element Vivaldi Array
Tracked vehicle
Accum.Radar –Airborne
Internal LayeringIce Thickness
750 MHz 300 MHz40 cm 300 m 10 W 20000 ft Patch Array
Vivaldi Array
Twin-OtterP-3Basler
SnowRadar
Snow CoverTopographyLayering
5 GHz 6 GHz4 cm 80 m 200 mW 30000 ft Horn
P-3DC-8Basler
Ku-Band TopographyLayering 15 GHz 6 GHz
4 cm 15 m 200 mW 20000 ft HornTwin-OtterDC-8Basler
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Accumulation Radar – Surface basedFrequency 500 – 2000 MHz
Sweep Time 4 msPRF 0.2 kHzTransmit Power 100 mWattNumber of Coherent Integrations
60
Antennas 12-element Vivaldi arrays
A/D Dynamic Range 12-bit, 72 dBSampling Rate 10 MHz
10.5 “
16 “
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Radar Range Profile
2845.01 r
r Relative Dielectric Constant
firn density (g cm-3)
Core density profile
Dielectric constant profile
(Kovacs et al., 1995)
The range profile is constructed as follows:
2)()1()( step
ncnrnr
where: r(n) = depth of the nth range binstep = time extent of 1 range bin
(Rink, 2006)
Antenna to snow surface = 2 m; r = 1
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Pass 1
Pass 2
.5 m
.6 m
Depth
.7 m
.8 m
.9 m
1.0 m
Depth
1.2 m
1.3 m
1.4 m
1.5 m
1.6 m
Snow Pit
Snow Pit
1.7 km
Greenland
Summit Camp,Greenland
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Tracked annual layers along traverse
375 kmIce Thickness = 3085 m Ice Thickness = 2542 m
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Radar Annual Accumulation: 1889 - 2007
1.2σ
1.2σ
0.6σ
0.6σ
185 km segment - Northern
(Chen, 2013)
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Radar Annual Accumulation: 1889 - 2007185 km segment - Southern
1.2σ
1.2σ
0.6σ
0.6σ
(Chen, 2013)
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Climate Index - NAONorth Atlantic Oscillation: a diagnostic quantity used to characterize atmospheric circulation patterns in the North Atlantic sector: 20°- 80° N; 90° W - 40° E.
Used Hurrell and Deser (2009) principal component (PC)-based indices of the NAO that are determined by the Empirical Orthogonal Function (EOF) of sea level pressure (SLP) anomalies in the domain.
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Connection between Greenland accumulation and PC-NAO?
Previous studies using ice core and model data say no.
Do the regional partitioning and spatial averaging advantages of radar determined accumulation show a connection?
The NAO shifts between a positive phase and a negative phase resulting in large changes in air temperature, storminess, winds, and precipitation.
Large pressure gradient
Weak pressure gradient
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Average Accumulation
NGRIP NEEM
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NAO versus Accumulation: 1958-2006 Radar annual accumulation• Gridded annual accumulation from Polar MM5
(Burgess et al., 2010)• NEEM ice core derived annual accumulations
PC-NAO time series examined:• Annual• Winter (DJFM and DJF)• Spring (MAM) Summer (JJA)• Fall (SON)
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Significant positive correlations between summer PC-NAO and 25 km-averaged
radar accumulation time series (49 years)
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Summer PC-NAO and annual accumulation
r= 0.391P-value= 0.005
25 km segment
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Conclusions• Accumulation radar provides spatial averaging to overcome
local redistribution of snow by wind.• Accumulation radar provides regional coverage allowing
examination of different precipitation regimes.• Positive correlation found between summer PC-NAO and
radar determined accumulation.• Climate models show summer NAO becomes increasingly
positive in a warming world (Folland et al., 2009).• Takes us beyond the Clausius–Clapeyron equation (es(T)) to
include large scale circulation for understanding future ice sheet mass balance.
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