Post on 24-Feb-2016
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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
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.
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
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 “
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
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
Tracked annual layers along traverse
375 kmIce Thickness = 3085 m Ice Thickness = 2542 m
Radar Annual Accumulation: 1889 - 2007
1.2σ
1.2σ
0.6σ
0.6σ
185 km segment - Northern
(Chen, 2013)
Radar Annual Accumulation: 1889 - 2007185 km segment - Southern
1.2σ
1.2σ
0.6σ
0.6σ
(Chen, 2013)
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.
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
Average Accumulation
NGRIP NEEM
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)
Significant positive correlations between summer PC-NAO and 25 km-averaged
radar accumulation time series (49 years)
Summer PC-NAO and annual accumulation
r= 0.391P-value= 0.005
25 km segment
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.