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.