1. 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

2. 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.

3. Radars InstrumentMeasurementFreq. BW/ Res. Depth PowerAltitudeAntennaInstalls Accum Radar - Surface Internal Layering Ice Thickness 1250 MHz 1500 MHz 10 cm 300 m 100 mWSurface 12-element Vivaldi Array Tracked vehicle Accum. Radar – Airborne Internal Layering Ice Thickness 750 MHz 300 MHz 40 cm 300 m 10 W20000 ft Patch Array Vivaldi Array Twin-Otter P-3 Basler Snow Radar Snow Cover Topography Layering 5 GHz 6 GHz 4 cm 80 m 200 mW30000 ftHorn P-3 DC-8 Basler Ku-Band Topography Layering 15 GHz 6 GHz 4 cm 15 m 200 mW20000 ftHorn Twin-Otter DC-8 Basler

4. Accumulation Radar – Surface based Frequency500 – 2000 MHz Sweep Time4 ms PRF0.2 kHz Transmit Power100 mWatt Number of Coherent Integrations 60 Antennas12-element Vivaldi arrays A/D Dynamic Range12-bit, 72 dB Sampling Rate10 MHz 10.5 “ 16 “

5. Radar Range Profile 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: where:r(n) = depth of the nth range bin  step = time extent of 1 range bin (Rink, 2006) Antenna to snow surface = 2 m;  r = 1

6. 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 1.7 km Greenland Summit Camp, Greenland

7. Tracked annual layers along traverse 375 km Ice Thickness = 3085 m Ice Thickness = 2542 m

8. Radar Annual Accumulation: 1889 - 2007 1.2σ 0.6σ 185 km segment - Northern (Chen, 2013)

9. Radar Annual Accumulation: 1889 - 2007 185 km segment - Southern 1.2σ 0.6σ (Chen, 2013)

10. Climate Index - NAO North 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.

11. 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

12. Average Accumulation NGRIP NEEM

13. 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)

14. Significant positive correlations between summer PC-NAO and 25 km-averaged radar accumulation time series (49 years)

15. Summer PC-NAO and annual accumulation r= 0.391 P-value= 0.005 25 km segment

16. 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 (e s (T)) to include large scale circulation for understanding future ice sheet mass balance.