Ice Shelf Melt Rates and 3D Imaging

Transcription

1 Ice Shelf Melt Rates and 3D Imaging Cameron Lewis University of Kansas 2335 Irving Hill Road Lawrence, KS Technical Report CReSIS TR

2 Ice Shelf Melt Rates and 3D Imaging Cameron Lewis 05/08/15

3 Presentation outline Background Recommendations from committee Radar system simulation Ice-shelf melt rates Petermann glacier Thickness profiles and thickness changes Surface vs basal melt rates 3D imaging Data set selection Bed maps and crossover analysis Summary and recommendations

4 What are ice shelves? Floating ice attached to land Distinct from sea ice which forms by freezing sea water Fed by glaciers and ice streams flowing off the land Typically found in embayments where side walls and pinning points provide protection from open ocean Largest (Ross and Ronne- Filchner) can cover 500,000 km 2 Thickness vary from 2 km at the grounding line to 100 m at the ocean edge Doake, 2001

5 Importance of ice shelves Play an important role in the global climate system Considered a sensitive indicators of climate change Two critical roles: Buttressing effect on glaciers and ice streams, regulating their flow velocities Ice shelf collapse could generate unchecked ice-sheet mass loss Loss of buttressing ice in Pine Island region has lead to significant glacial flow rates Basal melting and refreezing of large ice shelves influences the development of Antarctic Bottom Water Bottom water acts as a heat sink within the southern ocean circulation Thinning or loss of ice shelves can change bottom water development and subsequently global ocean circulation and ocean nutrient transport Monitoring changes in shelf mass balance can provide insight into shelf stability

6 Mass balance factors Lateral: Ice shelf mass balance Input glacial inflow from ice sheet Output ocean-edge calving Surface: Input snow accumulation Output blown snow, sublimation, surface melt Base: Input basal refreezing Output basal melting

7 Observed melt rates Region Melt Rate Thickness Reference George IV, Southern Antarctica ± m, measured over 12 days 450 m average Corr and others, 2002 peninsula 2.78 ± 0.08 m/yr equivalent Rutford Ice Stream grounding line, Ronne-Filchner ice shelf 1-4 m/yr 1.4 km average Jenkins and Doake, 1991 Ronne-Filchner ice shelf, 30 km from ice front, 50 km NW of Filchner Station 1.41 ± 0.45 m/yr, measured at a single point over 2 years 1.5 ± 0.15 m/yr, the mean of measurements from a 10 km square grid over 26 days 240 m average Grosfeld and others, 1992 Pine Island Glacier 44 ± 6 m/yr at grounding line 1 km average Rignot and Jacobs, 2002 Petermann Glacier 30 m/yr at grounding line 300 m average Rignot and Steffen, m/yr at ice front Petermann Glacier m/yr, spatially averaged 300 m average, 600 m maximum Münchow and others, 2014 Ross Ice Shelf, near Mercer ice stream grounding line 2.5 ± 0.7 m, measured over 1 month 900 m average Anandakrishnan and others, 2014 (AGU, unpublished)

8 UHF radar Wideband chirped-pulse, MHz Designed to work on ground-based and airborne platforms Designed to map layering in polar firn Field experiments confirmed its ability to sound shallow ice and ice shelves Cross-track array to allows for 3D processing Parameter Value Unit Center Frequency 750 MHz Bandwidth 300 MHz Waveform Pulsed Chirp Pulse Duration Airborne: µs Surface: Peak transmit power Airborne: 5 W Surface: 1 Pulse repetition frequency 50 khz Antennas Twin Otter: Vivaldi P-3: Elliptical Dipole Surface: PCB Dipole Polarization VV Sampling frequency 1 GHz Hardware coherent averages 128 Data rate 3.9 MB/s Dynamic Range 161 db DDS AMPS GPS TX MUX CLOCK RX MUX

9 Recommendations from Committee Demonstration of the technique to estimate basal melt rates from the UHF radar data and/or other depth sounder data Separate the effects of ice motion and surface accumulation from basal melt rate estimates Generate 3D basal images via the MUSIC algorithm using the UHF radar data Validation of 3D image results using cross lines Radar system response simulation using CAD tools and comparison with measurements

10 Radar system simulation Measured radar subsystems (transmitter, receiver, optical delay line) ADS used to create cascaded system response Transmitter BPF Filter Coupler Main Line Delay Line Isolators Attenuators τ Coupler Coupled Line BPF Filter Receiver

11 Radar system simulation ADS results exported to Matlab Ideal chirp: MHz, 2µs, -26 dbm, 20% tukey window Good main lobe and trailing sidelobe agreement First trailing sidelobe from delay line target Sidelobe pairs highlighted, high sidelobe response poor filter port VSWR Disagreement below -21 db level, 1/125 th of peak power Workflow Diagram Measured (S) Radar τ Measured Delay Line ADS Measured Response Matlab Plots

12 Petermann glacier Fast-flowing, marine-terminating glacier Drains roughly 4% of Greenland ice sheet Ice front at ~45 km from grounding line Average thickness: ~300 m Thickness at grounding line: ~550 m Major calving events in 2010 and 2012 Average velocity: ~1.2 km/yr Average reported surface melt: ~1.2 m/yr Average reported basal melt: m/yr

13 3 nearly identical flight lines May 7, 2011 April 20, 2013 May 12, 2014 Track follows flow streamline Thickness observed from grounding line to ice front Petermann glacier

14 Petermann glacier Basal interface traceable with UHF radar to ~5 km from grounding line Basal interface between 5 km and ground line filled in using VHF sounder

15 Thickness and velocity

16 Radar vs hydrostatic thickness

17 Thickness changes H t + uh = a m

18 Non-steady-state thickness change The change in ice thickness at a particular location (parcel) over a period of time Thickness changes H t + uh = a m Steady-state thickness change u H + H u Mass flux divergence + non-linear dynamic thinning External mass loss/gain a = surface accumulation ablation m = basal melt - freezing Advection of ice in/out of particular location Assume cross-track flow is negligible, u H = v 0 H y Non-linear dynamic thinning, H u, has been shown to be negligible

19 Consider steady-state thinning first Thickness changes y 1 2Hdy H = y 2 y 1 y 1 Create along-track cumulative average ice thickness to reduce noise of point-by-point thickness comparisons

20 Thickness changes Use cumulative average thickness to calculate flux divergence

21 Non-steady-state thinning Thickness changes Point-by-point subtraction of H divided by time difference

22 Total thickness loss Thickness changes

23 Bright internal layer tracked from yearto-year Internal layer assumed to be stable Thickness change calculation methods repeated Surface Layer Layer Bottom Surface vs basal melt

24 Melt rate summary UHF radar-tracked thickness UHF radar hydrostatic thickness ATM-derived hydrostatic thickness [Münchow and other, 2014] VHF radar thickness [Rignot and Steffen, 2008] Flux-gate [Rignot and others, 2001] Year v 0 H y [m/yr] H/ t [m/yr] Total [m/yr] Velocity [km/yr] Year Surface melt [m/yr] Basal melt [m/yr]

25 Major results: Melt rate summary In general, shelf in hydrostatic balance beyond 20km from grounding line Disagreement between radar-track and hydrostatic thickness close to grounding line suggests the shelf has some inertia after decoupling from the bedrock and needs time to come into general hydrostatic equilibrium Basal interface is heavily crevassed, hydrostatic assumption does not fully capture their number and magnitude Seaward accelerations jumped 15x (4m/yr 2 60m/yr 2 ) in the last decade Steady-state melt rates appear to be stable, non-steady-state melt appears to jump significantly following major calving events

26 3D image generation Ground-based version of UHF radar used to image shallow ice 12 or 16 virtual phase center cross-track antenna array MUSIC algorithm used to determine cross-track angle of arrival Two data sets collected in Antarctica Grid near exposed nunatak in Pirrit Hills region Square track on Kamb Ice Stream Basal interface exhibited off-nadir backscatter

27 Pirrit Hills Ice Surface Bedrock Ice Surface Bedrock 42 db

28 Pirrit Hills

29 Kamb Ice Stream Ice Surface Bedrock

30 Kamb Ice Stream

31 Crossover analysis Pirrit Hills parallel tracks Elevation error values range from 0 to 70 m Only 5% of pixels where error on the order of the radar resolution Largest elevation errors at steep transitions on the SW side of the ridge

32 Crossover analysis Pirrit Hills cross track Elevation error also varies between 0 and 70 m Only 2% of pixels where error on the order of the radar resolution Again, largest errors appear to occur mostly as sharp elevation transitions

33 Crossover analysis Kamb Ice Stream corner overlap Roughly 80m x 80m Errors range from 0 to 14 m Significant elevation error in one corner possibly due to low SNR

34 3D imaging summary Proposed 3D algorithm appears to produce coarse resolution topography Some major feature clearly distinguishable Shape and orientation of under-ice ridge in Pirrit Hills region Grooves in bedrock below Kamb Ice Stream parallel to flow Error analysis shows algorithm is not effective at imaging small scale features reliably MUSIC algorithm is calculating a spectrum based on the energy received from all targets within the radar footprint

35 Relative Power 3D imaging summary Antenna Array Footprint MUSIC divides footprint up into bins and evaluates the signal strength from each bin User can define the number of bins to divide the footprint into Spectrum from distributed target (red, green) leads to ambiguity Desired spectrum with distinct peaks response from a point target Spatial Frequency

36 Recommendations for Future Work Estimate cross-track angle of arrival using Fourier methods (e.g. periodogram Require large cross-track antenna array Need for agile platform, ability to perform repeatable tightly-spaced survey lines and accommodate a large antenna array Meridian UAV has 8 m wing span, 36 element antenna array, 5º resolution Sierra/Viking UAVs have 6 m wing span, 28 elements, 6.5º resolution Possible survey parameters: 100 m altitude ~8.7 m surface, ~35.8 m base of ice shelf 50% swath overlap, maximum swath width ±45º 340 m flight line spacing Resolution/swath width include 1 air/ice refraction at surface

37 Recommendations for Future Work Conformal PCB antennas built into wing skin Wideband 16-element VHF/UHF ( MHz) array successfully designed for Basler Elements could be scaled for MHz operation 16 element array size reduced to 0.98 m x 0.22 m Increased to 36 elements: 2.2 m length Ground plane height reduced to 4.4 cm, Meridian could accommodate with a wing thickness of about 11 cm 0.16 m 3.6 m 0.8 m Basler array dimensions

38 Contributions Refined radar hardware for both airborne and ground-based deployment Publication in high visibility journal: Lewis C, Gogineni SP, Rodriguez-Morales F, Panzer B, Stumpf T, Paden J, and Leuschen C (2015) Airborne fine-resolution UHF radar: an approach to the study of englacial reflections, firn compaction, and ice attenuation rates. Journal of Glaciology: Instruments and Methods, 61(225), Applied radar system to measure ice shelf basal melt rates Highly detailed look at Petermann glacier basal interface Ability to measure surface and basal melt separately through tracked internal layer Explored creation of 3D imagery Imaging of large-scale features possible Found significant limitations to fine-resolution imaging Sets a precedent for future use of a large antenna array aboard a UAV

39 Questions?