Assessment of Antarctic sea ice by surface validated satellite measurements

178 Pages·2014·5.32 MB·English

by  Daniel Price

Assessment of Antarctic sea ice by
surface validated satellite
measurements

A thesis submitted in partial fulfilment of the requirements for the Degree
of
Doctor of Philosophy
by
Daniel Price

Gateway Antarctica
University of Canterbury
2014

1
2
Table of contents

Table of contents i

List of Figures iii

List of Tables iv

Abstract v

Co-authorship Forms vi

Acknowledgements ix

1 Introduction .................................................................................................. 1

1.1 Rationale ............................................................................................ 1
1.2 Aims and objectives .......................................................................... 5
1.3 Antarctic sea ice thickness ................................................................ 6
1.4 Sea ice in the Southern Ocean ........................................................ 12
1.5 Sea ice in the vicinity of ice shelves ............................................... 16
1.6 Coastal polynyas ............................................................................. 19
1.7 A regional perspective – The Ross Sea .......................................... 19
1.8 McMurdo Sound ............................................................................. 22
1.9 Outline and structure of the thesis .................................................. 24

2 Satellite and in situ observation ............................................................... 28

2.1 Ocean topography ........................................................................... 29
2.2 Ice, Cloud, and land Elevation Satellite.......................................... 33
2.3 CryoSat-2 ......................................................................................... 34
2.4 Imaging platforms ........................................................................... 37
2.4.1 Environment satellite ................................................................ 37
2.4.2 Terra & Aqua ............................................................................ 38
2.4.3 TerraSAR-X .............................................................................. 38
2.5 In situ and near-surface measurements ........................................... 39
2.5.1 In situ measurements of sea ice properties ............................... 40
2.5.2 Global Navigation Satellite System .......................................... 40
2.5.3 Helicopter-borne electromagnetic induction sounding ............ 41

3 Sea ice freeboard derived by ICESat laser altimeter data ................... 42

3.1 Introduction ..................................................................................... 42
3.2 Sea ice types in the study area ........................................................ 46
3.3 Method-1 ICESat freeboard retrieval ............................................. 48
3.4 Evaluation of ICESat freeboard retrieval ....................................... 53
i
3.5 Method-2 ICESat freeboard retrieval .............................................. 57
3.6 Freeboard in McMurdo Sound 2003-2009 ..................................... 59
3.7 Comparison with airborne meas. and in situ information .............. 66
3.8 Discussion ........................................................................................ 71
3.9 Conclusions ...................................................................................... 76

4 The sub-ice platelet layers and freeboard to thickness conversion ..... 78

4.1 Introduction ...................................................................................... 78
4.2 Estimating solid fraction under the hydro. equilib. assum. ............ 81
4.3 In situ investigation ......................................................................... 83
4.3.1 Drill-hole measurements ........................................................... 83
4.3.2 Maps of sea ice and snow cover characteristics ....................... 84
4.4 The solid fraction in McMurdo Sound ........................................... 86
4.5 Sea ice thickness from GNSS derived surface elevation ............... 88
4.6 Discussion ........................................................................................ 94
4.7 Conclusions ...................................................................................... 95

5 Evaluation of CryoSat-2 derived sea ice freeboard ............................... 97

5.1 Introduction ...................................................................................... 97
5.2 CryoSat-2 assessment in McMurdo Sound ..................................103
5.2.1 In situ investigations ................................................................103
5.2.2 CryoSat-2 .................................................................................105
5.3 Tracking surface height from CS-2 waveforms ...........................106
5.3.1 European Space Agency Level 2 data product ......................106
5.3.2 Waveform Fitting Procedure data product ............................106
5.3.3 Threshold-First-Maximum-Retracker-Algorithm 40 ............107
5.3.4 Tracking of sea surface height ................................................108
5.4 Supervised freeboard retrieval procedure .....................................109
5.4.1 Supervised freeboard retrieval results ....................................111
5.5 Automatic freeboard retrieval procedure ......................................120
5.5.1 ESAL2 automatic SSH identification ......................................121
5.5.2 WfF automatic SSH identification ..........................................125
5.5.3 TFMRA40 automatic SSH identification ................................125
5.5.4 Automatic results .....................................................................126
5.6 Discussion ......................................................................................131
5.7 Conclusion .....................................................................................136

6 Synthesis and Conclusions ......................................................................137

6.1 ICESat investigation in McMurdo Sound .....................................139
6.2 The influence of a sub-ice platelet layer .......................................140
6.3 CryoSat-2 over Antarctic sea ice...................................................142
6.4 Research outlook ...........................................................................144

7 Appendices ................................................................................................148

8 Bibliography .............................................................................................150

ii
List of Figures
# Page
1.1 Trends in Antarctic sea ice extent 4
1.2 Principles of satellite laser altimetry 8
1.3 Albedo range over sea ice 15
1.4 Distribution of ice shelves and platelet observations 17
1.5 Sea ice in contact with ice shelf water 18
1.6 Sea ice thickness distribution, polynyas and McMurdo Sound 23
2.1 Measurement characteristics of ICESat & CryoSat-2 31
3.1 Location of study area with ICESat track information 45
3.2 Percentage cover of FY and MY sea ice 2003-2009 48
3.3 Surface elevation vs. EGM2008 in McMurdo Sound 51
3.4 Typical processing steps for ICESat Method-1 52
3.5 ICESat freeboard and reflectivity example 54
3.6 ICESat repeat pass and crossover analysis 56
3.7 Comparison of Scott Base Tide Gauge and modelled tide 59
3.8 ICESat derived FY and MY freeboard trends 2003-2009 62
3.9 ICESat derived MY freeboard trend for original 2003 area 63
3.10 Freeboard as a function of longitude using Method-1 64
3.11 MY sea ice area freeboard maps 65
3.12 ICESat, HL and EM freeboard and thickness PDFs 69
3.13 ICESat, HL and in situ distribution 2009 70
4.1 Location of study area with in situ measurement distribution 80
4.2 FY sea ice schematic with drill-hole measurement set up 84
4.3 Interpolated maps of drill-hole measurements 86
4.4 Solid fraction fits at different sea ice densities 87
4.5 GNSS sea ice thickness profiles 91
5.1 Components related to CS-2 freeboard retrieval 100
5.2 Typical CS-2 SIN mode waveform and retracking characteristics 102
5.3 Location of study area and CS-2 track distribution 104
5.4 CS-2 validation line 114
5.5 CS-2 waveform response to surface type, segments 1 – 4 117
5.6 Waveform examples for snow and no snow on sea ice 119
5.7 Surface type discrimination for ESAL2 124
5.8 McMurdo Sound fast ice freeboard trends from CS-2 129

iii
List of Tables
# Page
2.1 Satellite sensors used for sea ice investigation 32
2.2 SIRAL parameters 35
2.3 Utilised satellite platform 39
3.1 Mean September-December FY & MY freeboard from ICESat 60
3.2 Mean February-June FY & MY freeboard from ICESat 60
4.1 Drill-hole and GNSS derived sea ice thickness 90
5.1 CS-2 freeboards as derived by the supervised procedure 113
5.2 CS-2 freeboards as derived by the automatic procedure 126
5.3 CS-2 freeboards as derived by the automatic procedure for the fast ice are only 128

iv
Abstract

Satellite investigations have documented Antarctic sea ice area, but are restricted in their ability to
provide volume, as the procedure to derive thickness is still under development. This procedure
requires the measurement of sea ice freeboard, the segment of ice held above the ocean surface by
buoyancy. This measurement can be made by satellite altimeters and in conjunction with density
and snow depth information; sea ice thickness can be estimated via the hydrostatic equilibrium
assumption. The ability to monitor the spatial and temporal characteristics of the thickness
distribution must be improved as we strive to understand the linkages between the glaciological,
atmospheric and oceanic components of the Antarctic climate system. A key sector in which these
components interact is the Antarctic coast. There, offshore winds drive coastal polynyas creating
vast amounts of sea ice, and ice shelf interaction modifies ocean properties. Together they condition
the ocean for downwelling, driving the global oceanic circulation. In light of this, the coastal
Antarctic is a fundamental region in regard to Antarctic sea ice processes and the Earth climate
system. McMurdo Sound occupies a coastal area in proximity to an ice shelf in the south-western
corner of the Ross Sea. The sound has witnessed scientific investigation for over a century with a
fully established research programme since the 1960s. However, the sea ice research in this region is
spatially restricted. This thesis aims to expand the knowledge of sea ice in McMurdo Sound to a
larger area using space-borne remote sensing instrumentation and design of in situ measurement
campaigns. In doing so, this work evaluates the capabilities of satellite platforms to record sea ice
freeboard in the coastal Antarctic, whilst developing knowledge of ice shelf-sea ice interaction. This
work provides the first satellite altimeter based investigation of sea ice freeboard in McMurdo
Sound using ICESat over the period 2003-2009. No observable trend was observed for first-year sea
ice freeboard in the region in line with larger scale assessments in the Ross Sea. However, there was
significant increase in the freeboard of a temporary multiyear sea ice regime, the segment of the
largest increase linked to the outflow of supercooled Ice Shelf Water (ISW) from the McMurdo and
Ross Ice Shelf cavities. This remote sensing assessment supports the in situ and modelling work of
many others who have identified the influence of ISW on sea ice processes in this region, in
particular, that it is thicker than it would otherwise be. The influence of ISW on altimetric sea ice
thickness retrievals was also quantified using a Global Navigation Satellite System (GNSS)
evaluation of freeboard to thickness conversion. This revealed that a sub-ice platelet layer, created
by supercooled ISW and with an estimated solid fraction of 0.16, accumulates beneath the sea ice
cover and influences the thickness estimates from the GNSS-derived surface elevation. A cautionary
conclusion is reached that within 100 km of ice shelves this buoyant influence should be considered,
and in close proximity (< 50 km) can result in overestimations of sea ice thickness of ~ 12 %. It is
also suggested that the sea ice freeboard anomalies that result from enhanced growth, driven by
supercooled water advection could be used to map the presence of ISW in the coastal Antarctic.
Looking to future ability to monitor Southern Ocean sea ice thickness from space, the first
comprehensive evaluation of CryoSat-2 (CS-2) over Antarctic sea ice is provided. Using three
separate retracking procedures, CS-2 is shown to be capable of detecting the development of a fast
ice cover in McMurdo Sound. The role played by a snow cover with layering typical of the
Antarctic appears to cause a positive bias in the ice freeboard for a waveform fitting procedure
currently used over Arctic sea ice. The identification of open water and the establishment of
accurate sea surface heights are also indicated as causing errors (in the order of cms) in the study
region. CS-2 is shown to be capable of recording sea ice growth over two growth cycles in
McMurdo Sound. This work has advanced the application of satellite investigative techniques to
Antarctic sea ice, providing hope that such techniques may be capable of revealing larger scale
connections between sea ice and ice shelves.
v
Deputy Vice-Chancellor’s Office
Postgraduate Office

Co-authorship Form

This form is to accompany the submission of any PhD thesis that contains research reported
in co-authored work that has been published, accepted for publication, or submitted for
publication. A copy of this form should be included for each co-authored work that is
included in the PhD thesis. Completed forms should be included at the front (after the thesis
abstract) of each copy of the thesis submitted for examination and library deposit (including
electronic copy).

Please indicate the chapter/section/pages of this thesis that are extracted from co-authored
work and provide details of the publication or submission from which the extract comes:
Chapter 3 is derived from Price, D., W. Rack, C. Haas, P. J. Langhorne, and O. Marsh
(2013), Sea ice freeboard in McMurdo Sound, Antarctica, derived by surface-
validated ICESat laser altimeter data, J. Geophys. Res. Oceans, 118,
doi:10.1002/jgrc.20266.

Please detail the nature and extent (%) of contribution by the PhD candidate:
The candidate has performed all ICESat data analysis, developed the methods and written
the text. W. Rack, C. Hass and P.J. Langhorne provided supervisory guidance and proof
read while O. Marsh provided processing assistance.
Candidate contribution: 85 %

Certification by co-authors:

If there is more than one co-author then a single co-author can sign on behalf of all. The
undersigned certifies that:

The above statement correctly reflects the nature and extent of the PhD candidate’s
contribution to this co-authored work

In cases where the PhD candidate was the lead author of the co-authored work he or she
wrote the text.

Name: Wolfgang Rack Signature:

vi
Deputy Vice-Chancellor’s Office

Postgraduate Office

Co-authorship Form

This form is to accompany the submission of any PhD thesis that contains research reported
in co-authored work that has been published, accepted for publication, or submitted for
publication. A copy of this form should be included for each co-authored work that is
included in the PhD thesis. Completed forms should be included at the front (after the thesis
abstract) of each copy of the thesis submitted for examination and library deposit (including
electronic copy).

Please indicate the chapter/section/pages of this thesis that are extracted from co-authored
work and provide details of the publication or submission from which the extract comes:
Chapter 4 is derived from Price, D., W. Rack., P.J. Langhorne., C. Haas., G. Leonard
and K. Barnsdale. 2014. The sub-ice platelet layer and its influence on freeboard to
thickness conversion of Antarctic sea ice, The Cryosphere, 8, 1031-1039,
doi:10.5194/tc-8-1031-2014.

Please detail the nature and extent (%) of contribution by the PhD candidate:
The candidate has performed the analysis of processed GNSS and in situ data, developed
the method, written the text and contributed to the fieldwork. W. Rack provided supervisory
support and participated in discussions developing the method and scope of the work. All
authors partook in the collection of in situ information and provided comments and
suggestions to improve the paper.
Candidate contribution: 90 %

Certification by co-authors:

If there is more than one co-author then a single co-author can sign on behalf of all. The
undersigned certifies that:

The above statement correctly reflects the nature and extent of the PhD candidate’s
contribution to this co-authored work

In cases where the PhD candidate was the lead author of the co-authored work he or she
wrote the text.

Name: Wolfgang Rack Signature:
vii
Deputy Vice-Chancellor’s Office
Postgraduate Office

Co-authorship Form

This form is to accompany the submission of any PhD thesis that contains research reported
in co-authored work that has been published, accepted for publication, or submitted for
publication. A copy of this form should be included for each co-authored work that is
included in the PhD thesis. Completed forms should be included at the front (after the thesis
abstract) of each copy of the thesis submitted for examination and library deposit (including
electronic copy).

Please indicate the chapter/section/pages of this thesis that are extracted from co-authored
work and provide details of the publication or submission from which the extract comes:
Chapter 5 is derived from Price, D., J. Beckers., R. Ricker., N. Kurtz., W. Rack., V.
Helm., C. Haas., S. Hendricks., P.J. Langhorne., and G. Leonard. 2014. Evaluation of
CryoSat-2 derived sea ice freeboard over fast-ice in McMurdo Sound, Antarctica,
Journal of Glaciology, under review.

Please detail the nature and extent (%) of contribution by the PhD candidate:
The candidate has written the text and developed and processed the CryoSat-2 Level 2
freeboard method in collaboration with J. Beckers during a research stay at the University
of Alberta. The candidate has conceived and overseen the international collaboration
between UC, AWI and NASA who provided additional CryoSat-2 data. A major component
of the in situ validation campaign for CryoSat-2 was designed and executed by the
candidate. W. Rack participated in extensive discussions and all authors provided
suggestions and comments to improve the paper.
Candidate contribution: 80 %

Certification by co-authors:

If there is more than one co-author then a single co-author can sign on behalf of all. The
undersigned certifies that:

The above statement correctly reflects the nature and extent of the PhD candidate’s
contribution to this co-authored work

In cases where the PhD candidate was the lead author of the co-authored work he or she
wrote the text.

Name: Wolfgang Rack Signature:

viii