109 research outputs found
A long-term record of sea ice thickness in the Canadian Arctic
Sea ice plays a vital role in the Arctic region and affects numerous processes: it influences the radiative budget by reflecting sunlight and acts as a barrier for heat transport between atmosphere and ocean; it influences Arctic ecosystems as a habitat for different species; it is important for hunting and travel for local communities; and it acts as a hazard for marine shipping. Monitoring sea ice, specifically its thickness, is essential in understanding how it is changing with ongoing global warming.This thesis presents a novel method to create a long-term record (1996-2020) for sea ice thickness in the Canadian Arctic and assesses how sea ice thickness changed and what the impacts of these changes are.This thesis initially aimed to extract a long-term sea ice thickness record for the Canadian Arctic from satellite altimetry. However, it revealed that assumptions regarding the snowpack, sea ice density, and processing algorithms highly influence conclusions on sea ice thickness state and trends, and this approach was rejected. Instead, this thesis presents a proxy sea ice thickness product for the Canadian Arctic using ice charts, which for the first time consistently covers the Canadian Arctic Archipelago. In the final research chapter, this sea ice thickness proxy product and ice charts are used to assess sea ice changes in the Canadian Arctic Archipelago and their impact on accessibility.Sea ice has thinned across most of the Canadian Arctic region, with a mean change over the full area of 38.5 cm for November and 20.5 cm for April over the period 1996-2020. Moreover, the marine navigability is shown to increase in the access channels to the Canadian Arctic Archipelago, which enhances the possibilities for resupply for local communities. However, with continuing dynamic influx of old and thick sea ice, there is no change in full navigability of the Northwest Passage connecting the Atlantic and Pacific Oceans
Dual-polarization (HH/HV) RADARSAT-2 ScanSAR Observations of New, Young and First-year Sea Ice
Observations of sea ice from space are routinely used to monitor sea ice extent, concentration and type to support human marine activity and climate change studies. In this study, eight dual-polarization (dual-pol) (HH/HV) RADARSAT-2 ScanSAR images acquired over the Gulf of St. Lawrence during the winter of 2009 are analysed to determine what new or improved sea ice information is provided by dual-pol C-band synthetic aperture radar (SAR) data at wide swath widths, relative to single co-pol data. The objective of this study is to assess how dual-pol RADARSAT-2 ScanSAR data might improve operational ice charts and derived sea ice climate data records. In order to evaluate the dual-pol data, ice thickness and surface roughness measurements and optical remote sensing data were compared to backscatter signatures observed in the SAR data. The study found that: i) dual-pol data provide improved separation of ice and open water, particularly at steep incidence angles and high wind speeds; ii) the contrast between new, young and first-year (FY) ice types is reduced in the cross-pol channel; and iii) large areas of heavily deformed ice can reliably be separated from level ice in the dual-pol data, but areas of light and moderately ridged ice cannot be resolved and the thickness of heavily deformed ice cannot be determined. These results are limited to observations of new, young and FY ice types in winter conditions. From an operational perspective, the improved separation of ice and open water will increase the accuracy of ice edge and total ice concentration estimates while reducing the time required to produce image analysis charts. Further work is needed to determine if areas of heavily ridged ice can be separated from areas of heavily rafted ice based on knowledge of ice conditions in the days preceding the formation of high backscatter deformed ice. If rafted and ridged ice can be separated, tactical ridged ice information should be included on image analysis charts. The dual-pol data can also provide small improvements to ice extent and concentration data in derived climate data records. Further analysis of dual-pol RADARSAT-2 ScanSAR data over additional ice regimes and seasons is required
First-Year and Multiyear Sea Ice Incidence Angle Normalization of Dual-Polarized Sentinel-1 SAR Images in the Beaufort Sea
Automatic and visual sea ice classification of SAR imagery is impeded by the incidence angle dependence of backscatter intensities. Knowledge of the angular dependence of different ice types is therefore necessary to account for this effect. While consistent estimates exist for HH polarization for different ice types, they are lacking HV polarization data, especially for multiyear sea ice. Here we investigate the incidence angle dependence of smooth and rough/deformed first-year and multiyear ice of different ages for wintertime dual-polarization Sentinel-1 C-band SAR imagery in the Beaufort Sea. Assuming a linear relationship, this dependence is determined using the difference in incidence angle and backscatter intensities from ascending and descending images of the same area. At cross-polarization rough/deformed first-year sea ice shows the strongest angular dependence with -text{0.11} dB/1{circ } followed by multiyear sea ice with -text{0.07} dB/text{1}{circ }, and old multiyear ice (older than three years) with -text{0.04} dB/text{1}{circ }. The noise floor is found to have a strong impact on smooth first-year ice and estimated slopes are therefore not fully reliable. At co-polarization, we obtained slope values of -0.24, -0.20, -text{0.15}, and -text{0.10} dB/text{1}{circ } for smooth first-year, rough/deformed first-year, multiyear, and old multiyear sea ice, respectively. Furthermore, we show that imperfect noise correction of the first subswath influences the obtained slopes for multiyear sea ice. We demonstrate that incidence angle normalization should not only be applied to co-polarization but should also be considered for cross-polarization images to minimize intra ice type variation in backscatter intensity throughout the entire image swath
Arctic and Antarctic Sea Ice, 1978-1987: Satellite Passive-Microwave Observations and Analysis
This book contains a description and analysis of the spatial and temporal variations in the Arctic and Antarctic sea ice covers from October 26, 1978 through August 20, 1987. It is based on data collected by the Scanning Multichannel Microwave Radiometer (SMMR) onboard the NASA Nimbus 7 satellite. The 8.8-year period, together with the 4 years of the Nimbus 5 Electrically Scanning Microwave Radiometer (ESMR) observations presented in two earlier volumes, comprises a sea ice record spanning almost 15 years
Radar Altimetry Methods for Solid Earth Geodynamics Studies
This research is partially supported by grants from NOAA/NEDIS
NA06NES400007, NASA Earth Science program: NNX08AT52G, NNG04GN19G,
NNG04GN19G, JPL 1265252, 1283220, NSF's CMG Program (EAR0327633, and OCE-
0620885), Hydrology Program (EAR-044007), and by NGA NURI Program HM1582-
07-1-2024.Satellite radar altimetry, which was initially designed for accurate measurements of sea
surface height, has been demonstrated to be applicable to non-ocean surfaces as well. In
this study, three different applications of satellite altimetry to geodynamics studies have
been examined: solid Earth crustal deformation due to Glacial Isostatic Adjustment (GIA)
over Hudson Bay, North America, water level fluctuation over vegetated wetlands of
Louisiana, and ice sheet elevation change over the Amundsen Sea sector, West Antarctica.
For efficient altimetry data processing, high-rate (10-Hz for TOPEX, 18-Hz for
Environmental Satellite (Envisat)) regional stackfiles were developed based on the
original low-rate (1-Hz) global ocean stackfile. A modified threshold retracker has also
been developed for more accurate land radar waveform retracking. 90-m resolution Cband
Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) plays
an important role to be used as a reference surface to select an optimal retracker, to
correct surface gradient errors, and to calculate land surface anomalies over Hudson Bay.
As a result, the crustal vertical motion is estimated from TOPEX decadal (1992-2002)
time series over land surfaces with height variation (in terms of standard deviation) less
than 2 m. The estimated vertical motion has been compared with the vertical motion
derived from Gravity Recovery and Climate Experiment (GRACE) satellite and several
GIA models. It agrees well with the laterally varying 3D GIA model, RF3S20 (β=0.4)
with differences of -0.1 ± 2.2 mm/year. It is anticipated that the new observation from
this study can be used to provide additional constraints for GIA model improvement. The
10-Hz stackfile procedure has also been utilized to observe the Louisiana wetland water
level variations over each of 10-Hz stackfile bin with along-track spacing of ~660 meter
using TOPEX altimeter measurements. The feasibility of applying retracking has also
been examined. Specifically, the water level variation over the swamp forest along the
Atchafalaya River basin has been examined with the SRTM DEM elevation and L-band
Advanced Land Observing Satellite (ALOS) Synthetic Aperture Radar (SAR) imagery. It
has been found that the water level fluctuations in terms of amplitude of each 10-Hz
TOPEX time series becomes larger as the elevation decreases and the SAR backscattered
power increases. Over the Amundsen Sea sector, which suffers dynamic thinning due to
the recent acceleration of coastal glaciers, the 18-Hz stackfile has been built using
Envisat retracked measurements. The rates of ice sheet elevation changes have been
estimated over 1° × 1° regions with 1-km resolution Antarctic DEM which is used to
correct for the surface gradient error. The ice mass loss from September 2002 – May
2005 has been estimated to be -49 ± 5 Gigaton/year after correcting for the firn depth
changes, which correspond to equivalent sea level change of 0.14 ± 0.01 mm/year
Cryosphere Applications
Synthetic aperture radar (SAR) provides large coverage and high resolution, and it has been proven to be sensitive to both surface and near-surface features related to accumulation, ablation, and metamorphism of snow and firn. Exploiting this sensitivity, SAR polarimetry and polarimetric interferometry found application to land ice for instance for the estimation of wave extinction (which relates to sub surface ice volume structure) and for the estimation of snow water equivalent (which relates to snow density and depth). After presenting these applications, the Chapter proceeds by reviewing applications of SAR polarimetry to sea ice for the classification of different ice types, the estimation of thickness, and the characterisation of its surface. Finally, an application to the characterisation of permafrost regions is considered. For each application, the used (model-based) decomposition and polarimetric parameters are critically described, and real data results from relevant airborne campaigns and space borne acquisitions are reported
Ocean remote sensing techniques and applications: a review (Part II)
As discussed in the first part of this review paper, Remote Sensing (RS) systems are great tools to study various oceanographic parameters. Part I of this study described different passive and active RS systems and six applications of RS in ocean studies, including Ocean Surface Wind (OSW), Ocean Surface Current (OSC), Ocean Wave Height (OWH), Sea Level (SL), Ocean Tide (OT), and Ship Detection (SD). In Part II, the remaining nine important applications of RS systems for ocean environments, including Iceberg, Sea Ice (SI), Sea Surface temperature (SST), Ocean Surface Salinity (OSS), Ocean Color (OC), Ocean Chlorophyll (OCh), Ocean Oil Spill (OOS), Underwater Ocean, and Fishery are comprehensively reviewed and discussed. For each application, the applicable RS systems, their advantages and disadvantages, various RS and Machine Learning (ML) techniques, and several case studies are discussed.Peer ReviewedPostprint (published version
The Best of Both Worlds: Connecting Remote Sensing and Arctic Communities for Safe Sea Ice Travel
Northern communities are increasingly interested in technology that provides information about the sea ice environment for travel purposes. Synthetic aperture radar (SAR) remote sensing is widely used to observe sea ice independently of sunlight and cloud cover, however, access to SAR in northern communities has been limited. This study 1) defines the sea ice features that influence travel for two communities in the Western Canadian Arctic, 2) identifies the utility of SAR for enhancing mobility and safety while traversing environments with these features, and 3) describes methods for sharing SAR-based maps. Three field seasons (spring and fall 2017 and spring 2018) were used to engage residents in locally guided research, where applied outputs were evaluated by community members. We found that SAR image data inform and improve sea ice safety, trafficability, and education. Information from technology is desired to complement Inuit knowledge-based understanding of sea ice features, including surface roughness, thin sea ice, early and late season conditions, slush and water on sea ice, sea ice encountered by boats, and ice discontinuities. Floe edge information was not a priority. Sea ice surface roughness was identified as the main condition where benefits to trafficability from SAR-based mapping were regarded as substantial. Classified roughness maps are designed using thresholds representing domains of sea ice surface roughness (smooth ice/maniqtuk hiku, moderately rough ice/maniilrulik hiku, rough ice/maniittuq hiku; dialect is Inuinnaqtun). These maps show excellent agreement with local observations. Overall, SAR-based maps tailored for on-ice use are beneficial for and desired by northern community residents, and we recommend that high-resolution products be routinely made available in communities.
Les collectivités du Nord s’intéressent de plus en plus aux technologies qui leur fournissent de l’information au sujet de l’environnement de glace de mer à des fins de déplacements. La télédétection par radar à synthèse d’ouverture (SAR) est couramment utilisée pour observer la glace de mer, indépendamment de la lumière du soleil et de la nébulosité. Cependant, dans les collectivités du Nord, l’accès au SAR est restreint. Cette étude 1) définit les caractéristiques de la glace de mer qui exercent une influence sur les déplacements de deux collectivités dans l’ouest de l’Arctique canadien; 2) détermine l’utilité du SAR pour améliorer la mobilité et la sécurité quand vient le temps de traverser des environnements comportant ces caractéristiques; et 3) décrit les méthodes de partage de cartes établies à l’aide du SAR. Trois saisons sur le terrain (le printemps et l’automne de 2017, et le printemps de 2018) ont permis d’inciter les résidents à participer à une recherche locale guidée, là où les extrants appliqués ont été évalués par les membres de la collectivité. Nous avons trouvé que les données émanant des images du SAR éclairent et améliorent la sécurité de la glace de mer, l’aptitude à la circulation et l’éducation. L’information découlant de la technologie s’avère un complément désirable aux connaissances inuites en vue de la compréhension des caractéristiques de la glace de mer, dont la rugosité de la surface, la glace de mer mince, les conditions en début et en fin de saison, la bouillie de glace et la glace mouillée, la glace de mer rencontrée par les bateaux, et la discontinuité de la glace. Les données sur la glace de banc ne constituaient pas une priorité. La rugosité de la surface de la glace de mer était considérée comme la principale condition pour laquelle les avantages de la praticabilité déterminés au moyen des cartes établies à l’aide du SAR étaient substantiels. Les cartes indiquant la rugosité sont conçues en fonction de seuils représentant les caractéristiques de rugosité de la surface des glaces de mer (glace lisse/maniqtuk hiku, glace modérément rugueuse/maniilrulik hiku, glace rugueuse/maniittuq hiku; en dialecte inuinnaqtun). Ces cartes sont largement en accord avec les observations locales. Dans l’ensemble, les cartes établies à l’aide du SAR préparées en fonction des utilisations de la glace sont bénéfiques et désirées par les résidents des collectivités du Nord. Nous recommandons que des produits de haute résolution soient régulièrement mis à la disposition des collectivités
Toward Monitoring Surface and Subsurface Lakes on the Greenland Ice Sheet Using Sentinel-1 SAR and Landsat-8 OLI Imagery
Supraglacial lakes are an important component of the Greenland Ice Sheet's mass balance and hydrology, with their drainage affecting ice dynamics. This study uses imagery from the recently launched Sentinel-1A Synthetic Aperture Radar (SAR) satellite to investigate supraglacial lakes in West Greenland. A semi-automated algorithm is developed to detect surface lakes from Sentinel-1 images during the 2015 summer. A combined Landsat-8 and Sentinel-1 dataset, which has a comparable temporal resolution to MODIS (3 days vs. daily) but a higher spatial resolution (25–40 vs. 250–500 m), is then used together with a fully automated lake drainage detection algorithm. Rapid (<4 days) and slow (>4 days) drainages are investigated for both small (<0.125 km2, the minimum size detectable by MODIS) and large (≥0.125 km2) lakes through the summer. Drainage events of small lakes occur at lower elevations (mean 159 m), and slightly earlier (mean 4.5 days) in the melt season than those of large lakes. The analysis is extended manually into the early winter to calculate the dates and elevations of lake freeze-through more precisely than is possible with optical imagery (mean 30 August; 1,270 m mean elevation). Finally, the Sentinel-1 imagery is used to detect subsurface lakes and, for the first time, their dates of appearance and freeze-through (mean 9 August and 7 October, respectively). These subsurface lakes occur at higher elevations than the surface lakes detected in this study (mean 1,593 and 1,185 m, respectively). Sentinel-1 imagery therefore provides great potential for tracking melting, water movement and freezing within both the firn zone and ablation area of the Greenland Ice Sheet
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