184 research outputs found
Monitoring permafrost environments with Synthetic Aperture Radar (SAR) sensors
Permafrost occupies approximately 24% of the exposed land area in the Northern Hemisphere. It is an important element of the cryosphere and has strong impacts on hydrology, biological processes, land surface energy budget, and infrastructure.
For several decades, surface air temperatures in the high northern latitudes have warmed at approximately twice the global rate. Permafrost temperatures have increased in most regions since the early 1980s, the averaged warming north of 60°N has been 1-2°C.
In-situ measurements are essential to understanding physical processes in permafrost terrain, but they have several limitations, ranging from difficulties in drilling to the representativeness of limited single point measurements. Remote sensing is urgently needed to supplement ground-based measurements and extend the point observations to a broader spatial domain.
This thesis concentrates on the sub-arctic permafrost environment monitoring with SAR datasets. The study site is selected in a typical discontinuous permafrost region in the eastern Canadian sub-Arctic. Inuit communities in Nunavik and Nunatsiavut in the Canadian eastern sub-arctic are amongst the groups most affected by the impacts of climate change and permafrost degradation. Synthetic Aperture Radar (SAR) datasets have advantages for permafrost monitoring in the Arctic and sub-arctic regions because of its high resolution and independence of cloud cover and solar illumination. To date, permafrost environment monitoring methods and strategies with SAR datasets are still under development.
The variability of active layer thickness is a direct indication of permafrost thermal state changes. The Differential SAR Interferometry (D-InSAR) technique is applied in the study site to derive ground deformation, which is introduced by the thawing/freezing depth of active layer and underlying permafrost. The D-InSAR technique has been used for the mapping of ground surface deformation over large areas by interpreting the phase difference between two signals acquired at different times as ground motion information. It shows the ability to detect freeze/thaw-related ground motion over permafrost regions. However, to date, accuracy and value assessments of D-InSAR applications have focused mostly on the continuous permafrost region where the vegetation is less developed and causes fewer complicating factors for the D-InSAR application, less attention is laid on the discontinuous permafrost terrain. In this thesis, the influencing factors and application conditions for D-InSAR in the discontinuous permafrost environment are evaluated by using X- band and L-band data. Then, benefit from by the high-temporal resolution of C-band Sentinel-1 time series, the seasonal displacement is derived from small baseline subsets (SBAS)-InSAR.
Landforms are indicative of permafrost presence, with their changes inferring modifications to permafrost conditions. A permafrost landscape mapping method was developed which uses multi-temporal TerraSAR-X backscatter intensity and interferometric coherence information. The land cover map is generated through the combined use of object-based image analysis (OBIA) and classification and regression tree analysis (CART). An overall accuracy of 98% is achieved when classifying rock and water bodies, and an accuracy of 79% is achieved when discriminating between different vegetation types with one year of single-polarized acquisitions. This classification strategy can be transferred to other time-series SAR datasets, e.g., Sentinel-1, and other heterogeneous environments.
One predominant change in the landscape tied to the thaw of permafrost is the dynamics of thermokarst lakes. Dynamics of thermokarst lakes are developed through their lateral extent and vertical depth changes. Due to different water depth, ice cover over shallow thermokarst ponds/lakes can freeze completely to the lake bed in winter, resulting in grounded ice; while ice cover over deep thermokarst ponds/lakes cannot, which have liquid water persisting under the ice cover all winter, resulting in floating ice. Winter ice cover regimes are related to water depths and ice thickness. In the lakes having floating ice, the liquid water induces additional heat in the remaining permafrost underneath and surroundings, which contributes to further intensified permafrost thawing. SAR datasets are utilized to detect winter ice cover regimes based on the character that liquid water has a remarkably high dielectric constant, whereas pure ice has a low value. Patterns in the spatial distribution of ice-cover regimes of thermokarst ponds in a typical discontinuous permafrost region are first revealed. Then, the correlations of these ice-cover regimes with the permafrost degradation states and thermokarst pond development in two historical phases (Sheldrake catchment in the year 1957 and 2009, Tasiapik Valley 1994 and 2010) were explored. The results indicate that the ice-cover regimes of thermokarst ponds are affected by soil texture, permafrost degradation stage and permafrost depth. Permafrost degradation is difficult to directly assess from the coverage area of floating-ice ponds and the percentage of all thermokarst ponds consisting of such floating-ice ponds in a single year. Continuous monitoring of ice-cover regimes and surface areas is recommended to elucidate the hydrological trajectory of the thermokarst process.
Several operational monitoring methods have been developed in this thesis work. In the meanwhile, the spatial distribution of seasonal ground thaw subsidence, permafrost landscape, thermokarst ponds and their winter ice cover regimes are first revealed in the study area. The outcomes help understand the state and dynamics of permafrost environment.Der Permafrostboden bedeckt etwa 24% der exponierten LandflÀche in der nördlichen HemisphÀre. Es ist ein wichtiges Element der KryosphÀre und hat starke Auswirkungen auf die Hydrologie, die biologischen Prozesse, das Energie-Budget der LandoberflÀche und die Infrastruktur.
Seit mehreren Jahrzehnten erhöhen sich die OberflĂ€chenlufttemperaturen in den nördlichen hohen Breitengraden etwa doppelt so stark wie die globale Rate. Die Temperaturen der Permafrostböden sind in den meisten Regionen seit den frĂŒhen 1980er Jahren gestiegen. Die durchschnittliche ErwĂ€rmung nördlich von 60° N betrĂ€gt 1-2°C.
In-situ-Messungen sind essentiell fĂŒr das VerstĂ€ndnis der physischen Prozesse im PermafrostgelĂ€nde. Es gibt jedoch mehrere EinschrĂ€nkungen, die von Schwierigkeiten beim Bohren bis hin zur ReprĂ€sentativitĂ€t begrenzter Einzelpunktmessungen reichen. Fernerkundung ist dringend benötigt, um bodenbasierte Messungen zu ergĂ€nzen und punktuelle Beobachtungen auf einen breiteren rĂ€umlichen Bereich auszudehnen.
Diese Dissertation konzentriert sich auf die Umweltbeobachtung der subarktischen Permafrostböden mit SAR-DatensĂ€tzen. Das Untersuchungsgebiet wurde in einer typischen diskontinuierlichen Permafrostzone in der kanadischen östlichen Sub-Arktis ausgewĂ€hlt. Die Inuit-Gemeinschaften in den Regionen Nunavik und Nunatsiavut in der kanadischen östlichen Sub-Arktis gehören zu den Gruppen, die am stĂ€rksten von den Auswirkungen des Klimawandels und Permafrostdegradation betroffen sind. Synthetische Apertur Radar (SAR) DatensĂ€tze haben Vorteile fĂŒr das Permafrostmonitoring in den arktischen und subarktischen Regionen aufgrund der hohen Auflösung und der UnabhĂ€ngigkeit von Wolkendeckung und Sonnenstrahlung. Bis heute sind die Methoden und Strategien mit SAR-DatensĂ€tzen fĂŒr Umweltbeobachtung der Permafrostböden noch in der Entwicklung.
Die VariabilitĂ€t der Auftautiefe der aktiven Schicht ist eine direkte Indikation der VerĂ€nderung des thermischen Zustands der Permafrostböden. Die Differential-SAR-Interferometrie(D-Insar)-Technik wird im Untersuchungsgebiet zur Ableitung der Bodendeformation, die durch Auftau- / und Gefriertiefe der aktiven Schicht und des unterliegenden Permafrostbodens eingefĂŒhrt wird, eingesetzt. Die D-InSAR-Technik wurde fĂŒr Kartierung der LandoberflĂ€chendeformation ĂŒber groĂe FlĂ€chen verwendet, indem der Phasenunterschied zwischen zwei zu verschiedenen Zeitpunkten als Bodenbewegungsinformation erfassten Signalen interpretiert wurde. Es zeigt die FĂ€higkeit, tau- und gefrierprozessbedingte Bodenbewegungen ĂŒber Permafrostregionen zu detektieren. Jedoch fokussiert sich die Genauigkeit und WertschĂ€tzung der D-InSAR-Anwendung bis heute hauptsĂ€chlich auf kontinuierliche Permafrostregion, wo die Vegetation wenig entwickelt ist und weniger komplizierte Faktoren fĂŒr D-InSAR-Anwendung verursacht. Das diskontinuierliche PermafrostgelĂ€nde wurde nur weniger berĂŒcksichtigt. In dieser Dissertation wurden die Einflussfaktoren und Anwendungsbedingungen fĂŒr D-InSAR im diskontinuierlichen Permafrostgebiet mittels X-Band und L-Band Daten ausgewertet. Dann wurde die saisonale Verschiebung dank der hohen Auflösung der C-Band Sentinel-1 Zeitreihe von âSmall Baseline Subsets (SBAS)-InSARâ abgeleitet.
Landformen weisen auf die PrĂ€senz des Permafrosts hin, wobei deren VerĂ€nderungen auf die Modifikation der Permafrostbedingungen schlieĂen. Eine Kartierungsmethode der Permafrostlandschaft wurde entwickelt, dabei wurde Multi-temporal TerraSAR-X RĂŒckstreuungsintensitĂ€t und interferometrische KohĂ€renzinformationen verwendet. Die Landbedeckungskarte wurde durch kombinierte Anwendung objektbasierter Bildanalyse (OBIA) und Klassifikations- und Regressionsbaum Analyse (CART) generiert. Eine Gesamtgenauigkeit in Höhe von 98% wurde bei Klassifikation der Gesteine und Wasserkörper erreicht. Bei Unterscheidung zwischen verschiedenen Vegetationstypen mit einem Jahr einzelpolarisierte Akquisitionen wurde eine Genauigkeit von 79% erreicht. Diese Klassifikationsstrategie kann auf andere Zeitreihen der SAR-DatensĂ€tzen, z.B. Sentinel-1, und auch anderen heterogenen Umwelten ĂŒbertragen werden.
Eine vorherrschende VerĂ€nderung in der Landschaft, die mit dem Auftauen des Permafrosts verbunden ist, ist die Dynamik der Thermokarstseen. Die Dynamik der Thermokarstseen ist durch VerĂ€nderungen der seitlichen Ausdehnung und der vertikalen Tiefe entwickelt. Aufgrund der unterschiedlichen Wassertiefen kann die Eisdecke ĂŒber den flachen Thermokarstteichen/-seen im Winter bis auf den Wasserboden vollstĂ€ndig gefroren sein, was zum geerdeten Eis fĂŒhrt, wĂ€hrend die Eisdecke ĂŒber den tiefen Thermokarstteichen/-seen es nicht kann. In den tiefen Thermokarstteichen/-seen bleibt den ganzen Winter flĂŒssiges Wasser unter der Eisdecke bestehen, was zum Treibeis fĂŒhrt. Das Wintereisdeckenregime bezieht sich auf die Wassertiefe und die Eisdicke. In den Seen mit Treibeis leitet das flĂŒssige Wasser zusĂ€tzliche WĂ€rme in den restlichen Permafrost darunter oder in der Umgebung, was zur weiteren VerstĂ€rkung des Permafrostauftauen beitrĂ€gt. Basiert auf den Charakter, dass das flĂŒssige Wasser eine bemerkenswert hohe DielektrizitĂ€tskonstante besitzt, wĂ€hrend reines Eis einen niedrigen Wert hat, wurden die SAR DatensĂ€tzen zur Erkennung des Wintereisdeckenregimes verwendet. ZunĂ€chst wurden Schemen in der rĂ€umlichen Verteilung der Eisdeckenregimes der Thermokarstteiche in einer typischen diskontinuierlichen Permafrostregion abgeleitet. Dann wurden die ZusammenhĂ€nge dieser Eisdeckenregimes mit dem Degradationszustand des Permafrosts und der Entwicklung der Thermokarstteiche in zwei historischen Phasen (Sheldrake Einzugsgebiet in 1957 und 2009, Tasiapik Tal in 1994 und 2010) erforscht. Die Ergebnisse deuten darauf, dass die Eisdeckenregimes der Thermokarstteiche von der Bodenart, dem Degradationszustand des Permafrosts und der Permafrosttiefe beeinflusst werden. Es ist schwer, die Permafrostdegradation in einem einzelnen Jahr direkt durch den Abdeckungsbereich der Treibeis-Teiche und die Prozentzahl aller aus solchen Treibeis-Teichen bestehenden Thermokarstteiche abzuschĂ€tzen. Ein kontinuierliches Monitoring der Eisdeckenregimes und -oberflĂ€chen ist empfehlenswert, um den hydrologischen Verlauf des Thermokarstprozesses zu erlĂ€utern.
In dieser Dissertation wurden mehrere operativen Monitoringsmethoden entwickelt. In der Zwischenzeit wurden die rÀumliche Verteilung der saisonalen Bodentauabsenkung, die Permafrostlandschaft, die Thermokarstteiche und ihre Wintereisdeckenregimes erstmals in diesem Untersuchungsgebiet aufgedeckt. Die Ergebnisse tragen dazu bei, den Zustand und die Dynamik der Permafrostumwelt zu verstehen
Monitoring permafrost environments with Synthetic Aperture Radar (SAR) sensors
Permafrost occupies approximately 24% of the exposed land area in the Northern Hemisphere. It is an important element of the cryosphere and has strong impacts on hydrology, biological processes, land surface energy budget, and infrastructure.
For several decades, surface air temperatures in the high northern latitudes have warmed at approximately twice the global rate. Permafrost temperatures have increased in most regions since the early 1980s, the averaged warming north of 60°N has been 1-2°C.
In-situ measurements are essential to understanding physical processes in permafrost terrain, but they have several limitations, ranging from difficulties in drilling to the representativeness of limited single point measurements. Remote sensing is urgently needed to supplement ground-based measurements and extend the point observations to a broader spatial domain.
This thesis concentrates on the sub-arctic permafrost environment monitoring with SAR datasets. The study site is selected in a typical discontinuous permafrost region in the eastern Canadian sub-Arctic. Inuit communities in Nunavik and Nunatsiavut in the Canadian eastern sub-arctic are amongst the groups most affected by the impacts of climate change and permafrost degradation. Synthetic Aperture Radar (SAR) datasets have advantages for permafrost monitoring in the Arctic and sub-arctic regions because of its high resolution and independence of cloud cover and solar illumination. To date, permafrost environment monitoring methods and strategies with SAR datasets are still under development.
The variability of active layer thickness is a direct indication of permafrost thermal state changes. The Differential SAR Interferometry (D-InSAR) technique is applied in the study site to derive ground deformation, which is introduced by the thawing/freezing depth of active layer and underlying permafrost. The D-InSAR technique has been used for the mapping of ground surface deformation over large areas by interpreting the phase difference between two signals acquired at different times as ground motion information. It shows the ability to detect freeze/thaw-related ground motion over permafrost regions. However, to date, accuracy and value assessments of D-InSAR applications have focused mostly on the continuous permafrost region where the vegetation is less developed and causes fewer complicating factors for the D-InSAR application, less attention is laid on the discontinuous permafrost terrain. In this thesis, the influencing factors and application conditions for D-InSAR in the discontinuous permafrost environment are evaluated by using X- band and L-band data. Then, benefit from by the high-temporal resolution of C-band Sentinel-1 time series, the seasonal displacement is derived from small baseline subsets (SBAS)-InSAR.
Landforms are indicative of permafrost presence, with their changes inferring modifications to permafrost conditions. A permafrost landscape mapping method was developed which uses multi-temporal TerraSAR-X backscatter intensity and interferometric coherence information. The land cover map is generated through the combined use of object-based image analysis (OBIA) and classification and regression tree analysis (CART). An overall accuracy of 98% is achieved when classifying rock and water bodies, and an accuracy of 79% is achieved when discriminating between different vegetation types with one year of single-polarized acquisitions. This classification strategy can be transferred to other time-series SAR datasets, e.g., Sentinel-1, and other heterogeneous environments.
One predominant change in the landscape tied to the thaw of permafrost is the dynamics of thermokarst lakes. Dynamics of thermokarst lakes are developed through their lateral extent and vertical depth changes. Due to different water depth, ice cover over shallow thermokarst ponds/lakes can freeze completely to the lake bed in winter, resulting in grounded ice; while ice cover over deep thermokarst ponds/lakes cannot, which have liquid water persisting under the ice cover all winter, resulting in floating ice. Winter ice cover regimes are related to water depths and ice thickness. In the lakes having floating ice, the liquid water induces additional heat in the remaining permafrost underneath and surroundings, which contributes to further intensified permafrost thawing. SAR datasets are utilized to detect winter ice cover regimes based on the character that liquid water has a remarkably high dielectric constant, whereas pure ice has a low value. Patterns in the spatial distribution of ice-cover regimes of thermokarst ponds in a typical discontinuous permafrost region are first revealed. Then, the correlations of these ice-cover regimes with the permafrost degradation states and thermokarst pond development in two historical phases (Sheldrake catchment in the year 1957 and 2009, Tasiapik Valley 1994 and 2010) were explored. The results indicate that the ice-cover regimes of thermokarst ponds are affected by soil texture, permafrost degradation stage and permafrost depth. Permafrost degradation is difficult to directly assess from the coverage area of floating-ice ponds and the percentage of all thermokarst ponds consisting of such floating-ice ponds in a single year. Continuous monitoring of ice-cover regimes and surface areas is recommended to elucidate the hydrological trajectory of the thermokarst process.
Several operational monitoring methods have been developed in this thesis work. In the meanwhile, the spatial distribution of seasonal ground thaw subsidence, permafrost landscape, thermokarst ponds and their winter ice cover regimes are first revealed in the study area. The outcomes help understand the state and dynamics of permafrost environment.Der Permafrostboden bedeckt etwa 24% der exponierten LandflÀche in der nördlichen HemisphÀre. Es ist ein wichtiges Element der KryosphÀre und hat starke Auswirkungen auf die Hydrologie, die biologischen Prozesse, das Energie-Budget der LandoberflÀche und die Infrastruktur.
Seit mehreren Jahrzehnten erhöhen sich die OberflĂ€chenlufttemperaturen in den nördlichen hohen Breitengraden etwa doppelt so stark wie die globale Rate. Die Temperaturen der Permafrostböden sind in den meisten Regionen seit den frĂŒhen 1980er Jahren gestiegen. Die durchschnittliche ErwĂ€rmung nördlich von 60° N betrĂ€gt 1-2°C.
In-situ-Messungen sind essentiell fĂŒr das VerstĂ€ndnis der physischen Prozesse im PermafrostgelĂ€nde. Es gibt jedoch mehrere EinschrĂ€nkungen, die von Schwierigkeiten beim Bohren bis hin zur ReprĂ€sentativitĂ€t begrenzter Einzelpunktmessungen reichen. Fernerkundung ist dringend benötigt, um bodenbasierte Messungen zu ergĂ€nzen und punktuelle Beobachtungen auf einen breiteren rĂ€umlichen Bereich auszudehnen.
Diese Dissertation konzentriert sich auf die Umweltbeobachtung der subarktischen Permafrostböden mit SAR-DatensĂ€tzen. Das Untersuchungsgebiet wurde in einer typischen diskontinuierlichen Permafrostzone in der kanadischen östlichen Sub-Arktis ausgewĂ€hlt. Die Inuit-Gemeinschaften in den Regionen Nunavik und Nunatsiavut in der kanadischen östlichen Sub-Arktis gehören zu den Gruppen, die am stĂ€rksten von den Auswirkungen des Klimawandels und Permafrostdegradation betroffen sind. Synthetische Apertur Radar (SAR) DatensĂ€tze haben Vorteile fĂŒr das Permafrostmonitoring in den arktischen und subarktischen Regionen aufgrund der hohen Auflösung und der UnabhĂ€ngigkeit von Wolkendeckung und Sonnenstrahlung. Bis heute sind die Methoden und Strategien mit SAR-DatensĂ€tzen fĂŒr Umweltbeobachtung der Permafrostböden noch in der Entwicklung.
Die VariabilitĂ€t der Auftautiefe der aktiven Schicht ist eine direkte Indikation der VerĂ€nderung des thermischen Zustands der Permafrostböden. Die Differential-SAR-Interferometrie(D-Insar)-Technik wird im Untersuchungsgebiet zur Ableitung der Bodendeformation, die durch Auftau- / und Gefriertiefe der aktiven Schicht und des unterliegenden Permafrostbodens eingefĂŒhrt wird, eingesetzt. Die D-InSAR-Technik wurde fĂŒr Kartierung der LandoberflĂ€chendeformation ĂŒber groĂe FlĂ€chen verwendet, indem der Phasenunterschied zwischen zwei zu verschiedenen Zeitpunkten als Bodenbewegungsinformation erfassten Signalen interpretiert wurde. Es zeigt die FĂ€higkeit, tau- und gefrierprozessbedingte Bodenbewegungen ĂŒber Permafrostregionen zu detektieren. Jedoch fokussiert sich die Genauigkeit und WertschĂ€tzung der D-InSAR-Anwendung bis heute hauptsĂ€chlich auf kontinuierliche Permafrostregion, wo die Vegetation wenig entwickelt ist und weniger komplizierte Faktoren fĂŒr D-InSAR-Anwendung verursacht. Das diskontinuierliche PermafrostgelĂ€nde wurde nur weniger berĂŒcksichtigt. In dieser Dissertation wurden die Einflussfaktoren und Anwendungsbedingungen fĂŒr D-InSAR im diskontinuierlichen Permafrostgebiet mittels X-Band und L-Band Daten ausgewertet. Dann wurde die saisonale Verschiebung dank der hohen Auflösung der C-Band Sentinel-1 Zeitreihe von âSmall Baseline Subsets (SBAS)-InSARâ abgeleitet.
Landformen weisen auf die PrĂ€senz des Permafrosts hin, wobei deren VerĂ€nderungen auf die Modifikation der Permafrostbedingungen schlieĂen. Eine Kartierungsmethode der Permafrostlandschaft wurde entwickelt, dabei wurde Multi-temporal TerraSAR-X RĂŒckstreuungsintensitĂ€t und interferometrische KohĂ€renzinformationen verwendet. Die Landbedeckungskarte wurde durch kombinierte Anwendung objektbasierter Bildanalyse (OBIA) und Klassifikations- und Regressionsbaum Analyse (CART) generiert. Eine Gesamtgenauigkeit in Höhe von 98% wurde bei Klassifikation der Gesteine und Wasserkörper erreicht. Bei Unterscheidung zwischen verschiedenen Vegetationstypen mit einem Jahr einzelpolarisierte Akquisitionen wurde eine Genauigkeit von 79% erreicht. Diese Klassifikationsstrategie kann auf andere Zeitreihen der SAR-DatensĂ€tzen, z.B. Sentinel-1, und auch anderen heterogenen Umwelten ĂŒbertragen werden.
Eine vorherrschende VerĂ€nderung in der Landschaft, die mit dem Auftauen des Permafrosts verbunden ist, ist die Dynamik der Thermokarstseen. Die Dynamik der Thermokarstseen ist durch VerĂ€nderungen der seitlichen Ausdehnung und der vertikalen Tiefe entwickelt. Aufgrund der unterschiedlichen Wassertiefen kann die Eisdecke ĂŒber den flachen Thermokarstteichen/-seen im Winter bis auf den Wasserboden vollstĂ€ndig gefroren sein, was zum geerdeten Eis fĂŒhrt, wĂ€hrend die Eisdecke ĂŒber den tiefen Thermokarstteichen/-seen es nicht kann. In den tiefen Thermokarstteichen/-seen bleibt den ganzen Winter flĂŒssiges Wasser unter der Eisdecke bestehen, was zum Treibeis fĂŒhrt. Das Wintereisdeckenregime bezieht sich auf die Wassertiefe und die Eisdicke. In den Seen mit Treibeis leitet das flĂŒssige Wasser zusĂ€tzliche WĂ€rme in den restlichen Permafrost darunter oder in der Umgebung, was zur weiteren VerstĂ€rkung des Permafrostauftauen beitrĂ€gt. Basiert auf den Charakter, dass das flĂŒssige Wasser eine bemerkenswert hohe DielektrizitĂ€tskonstante besitzt, wĂ€hrend reines Eis einen niedrigen Wert hat, wurden die SAR DatensĂ€tzen zur Erkennung des Wintereisdeckenregimes verwendet. ZunĂ€chst wurden Schemen in der rĂ€umlichen Verteilung der Eisdeckenregimes der Thermokarstteiche in einer typischen diskontinuierlichen Permafrostregion abgeleitet. Dann wurden die ZusammenhĂ€nge dieser Eisdeckenregimes mit dem Degradationszustand des Permafrosts und der Entwicklung der Thermokarstteiche in zwei historischen Phasen (Sheldrake Einzugsgebiet in 1957 und 2009, Tasiapik Tal in 1994 und 2010) erforscht. Die Ergebnisse deuten darauf, dass die Eisdeckenregimes der Thermokarstteiche von der Bodenart, dem Degradationszustand des Permafrosts und der Permafrosttiefe beeinflusst werden. Es ist schwer, die Permafrostdegradation in einem einzelnen Jahr direkt durch den Abdeckungsbereich der Treibeis-Teiche und die Prozentzahl aller aus solchen Treibeis-Teichen bestehenden Thermokarstteiche abzuschĂ€tzen. Ein kontinuierliches Monitoring der Eisdeckenregimes und -oberflĂ€chen ist empfehlenswert, um den hydrologischen Verlauf des Thermokarstprozesses zu erlĂ€utern.
In dieser Dissertation wurden mehrere operativen Monitoringsmethoden entwickelt. In der Zwischenzeit wurden die rÀumliche Verteilung der saisonalen Bodentauabsenkung, die Permafrostlandschaft, die Thermokarstteiche und ihre Wintereisdeckenregimes erstmals in diesem Untersuchungsgebiet aufgedeckt. Die Ergebnisse tragen dazu bei, den Zustand und die Dynamik der Permafrostumwelt zu verstehen
Remote Sensing of Environmental Changes in Cold Regions
This Special Issue gathers papers reporting recent advances in the remote sensing of cold regions. It includes contributions presenting improvements in modeling microwave emissions from snow, assessment of satellite-based sea ice concentration products, satellite monitoring of ice jam and glacier lake outburst floods, satellite mapping of snow depth and soil freeze/thaw states, near-nadir interferometric imaging of surface water bodies, and remote sensing-based assessment of high arctic lake environment and vegetation recovery from wildfire disturbances in Alaska. A comprehensive review is presented to summarize the achievements, challenges, and opportunities of cold land remote sensing
Data Fusion and Synergy of Active and Passive Remote Sensing; An application for Freeze Thaw Detections
There has been a recent evolvement in the field of remote sensing after increase of number satellites and sensors data which could be fused to produce new data and products. These efforts are mainly focused on using of simultaneous observations from different platforms with different spatial and temporal resolutions. The research dissertation aims to enhance the synergy use of active and passive microwave observations and examine the results in detection land freeze and thaw (FT) predictions. Freeze thaw cycles particularly in high-latitude regions have a crucial role in many applications such as agriculture, biogeochemical transitions, hydrology and ecosystem studies. The dielectric change between frozen ice and melted water can dramatically affect the brightness temperature (TB) signal when water transits from the liquid to the solid phase which makes satellite-based microwave remote sensing unique for characterizing the surface freeze thaw status. Passive microwave (PMW) sensors with coarse resolution (about 25 km) but more frequent observations (at least twice a day and more frequent in polar regions) have been successfully utilized to define surface state in terms of freeze and thaw in the past. Alternatively, active microwave (AMW) sensors provide much higher spatial resolution (about 100 m or less) though with less temporal resolution (each 12 days). Therefore, an integration of microwave data coming from different sensors may provide a more complete estimation of land freeze thaw state. In this regard, the overarching goal of this research is to explore estimating high spatiotemporal freeze and thaw states using the combination of passive and active microwave observations. To obtain a high temporal resolution TB, this study primarily builds an improved diurnal variation of land surface temperature from integration of infrared sensors. In the next step, a half an hourly diurnal cycle of TB based on fusion of different passive sensors is estimated. It should be mentioned that each instrument has its own footprint, resolution, viewing angle, as well as frequency and consequently their data need to be harmonized in order to be combined. Later, data from an AMW sensor with fine spatial resolution are merged and compared to the corresponding passive data in order to find a relation between TB and backscatter data. Subsequently, PMW TB map can be downscaled to a higher spatial resolution or AMW backscatter timeseries can be generalized to high temporal resolution. Eventually, the final high spatiotemporal resolution TB product is used to examine the freeze thaw state for case studies areas in Northern latitudes
DĂ©tection des cycles de gel/dĂ©gel de la couche active du sol en toundra arctique Ă partir dâimageries radar Ă synthĂšse dâouverture (RSO) multicapteur en bande C
Lâaugmentation de la tempĂ©rature de lâair moyenne annuelle, chiffrĂ©e Ă +2,3 °C pour les rĂ©gions de lâarctique Canadien entre 1948 et 2016, a des impacts considĂ©rables sur le couvert nival arctique et sur la vĂ©gĂ©tation en place. Ces deux paramĂštres influencent le rĂ©gime thermique du sol et donc, les cycles de gel/dĂ©gel de sa couche active dans lâĂ©cosystĂšme arctique. Lâimportance du suivi de ces cycles rĂ©side dans leur influence sur plusieurs paramĂštres de la cryosphĂšre tels que le cycle hydrologique et du carbone, la saison de croissance de la vĂ©gĂ©tation, lâĂ©tat du pergĂ©lisol sous-jacent ainsi que lâĂ©paisseur de sa couche active. Lâutilisation de donnĂ©es ponctuelles ou provenant de capteurs micro-onde passive Ă basse rĂ©solution prĂ©sente un enjeu pour le suivi spatial et temporel de ces cycles. Le projet vise Ă dĂ©velopper un algorithme de dĂ©tection des cycles de gel/dĂ©gel du sol en toundra arctique Ă partir dâimageries RSO multicapteur (i.e., Sentinel-1 et RADARSAT-2) ayant une couverture temporelle quasi journaliĂšre en bande C, afin dâĂ©valuer lâimpact de la variabilitĂ© spatiale et temporelle des paramĂštres influençant le rĂ©gime thermique du sol tel que, les Ă©cosystĂšmes terrestres (i.e., Ă©cotype) et la prĂ©sence de neige. LâĂ©tude se concentre sur une zone Ă lâintĂ©rieur du bassin versant du lac Greiner Ă proximitĂ© de la ville de Cambridge Bay au Nunavut. La normalisation de lâangle dâincidence a permis de diminuer le bruit dans les sĂ©ries temporelles ainsi que de rendre possible lâutilisation d'images acquises Ă l'intĂ©rieur de plusieurs orbites dâobservation. Cela a aussi permis dâuniformiser les donnĂ©es des deux capteurs pour les combiner en une seule sĂ©rie temporelle. Deux algorithmes de dĂ©tections ont Ă©tĂ© utilisĂ©s, soit un algorithme de seuil saisonnier (STA) ainsi quâun algorithme de dĂ©tection de changement (CPD). La validation sâest faite Ă partir des donnĂ©es spatialement distribuĂ©es de tempĂ©rature du sol et de lâair indĂ©pendamment sous forme de prĂ©cision (%) et de dĂ©lai (#jours) de dĂ©tection. Les deux algorithmes ont permis dâobtenir une prĂ©cision de dĂ©tection de plus de 97% sur les sites de rĂ©fĂ©rence. Une spatialisation, pixel par pixel, de la mĂ©thode STA a permis la crĂ©ation de cartes de jour de gel/dĂ©gel pour le site dâĂ©tude. La combinaison des cartes de jour de transition avec la carte dâĂ©cotype a permis de modĂ©liser lâimpact des caractĂ©ristiques des Ă©cotypes sur le jour de transition. Les rĂ©sultats obtenus dans ce projet dĂ©montrent clairement le potentiel de lâutilisation des donnĂ©es RSO en bande C pour la dĂ©tection des cycles de gel/dĂ©gel, ce qui constitue un rĂ©sultat important en raison de la quantitĂ© grandissante de donnĂ©es Ă cette frĂ©quence (e.g., RCM, Sentinel-1A-C-D). La mĂ©thode prĂ©sentĂ©e dans ce projet pourrait permettre de crĂ©er des cartes de transition pour tout le bassin versant du lac Greiner Ă partir de donnĂ©es RSO en bande C.Abstract : The observed average annual surface temperature increase of 2.3°C in the Canadian Arctic regions between 1948 and 2016 has significant effects on the Arctic snow cover and on the vegetation in place. Those two parameters influence the thermal regime of the ground and therefore the freeze and thaw (F/T) cycles of the soil active layer in the Arctic tundra ecosystem. The importance of monitoring these cycles lies in their influence on several parameters of the cryosphere such as the hydrological and carbon cycle, the vegetation growing season, the state of the underlying permafrost and the thickness of its active layer. The use of punctual data or low-resolution passive microwave sensors presents a challenge for the spatial and temporal monitoring of these cycles. The project aims to develop an algorithm for soil freeze/thaw cycles detection in arctic tundra from multisensor C-band imagery (i.e., Sentinel-1 and RADARSAT-2) to assess the impact of the spatial and temporal variability of the parameters influencing the thermal regime of the ground, such as the terrestrial ecosystems (i.e., ecotype) and the snow cover. The study focused on a region of the Greiner lake watershed on Victoria Island in Nunavut. An incidence angle normalization was applied to the backscatter time series to remove influence of the acquisition angle on backscatter and to allow for the use of images acquired within several orbits of observation. This also standardized the data from the two sensors to combine them into a single time series. Two detection algorithms were used on the normalized backscatter coefficient data, namely a seasonal threshold algorithm (STA) and a change point detection algorithm (CPD). A spatially distributed network of soil and air temperature were used for validation in the form of accuracy (%) and delay (#days) of detection. Both algorithms achieved a detection accuracy of more than 97% for the entire analysis period on the reference sites. A pixel-by-pixel spatialization of the STA method allowed to create F/T transition maps for the extended study site. The combination of the transition maps with the ecotype data made it possible to model the impact of ecotype characteristics on the day of transition. The results obtained in this project clearly demonstrate the potential of using C-band for the detection of F/T cycles, which is an important aspect due to the increasing number of data at this frequency (e.g., RCM, Sentinel -1A-C-D). The method presented in this project could then make it possible to create transition maps for the entire Greiner Lake watershed from C-band SAR data and thus improve the integration of this parameter in climate models
Mapping Snowmelt Progression in the Upper Indus Basin with Synthetic Aperture Radar
The Indus River is a vital resource for food security, ecosystem services, hydropower, and economy for millions of people living in Pakistan, India, China, and Afghanistan. Glacier and snowmelt from the high altitude Himalaya, Karakoram, and Hindu Kush mountain ranges are the largest drivers of discharge in the upper Indus Basin (UIB), and contribute significantly to Indus flows. Complex climatology and topography, coupled with the challenges of field study and meteorological measurement in these rugged ranges, elicit notable uncertainties in predicting seasonal runoff as well as cryospheric response to changes in climate. Here we utilize Sentinel-1 synthetic aperture radar (SAR) imagery to track ablation season development of wet snow in the Shigar Watershed of the Karakoram Mountains in Pakistan from 2015 to 2018. We exploit opportune local image acquisition times to highlight diurnal differences in radar indications of wet snow, and examine the spatial and temporal contexts of radar diurnal differences for 2015, 2017, and 2018 ablation seasons. Radar classifications for each ablation season show spatial and temporal patterns that indicate a dry winter snowpack undergoing diurnal surface melt-refreeze cycles, transitioning to surface snow that remains wet both day and night, and finally snow free conditions following melt out. Diurnally differing SAR signals may offer insights into important snowpack energy balance processes that precede melt out, which could provide useful constraints for both glacier mass balance modeling and runoff forecasting in remote alpine watersheds
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 collectivitĂ©s du Nord sâintĂ©ressent de plus en plus aux technologies qui leur fournissent de lâinformation au sujet de lâenvironnement de glace de mer Ă des fins de dĂ©placements. La tĂ©lĂ©dĂ©tection par radar Ă synthĂšse dâouverture (SAR) est couramment utilisĂ©e pour observer la glace de mer, indĂ©pendamment de la lumiĂšre du soleil et de la nĂ©bulositĂ©. Cependant, dans les collectivitĂ©s du Nord, lâaccĂšs au SAR est restreint. Cette Ă©tude 1) dĂ©finit les caractĂ©ristiques de la glace de mer qui exercent une influence sur les dĂ©placements de deux collectivitĂ©s dans lâouest de lâArctique canadien; 2) dĂ©termine lâutilitĂ© du SAR pour amĂ©liorer la mobilitĂ© et la sĂ©curitĂ© quand vient le temps de traverser des environnements comportant ces caractĂ©ristiques; et 3) dĂ©crit les mĂ©thodes de partage de cartes Ă©tablies Ă lâaide du SAR. Trois saisons sur le terrain (le printemps et lâautomne de 2017, et le printemps de 2018) ont permis dâinciter les rĂ©sidents Ă participer Ă une recherche locale guidĂ©e, lĂ oĂč les extrants appliquĂ©s ont Ă©tĂ© Ă©valuĂ©s par les membres de la collectivitĂ©. Nous avons trouvĂ© que les donnĂ©es Ă©manant des images du SAR Ă©clairent et amĂ©liorent la sĂ©curitĂ© de la glace de mer, lâaptitude Ă la circulation et lâĂ©ducation. Lâinformation dĂ©coulant de la technologie sâavĂšre un complĂ©ment dĂ©sirable aux connaissances inuites en vue de la comprĂ©hension des caractĂ©ristiques de la glace de mer, dont la rugositĂ© de la surface, la glace de mer mince, les conditions en dĂ©but et en fin de saison, la bouillie de glace et la glace mouillĂ©e, la glace de mer rencontrĂ©e par les bateaux, et la discontinuitĂ© de la glace. Les donnĂ©es sur la glace de banc ne constituaient pas une prioritĂ©. La rugositĂ© de la surface de la glace de mer Ă©tait considĂ©rĂ©e comme la principale condition pour laquelle les avantages de la praticabilitĂ© dĂ©terminĂ©s au moyen des cartes Ă©tablies Ă lâaide du SAR Ă©taient substantiels. Les cartes indiquant la rugositĂ© sont conçues en fonction de seuils reprĂ©sentant les caractĂ©ristiques de rugositĂ© de la surface des glaces de mer (glace lisse/maniqtuk hiku, glace modĂ©rĂ©ment rugueuse/maniilrulik hiku, glace rugueuse/maniittuq hiku; en dialecte inuinnaqtun). Ces cartes sont largement en accord avec les observations locales. Dans lâensemble, les cartes Ă©tablies Ă lâaide du SAR prĂ©parĂ©es en fonction des utilisations de la glace sont bĂ©nĂ©fiques et dĂ©sirĂ©es par les rĂ©sidents des collectivitĂ©s du Nord. Nous recommandons que des produits de haute rĂ©solution soient rĂ©guliĂšrement mis Ă la disposition des collectivitĂ©s
Spaceborne monitoring of Arctic lake ice in a changing climate
Lake ice phenology (timing of ice-on and ice-off) and thickness are changing in response to generally warmer climate conditions at high northern latitudes observed during recent decades. Monitoring changes in the lake ice cover provides valuable evidence in assessing climate variability in the Arctic. To enhance our understanding of the role of lake ice in the Arctic cryosphere and to evaluate the extent to which Arctic lakes have been impacted by the contemporary changing climate, development of a lake ice monitoring system at pan-Arctic scale is needed. While large lakes across the Arctic are currently being monitored through satellite observations, there are extremely sparse and mostly non-existent records tracking the changes in small high-latitude lakes.
Employing a combination of spaceborne observations from synthetic aperture radar (SAR) and optical sensors, and simulations from the Canadian Lake Ice Model (CLIMo), this researched aimed to investigate changes in winter ice growth and ice phenology of lakes across the Arctic, focus being given to smaller lakes on the North Slope of Alaska (NSA) and lakes of various sizes in the Canadian Arctic Archipelago (CAA).
To determine the changes in the fraction of lakes that freeze to bed (grounded ice) in late winter on the NSA from 1991 to 2011, a time series of ERS-1/2 was analysed. Results show a trend toward increasing floating ice fractions from 1991 to 2011, with the greatest change occurring in April, when the grounded ice fraction declined by 22% (α = 0.01). This finding is in good agreement with the decrease in ice thickness simulated with CLIMo, a lower fraction of lakes frozen to the bed corresponding to a thinner ice cover. Model simulations over the same period as SAR acquisitions (1991-2011) indicate a trend toward thinner ice covers by 18-22 cm (no-snow and 53% snow depth scenarios, α = 0.01). The results emphasize the regime shifts that these lakes are currently undergoing, including shorter ice seasons. The longer-term trends (1950-2011) derived from model simulations show a decrease in the ice cover duration by ~ 24 days consequent to later freeze-up dates by 5.9 days (α = 0.1) and earlier break-up dates by 17.7-18.6 days (α = 0.001).
The temporal evolution of backscatter (Ï0) from two C-band SAR sensors â Advanced Synthetic Aperture Radar (ASAR) Wide Swath and RADARSAT-2 ScanSAR Wide Swath â was then used to investigate the potential of high temporal-frequency SAR for determining lake ice phenological events (e.g. freeze onset, melt onset and water-clear-of-ice). Results show that combined SAR observations are generally suitable for detection of important lake ice events timing. However, the wide range of incidence angles and to a certain extent the orbit differences between the observations, the wind effect, particularly during fall freeze-up, the low differences in Ï0 during transition from a grounded-ice cover to melt onset of ice in early spring, complicate the detection of lake ice phenological events.
In order to order to document the response of ice cover of lakes in the Canadian High Arctic to climate conditions during recent years, a 15-year time series (1997-2011) of RADARSAT-1/2 ScanSAR Wide Swath, ASAR Wide Swath and Landsat acquisitions were analyzed. Results show that earlier melt onset occurred earlier for all 11 polar-desert and polar-oasis lakes that were investigated. With the exception of Lower Murray Lake, all lakes experienced earlier ice-minimum and water-clear-of-ice dates, with greater changes being observed for polar-oasis lakes (9-23.6 days earlier water-clear-of-ice for lakes located in polar oases and 1.6-20 days earlier water-clear-of-ice for polar-desert lakes). Additionally, results suggest that some lakes may be transitioning from a perennial to a seasonal ice regime, with only a few lakes maintaining a perennial ice cover on occasional years. Aside Lake Hazen and Murray Lakes that preserved their ice cover during the summer of 2009, no residual ice was observed on any of the other lakes from 2007 to 2011.
This research provides the foundation of a lake-ice monitoring network that can be built on with the newly launched and future SAR and multispectral missions. Additionally, this study shows that in response to warmer climate conditions, Arctic lakes are experiencing regime shifts with overall shorter ice seasons, thinner ice covers, fewer lakes that freeze to the bottom and more lakes that lose the perennial ice cover and experience a seasonal ice regime
Mapping bedfast and floating thermokarst lake ice and determining lake depth using Sentinel 1 Synthetic Aperture Radar Remote Sensing on the west shore of Hudson Bay, Canada and Prudhoe Bay, Alaska
Thermokarst lakes are an abundant feature in Arctic permafrost regions and cover up to 40 percent of the land area. During winter shallow lakes freeze to the bed (bedfast ice) while lakes which are deeper than the maximum ice thickness up to 2 m preserve perennial liquid water below the ice (floating ice). The different lake ice regimes have an impact on the energy distribution to the surrounding permafrost, available aquatic habitat and geomorphological processes. Completely frozen lakes contribute less energy and gas fluxes to the landscape and atmosphere while floating ice conditions support the development of a talik, a continuously unfrozen layer, as the remaining liquid water provides energy to the surrounding permafrost. This has an impact on permafrost thawing and geomorphological development as taliks can favour subsurface lake drainage, permafrost degradation and lateral lake erosion. Bedfast or floating ice conditions are dependant on the maximum ice thickness. Ice growth is determined by winter temperatures and snow conditions as a thicker snow cover provides insulation and reduce ice growth. In this study Sentinel 1 synthetic aperture radar (SAR) data for four winters from 2015 to 2018 was used to investigate thermokarst lakes and compare lake ice regimes in two study areas with permafrost conditions. One is in the area of Prudhoe Bay, North Slope Borough, Alaska and the other on the west shore of Hudson Bay near Churchill, Manitoba, Canada. Synthetic aperture radar remote sensing allows to distinguish between bedfast and floating ice due to different backscatter intensities. While bedfast ice absorbs the radar signal and appears dark on the radar image, floating ice shows a strong reflectance and appears bright. This is due to differences in the dielectric contrast between ice and sediment (lake bed) and ice and liquid water, respectively.
Additionally the maximum ice thickness was approximated by calculating ice growth based on freezing degree days from MODIS land surface temperature data. With the resulting ice growth curve the maximum water depth of lakes which freeze completely to the ground was determined through the date when they became bedfast. Bedfast lake ice percentages decreased over the study period in Prudhoe Bay while they varied widely in Churchill. The average proportions were similar for both study areas with 68 % in Prudhoe Bay and 62 % in Churchill. The lakes in Prudhoe Bay showed a trend towards floating ice regimes which was not detectable in Churchill. Relationships between winter temperatures and the amount of bedfast ice were not linear and indicate the presence of tipping points. Maximum ice thickness was estimated to be 160 cm in Prudhoe Bay which seems valid, while the similar ice thickness in Churchill is most likely overestimated by the used method. Future work in permafrost regions and the establishment of long term observations should help to understand trends more reliable and detect relationships between climate and resulting landscape response
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