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Deep learning estimation of northern hemisphere soil freeze-thaw dynamics using satellite multi-frequency microwave brightness temperature observations
Satellite microwave sensors are well suited for monitoring landscape freeze-thaw (FT) transitions owing to the strong brightness temperature (TB) or backscatter response to changes in liquid water abundance between predominantly frozen and thawed conditions. The FT retrieval is also a sensitive climate indicator with strong biophysical importance. However, retrieval algorithms can have difficulty distinguishing the FT status of soils from that of overlying features such as snow and vegetation, while variable land conditions can also degrade performance. Here, we applied a deep learning model using a multilayer convolutional neural network driven by AMSR2 and SMAP TB records, and trained on surface (~0â5 cm depth) soil temperature FT observations. Soil FT states were classified for the local morning (6 a.m.) and evening (6 p.m.) conditions corresponding to SMAP descending and ascending orbital overpasses, mapped to a 9 km polar grid spanning a five-year (2016â2020) record and Northern Hemisphere domain. Continuous variable estimates of the probability of frozen or thawed conditions were derived using a model cost function optimized against FT observational training data. Model results derived using combined multi-frequency (1.4, 18.7, 36.5 GHz) TBs produced the highest soil FT accuracy over other models derived using only single sensor or single frequency TB inputs. Moreover, SMAP L-band (1.4 GHz) TBs provided enhanced soil FT information and performance gain over model results derived using only AMSR2 TB inputs. The resulting soil FT classification showed favorable and consistent performance against soil FT observations from ERA5 reanalysis (mean percent accuracy, MPA: 92.7%) and in situ weather stations (MPA: 91.0%). The soil FT accuracy was generally consistent between morning and afternoon predictions and across different land covers and seasons. The model also showed better FT accuracy than ERA5 against regional weather station measurements (91.0% vs. 86.1% MPA). However, model confidence was lower in complex terrain where FT spatial heterogeneity was likely beneath the effective model grain size. Our results provide a high level of precision in mapping soil FT dynamics to improve understanding of complex seasonal transitions and their influence on ecological processes and climate feedbacks, with the potential to inform Earth system model predictions
Retrieving landscape freeze/thaw state fromSoil Moisture Active Passive (SMAP) radar and radiometer measurements
Over one-third of the global land area undergoes a seasonal transition between predominantly frozen and non-frozen conditions each year, with the land surface freeze/thaw (FT) state a significant control on hydrological and biospheric processes over northern land areas and at high elevations. The NASA Soil Moisture Active Passive (SMAP) mission produced a daily landscape FT product at 3-km spatial resolution derived from ascending and descending orbits of SMAP high-resolution L-band (1.4 GHz) radar measurements. Following the failure of the SMAP radar in July 2015, coarser (36-km) footprint SMAP radiometer inputs were used to develop an alternative daily passive microwave freeze/thaw product. In this study, in situ observations are used to examine differences in the sensitivity of the 3-km radar versus the 36-km radiometer measurements to the landscape freeze/thaw state during the period of overlapping instrument operation. Assessment of the retrievals at high-latitude SMAP core validation sites showed excellent agreement with in situ flags, exceeding the 80% SMAP mission accuracy requirement. Similar performance was found for the radar and radiometer products using both air temperature and soil temperature derived FT reference flags. There was a tendency for SMAP thaw retrievals to lead the surface flags due to the influence of wet snow cover conditions on both the radar and radiometer signal. Comparison with other satellite derived FT products showed those derived from passive measurements (SMAP radiometer; Aquarius radiometer; Advanced Microwave Scanning Radiometer - 2) retrieved less frozen area than the active products (SMAP radar; Aquarius radar)
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
Review Article: Global Monitoring of Snow Water Equivalent Using High-Frequency Radar Remote Sensing
Seasonal snow cover is the largest single component of the cryosphere in areal extent, covering an average of 46âĂâ106âkm2 of Earth\u27s surface (31â% of the land area) each year, and is thus an important expression and driver of the Earth\u27s climate. In recent years, Northern Hemisphere spring snow cover has been declining at about the same rate (âŒââ13â% per decade) as Arctic summer sea ice. More than one-sixth of the world\u27s population relies on seasonal snowpack and glaciers for a water supply that is likely to decrease this century. Snow is also a critical component of Earth\u27s cold regions\u27 ecosystems, in which wildlife, vegetation, and snow are strongly interconnected. Snow water equivalent (SWE) describes the quantity of water stored as snow on the land surface and is of fundamental importance to water, energy, and geochemical cycles. Quality global SWE estimates are lacking. Given the vast seasonal extent combined with the spatially variable nature of snow distribution at regional and local scales, surface observations are not able to provide sufficient SWE information. Satellite observations presently cannot provide SWE information at the spatial and temporal resolutions required to address science and high-socio-economic-value applications such as water resource management and streamflow forecasting. In this paper, we review the potential contribution of X- and Ku-band synthetic aperture radar (SAR) for global monitoring of SWE. SAR can image the surface during both day and night regardless of cloud cover, allowing high-frequency revisit at high spatial resolution as demonstrated by missions such as Sentinel-1. The physical basis for estimating SWE from X- and Ku-band radar measurements at local scales is volume scattering by millimeter-scale snow grains. Inference of global snow properties from SAR requires an interdisciplinary approach based on field observations of snow microstructure, physical snow modeling, electromagnetic theory, and retrieval strategies over a range of scales. New field measurement capabilities have enabled significant advances in understanding snow microstructure such as grain size, density, and layering. We describe radar interactions with snow-covered landscapes, the small but rapidly growing number of field datasets used to evaluate retrieval algorithms, the characterization of snowpack properties using radar measurements, and the refinement of retrieval algorithms via synergy with other microwave remote sensing approaches. This review serves to inform the broader snow research, monitoring, and application communities on progress made in recent decades and sets the stage for a new era in SWE remote sensing from SAR measurements
Caractérisation diélectrique micro-onde (1,4 GHz) des arbres et des sols
Le dĂ©couplage du signal dâĂ©mission micro-onde entre la vĂ©gĂ©tation et le sol demeure
une difficulté omniprésente pour toutes applications en télédétection. Pour améliorer les
produits micro-ondes globaux (e.g. humidité du sol, état de gel/dégel du sol) en milieu
forestier, une meilleure estimation de la permittivité électrique de la végétation et du
sol est requise. Dans le cadre de ce projet, un nouveau prototype de sonde coaxiale Ă
terminaison ouverte adaptée aux mesures sur le terrain a été développé. Nous montrons
dans ce travail que la sonde est apte à mesurer la permittivité électrique en bande L (1.4
GHz) de la végétation et du sol.
La sonde affiche des incertitudes maximales de 3,3% pour une large plage de valeurs
de permittivitĂ©. La permittivitĂ© complexe de sept espĂšces dâarbres diffĂ©rentes a Ă©tĂ© caractĂ©risĂ©e
dans des conditions de gel et de dégel. Les résultats montrent que la permittivité
Ă©lectrique du tronc des arbres est fortement corrĂ©lĂ©e avec lâĂ©tat de gel/dĂ©gel de la vĂ©gĂ©tation
et que cet état de gel/dégel de la végétation est sensible aux courts événements
de dégel hivernal. Il a aussi été démontré que les différences de permittivité électrique
interespĂšces sont importantes. La sonde coaxiale Ă terminaison ouverte sâest Ă©galement
rĂ©vĂ©lĂ©e suffisamment prĂ©cise pour capturer le cycle diurne de teneur en eau Ă lâintĂ©rieur
du tronc des arbres.
Les mesures de permittivité électrique de sols organiques en chambre froide mettent
en évidence une hystérésis importante entre le cycle de gel et de dégel du sol. Un tel phénomÚne
nâest pas considĂ©rĂ© dans les modĂšles de permittivitĂ© du sol actuel ni dans les
algorithmes de dĂ©tection du gel/dĂ©gel des sols. La sonde devrait permettre dâamĂ©liorer
la modĂ©lisation du transfert radiatif en milieu forestier et ainsi permettre dâamĂ©liorer les
produits satellitaires en bande L.The decoupling of the signal between vegetation and soil remains an omnipresent
difficulty for all remote sensing applications in the microwave spectrum. To improve global
microwave products (e.g. soil moisture, freeze/thaw soil state) in the forest environment,
a better estimate of the permittivity of vegetation and soil is required. As part of this
project, a new prototype of open-ended coaxial probe adapted for field measurements has
been developed.
The probe is designed to measure the L-band permittivity (1.4 GHz) of vegetation
and soil. The probe displays maximum uncertainties of 3.3% for a wide range of
permittivity values. The complex permittivity of seven different tree species was characterized
under freezing and thawing conditions. The results show that the permittivity
of tree trunks is strongly correlated with the freeze/thaw state of vegetation, the tree
freeze/thaw state is sensitive to short winter thawing events and the inter species differences
in permittivity are important. The open-ended coaxial probe is also precise enough
to capture the diurnal cycle of water content within the tree trunks.
The permittivity measurements of organic soils in cold chamber show a significant
hysteresis between the freezing and thawing cycles. Such phenomenon is not considered
in current soil permittivity models or in soil freeze/thaw detection algorithms. The probe
will allow to improve radiative transfer models in forest environment and thus improve
L-band satellite products
HUMAN AND CLIMATE IMPACTS ON FLOODING VIA REMOTE SENSING, BIG DATA ANALYTICS, AND MODELING
Over the last 20 years, the amount of streamflow has greatly increased and spring snowmelt floods have occurred more frequently in the north-central U.S. In the Red River of the North Basin (RRB) overlying portions of North Dakota and Minnesota, six of the 13 major floods over the past 100 years have occurred since the late 1990s. Based on numerous previous studies as well as senior flood forecastersâ experiences, recent hydrological changes related to human modifications [e.g. artificial subsurface drainage (SSD) expansion] and climate change are potential causes of notable forecasting failures over the past decade. My dissertation focuses on the operational and scientific gaps in current forecasting models and observational data and provides insights and value to both the practitioner and the research community. First, the current flood forecasting model needs both the location and installation timing of SSD and SSD physics. SSD maps were developed using satellite âbigâ data and a machine learning technique. Next, using the maps with a land surface model, the impacts of SSD expansion on regional hydrological changes were quantified. In combination with model physics, the inherent uncertainty in the airborne gamma snow survey observations hinders the accurate flood forecasting model. The operational airborne gamma snow water equivalent (SWE) measurements were improved by updating antecedent surface moisture conditions using satellite observations on soil moisture. From a long-term perspective, flood forecasters and state governments need knowledge of historical changes in snowpack and snowmelt to help flood management and to develop strategies to adapt to climate changes. However, historical snowmelt trends have not been quantified in the north-central U.S. due to the limited historical snow data. To overcome this, the current available historical long-term SWE products were evaluated across diverse regions and conditions. Using the most reliable SWE product, a trend analysis quantified the magnitude of change extreme snowpack and melt events over the past 36 years. Collectively, this body of research demonstrates that human and climate impacts, as well as limited and noisy data, cause uncertainties in flood prediction in the great plains, but integrated approaches using remote sensing, big data analytics, and modeling can quantify the hydrological changes and reduce the uncertainties. This dissertation improves the practice of flood forecasting in Red River of the North Basin and advances research in hydrology and snow science
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
AmĂ©lioration de la caractĂ©risation de la neige et du sol arctique afin dâamĂ©liorer la prĂ©diction de lâĂ©quivalent en eau de la neige en tĂ©lĂ©dĂ©tection micro-ondes
Le phĂ©nomĂšne de lâamplification arctique consiste en une augmentation plus prononcĂ©e des tempĂ©ratures de surface dans cette rĂ©gion que sur le reste du globe. Ce phĂ©nomĂšne est notamment dĂ» Ă la diminution marquĂ©e du couvert nival provoquant un dĂ©sĂ©quilibre dans le bilan dâĂ©nergie de surface via une rĂ©duction gĂ©nĂ©ralisĂ©e de lâalbĂ©do (rĂ©troaction positive). LâaccĂ©lĂ©ration du rĂ©chauffement est jusquâĂ trois fois plus Ă©levĂ©e dans ces rĂ©gions. Il est donc primordial, dans un contexte de changement climatique arctique, de poursuivre et dâamĂ©liorer le suivi Ă grande Ă©chelle du couvert nival afin de mieux comprendre les processus gouvernant la variabilitĂ© spatio-temporelle du manteau neigeux. Plus spĂ©cifiquement, lâĂquivalent en Eau de la Neige (EEN) est gĂ©nĂ©ralement utilisĂ© pour quantifier deux propriĂ©tĂ©s (hauteur et densitĂ©) de la neige. Son estimation Ă grande Ă©chelle dans les rĂ©gions Ă©loignĂ©es tel que lâArctique provient actuellement essentiellement de produits en micro-ondes passives satellitaires. Cependant, il existe encore beaucoup dâincertitudes sur les techniques dâassimilation de lâĂEN par satellite et ce projet vise une rĂ©duction de lâerreur liĂ©e Ă lâestimation de lâĂEN en explorant deux des principales sources de biais tels que : 1) la variabilitĂ© spatiale de lâĂ©paisseur et des diffĂ©rentes couches du manteau neigeux arctique liĂ©es Ă la topographie et la vĂ©gĂ©tation au sol influençant lâestimation de lâĂEN; et 2) les modĂšles de transfert radiatif micro-ondes de la neige et du sol ne bĂ©nĂ©ficient pas actuellement dâune bonne paramĂ©trisation en conditions arctiques, lĂ oĂč les erreurs liĂ©es Ă lâassimilation de lâĂEN sont les plus importantes. Lâobjectif global est donc dâanalyser les propriĂ©tĂ©s gĂ©ophysiques du couvert nival en utilisant des outils de tĂ©lĂ©dĂ©tection et de modĂ©lisation pour diminuer lâerreur liĂ©e Ă la variabilitĂ© spatiale locale dans lâestimation du ĂEN Ă grande Ă©chelle, tout en amĂ©liorant la comprĂ©hension des processus locaux qui affectent cette variabilitĂ©. PremiĂšrement, une analyse haute rĂ©solution Ă lâaide de lâalgorithme Random Forest a permis de prĂ©dire la hauteur de neige Ă une rĂ©solution spatiale de 10 m avec une RMSE de 8 cm (23%) et dâen apprendre davantage sur les processus de distribution de la neige en Arctique. DeuxiĂšmement, la variabilitĂ© du manteaux neigeux arctique (hauteur et microstructure) a Ă©tĂ© incorporĂ©e dans des simulations en transfert radiatif micro-ondes de la neige et comparĂ©e au capteur satellitaire SSMIS. Lâajout de variabilitĂ© amĂ©liore la RMSE des simulations de 8K par rapport Ă un manteau neigeux uniforme. Finalement, une paramĂ©trisation du sol gelĂ© est prĂ©sentĂ©e Ă lâaide de mesures de rugositĂ© provenant de photogrammĂ©trie (Structure-from-Motion). Cela a permis dâinvestiguer trois modĂšles de rĂ©flectivitĂ© micro-ondes du sol ainsi que la permittivitĂ© effective du sol gelĂ© avec une rugositĂ© SfM dâune prĂ©cision de 0.1 mm. Ces donnĂ©es de rugositĂ© SfM avec une permittivitĂ© optimisĂ©e (Δ'_19 = 3.3, Δ'_37 = 3.6) rĂ©duisent significativement lâerreur des tempĂ©ratures de brillance simulĂ©es par rapport Ă des mesures au sols (RMSE = 3.1K, R^2 = 0.71) pour toutes les frĂ©quences et polarisations. Cette thĂšse offre une caractĂ©risation des variables de surface (neige et sol) en Arctique en transfert radiatif micro-ondes qui bĂ©nĂ©ficie aux multiples modĂ©lisations (climatiques et hydrologiques) de la cryosphĂšre
Improving estimates of net ecosystem CO2 exchange between the Arctic land surface and the atmosphere
Feedbacks between the climate system and the high-latitude carbon
cycle will substantially influence the intensity
of future climate change. It is therefore crucial that the net ecosystem
exchange of CO2 (NEE) between the high-latitude land surface and the
atmosphere is accurately quantified, where NEE refers to the difference
between ecosystem respiration (R) and photosynthesis (gross ecosystem
exchange, GEE): NEE=-GEE+R in umol/m^2/s. NEE can only be directly
measured over areas of 1 km^2 through eddy covariance, and modeling
approaches such as the Vegetation Photosynthesis Respiration Model (VPRM) are
required to upscale NEE. VPRM
is a remote
sensing based model that calculates R as a linear function of air
temperature (Ta) when air
temperature is above a given threshold (Tlow), and sets respiration to a
constant
value when Ta<Tlow. GEE is estimated according to remote sensing
observations of vegetation indices, shortwave radiation, air temperature, and
soil moisture. Although in situ findings have shown
that snow and Arctic species composition have a
substantial
influence on high-latitude NEE, model estimates of high-latitude NEE have
typically been generated without Arctic-specific vegetation classes, and
without using remote sensing observations to represent
the effects of snow on NEE. The hypothesis driving this
work was therefore that uncertainty in estimates of high-latitude NEE could
be reduced by representing the influences of Arctic
vegetation classes and snow. The central objectives were
to determine feasible approaches for reducing uncertainty in VPRM estimates
of NEE by representing the influences of snow and Arctic vegetation,
create PolarVPRM accordingly, and analyze inter-annual variability in PolarVPRM
estimates of high-latitude North American NEE (2001-2012).
The associations between snow and NEE, and the potential to describe
these influences on NEE using remote sensing observations, were
examined using time lapse camera observations of snow cover area (SCA) and eddy
covariance measurements of NEE from Daring Lake, Northwest Territories,
Canada. Analyses indicated
good agreement between SCA derived from camera, Landsat and Moderate Resolution
Imaging Spectroradiometer (MODIS) observations. SCA was also found to influence
the timing and magnitude of NEE. MODIS SCA was therefore incorporated into VPRM,
and VPRM was calibrated using eddy covariance and meteorological observations
collected in
2005 at Daring Lake. VPRM was run through years
2004-2007 over both Daring Lake and Ivotuk, Alaska, USA, using four model
formulations, three of which represented the effects of SCA on respiration
and/or photosynthesis, and another which did not use MODIS SCA. Comparisons
against eddy covariance observations indicated that uncertainty was reduced in
VPRM estimates of NEE when respiration was calculated as a linear function of
soil temperature when
SCA>50%, and as a linear function of air temperature when SCA<50%,
thereby reflecting the influence of snow on decoupling soil/air temperatures.
Representing the effect of SCA on NEE therefore reduced uncertainty in VPRM
estimates of NEE.
In order to represent spatial variability in high-latitude
estimates of NEE due to vegetation type, Arctic-specific vegetation classes were
created for PolarVPRM by combining
and aggregating two existing vegetation classifications: the Synergetic Land
Cover Product and the Circumpolar Arctic Vegetation Map. Levene's test
indicated that the PolarVPRM vegetation classes divided the pan-Arctic
region into
heterogeneous distributions
in terms of net primary productivity, and passive microwave derived
estimates of snow and growing season influences on NEE. A
non-parametric statistical approach of Alternating Conditional Expectations
found significant, non-linear associations to exist between passive microwave
derived estimates of snow and growing season drivers of NEE. Furthermore,
the shape of these associations varied according to the vegetation class over
which they were examined. Further support was therefore provided to the idea
that uncertainty in model estimates of NEE could be reduced by calculating snow
and growing season NEE separately within each vegetation class.
PolarVPRM estimates of NEE in 2001-2012 were
generated at
a three hourly and 1/6 x 1/4 degree resolution across
polar North
America (55-170 W, 55-83 N). Model
calibration was conducted over three sites: Daring Lake, Ivotuk, and Atqasuk,
Alaska, USA. Model validation was then conducted by comparing PolarVPRM
estimates of year-round daily average NEE
to non-gap-filled eddy covariance observations of daily average NEE acquired
over the three calibration sites, as well as six other Arctic sites.
PolarVPRM performed well over all sites, with an average mean absolute
error (MAE) of 0.20 umol/m^2/s, and had
diminished
error rates when the influence of SCA on
respiration was explicitly represented. Error
analysis indicated that peak growing season GEE was underestimated at Barrow
because GEE at this site showed a stronger response to the amount
of incoming shortwave radiation than at the calibration site, suggesting
that PolarVPRM may underestimate GEE over wetland and barren vegetated
regions. Despite these uncertainties, PolarVPRM was found to generate more
accurate estimates of monthly and three-hourly NEE relative to eddy covariance
observations than two established models, FLUXNET Model-Tree Ensemble (MTE) and CarbonTracker.
Relative to eddy covariance observations and PolarVPRM estimates, MTE
tended to overestimate snow season respiration, and CarbonTracker tended to
overestimate the amount of midday photosynthesis. Analysis of PolarVPRM output
across North America (north of 55 N) found an increase in net annual carbon
efflux over over time (2001-2012). Specifically, increased rates of respiration
are estimated when soil and air temperatures are warmer. Although
increases in growing season vegetation indices and air temperature enable
greater
photosynthetic uptake by Arctic vegetation, forests and shrublands
uptake less CO2 in the middle of the growing season when air temperatures rise
above the physiological optima for photosynthesis. As a result, PolarVPRM
estimated a decline in net photosynthetic uptake over time. Overall, PolarVPRM
output indicates that North American regions north of 55 N are
losing strength as a carbon sink in response to rising air temperatures.1 yea