The Experts below are selected from a list of 7968 Experts worldwide ranked by ideXlab platform
Miska Luoto - One of the best experts on this subject based on the ideXlab platform.
-
Evaluating sources of uncertainty in modelling the impact of probabilistic climate change on sub-arctic palsa mires
Natural Hazards and Earth System Sciences, 2011Co-Authors: Stefan Fronzek, Timothy R. Carter, Miska LuotoAbstract:Abstract. We present an analysis of different sources of impact model uncertainty and combine this with probabilistic projections of climate change. Climatic envelope models describing the spatial distribution of palsa mires (mire complexes with permafrost peat hummocks) in northern Fennoscandia were calibrated for three baseline periods, eight state-of-the-art modelling techniques and 25 versions sampling the parameter uncertainty of each technique – a total of 600 models. The sensitivity of these models to changes in temperature and precipitation was analysed to construct impact response surfaces. These were used to assess the behaviour of models when extrapolated into changed climate conditions, so that new criteria, in addition to conventional model evaluation statistics, could be defined for determining model reliability. Impact response surfaces were also combined with climate change projections to estimate the risk of areas suitable for palsas disappearing during the 21st century. Structural differences in impact models appeared to be a major source of uncertainty, with 60% of the models giving implausible projections. Generalized additive modelling (GAM) was judged to be the most reliable technique for model extrapolation. Using GAM, it was estimated as very likely (>90% probability) that the area suitable for palsas is reduced to less than half the baseline area by the period 2030–2049 and as likely (>66% probability) that the entire area becomes unsuitable by 2080–2099 (A1B emission scenario). The risk of total loss of palsa area was reduced for a mitigation scenario under which global warming was constrained to below 2 °C relative to pre-industrial climate, although it too implied a considerable reduction in area suitable for palsas.
-
Applying probabilistic projections of climate change with impact models: a case study for sub-arctic palsa mires in Fennoscandia
Climatic Change, 2009Co-Authors: Stefan Fronzek, Timothy R. Carter, Jouni Räisänen, Leena Ruokolainen, Miska LuotoAbstract:A comparison of two approaches for determining probabilistic climate change impacts is presented. In the first approach, ensemble climate projections are applied directly as inputs to an impact model and the risk of impact is computed from the resulting ensemble of outcomes. As this can involve large numbers of projections, the approach may prove to be impractical when applied to complex impact models with demanding input requirements. The second approach is to construct an impact response surface based on a sensitivity analysis of the impact model with respect to changes in key climatic variables, and then to superimpose probabilistic projections of future climate onto the response surface to assess the risk of impact. To illustrate this comparison, an impact model describing the spatial distribution of palsas in Fennoscandia was applied to estimate the risk of palsa disappearance. Palsas are northern mire complexes with permanently frozen peat hummocks, located at the outer limit of the permafrost zone and susceptible to rapid decline due to regional warming. Probabilities of climate changes were derived from an ensemble of coupled atmosphere–ocean general circulation model (AOGCM) projections using a re-sampling method. Results indicated that the response surface approach, though introducing additional uncertainty, gave risk estimates of area decline for palsa suitability that were comparable to those obtained using multiple simulations with the original palsa model. It was estimated as very likely (>90% probability) that a decline of area suitable for palsas to less than half of the baseline distribution will occur by the 2030s and likely (>66%) that all suitable areas will disappear by the end of the twenty-first century under scenarios of medium (A1B) and moderately high (A2) emissions. For a low emissions (B1) scenario, it was more likely than not (>50%) that conditions over a small fraction of the current palsa distribution would remain suitable until the end of the twenty-first century.
-
Potential effect of climate change on the distribution of palsa mires in subarctic Fennoscandia
Climate Research, 2006Co-Authors: Stefan Fronzek, Miska Luoto, Timothy R. CarterAbstract:Palsa mires are northern mire complexes with permanently frozen peat hummocks, located at the outer limit of the permafrost zone. Palsa mires have high conservation status, being characterized by a rich diversity of bird species and unique geomorphological processes. They are currently degrading throughout their distributional range, probably because of regional climatic warming. Distributions of palsas in Fennoscandia were modelled using 5 climate envelope tech- niques (generalized linear modelling, generalized additive modelling, classification tree analysis, artificial neural networks and multiple adaptive regression splines). The models were studied with respect to their sensitivity to altered climate. Climate change scenarios were applied to assess possi- ble impacts on the palsa distribution during the 21st century. The models achieved a good to very good agreement with the observed palsa distribution and thus suggest a strong dependency on cli- mate. Even small increases of temperature (1°C) and precipitation (10%) resulted in considerable losses of areas suitable for palsa development. Of the 5 models tested, 3 predicted the total disappear- ance of regions suitable for palsa development with an increased mean annual temperature of 4°C. Under climate change scenarios based on 7 Atmosphere-Ocean General Circulation Models (AOGCMs) the models indicated that the degradation of palsas will proceed very quickly. All but one climate scenario resulted in the total disappearance of suitable regions for palsa development by the end of the 21st century. Potential impacts of degrading palsa mires on biodiversity and carbon balance are the loss of habitat for migrating bird species and the increase in CH4 emissions.
-
Loss of palsa mires in Europe and biological consequences
Environmental Conservation, 2004Co-Authors: Miska Luoto, Risto K. Heikkinen, Timothy R. CarterAbstract:Palsa mires are northern mire complexes with permanently frozen peat hummocks. These are degrading throughout their distribution range, probably because of regional climatic warming. This review of the current understanding of the geographical, climatic and biological characteristics of palsa mires focuses on Europe. Recent studies have reported a drastic decrease in the extent of palsa mires in Fennoscandia; in Finland, the distribution of palsas was formerly about three times that at present. With continued or accelerated warming, as predicted for high latitudes, further extensive degradation or the wholesale disappearance of palsa mires seems inevitable. Palsa mires are known to be biologically heterogeneous environments with a rich diversity of bird species, and they are listed as a priority habitat type by the European Union. However, their role as habitats for other organisms is still poorly understood. There is urgent need for research and monitoring to assess the ecological and biological consequences of the decline of palsa mires in Europe.
-
Thermokarst ponds as indicators of the former distribution of palsas in Finnish Lapland
Permafrost and Periglacial Processes, 2003Co-Authors: Miska Luoto, Matti SeppäläAbstract:Thermokarst ponds resulting from thawing of palsas were mapped in a 95 km-long transect area in northern Finland. The spatial distribution of thermokarst was related to palsa distribution and to geographical variables using GIS techniques and multivariate spatial modelling. In our 3370 km2 study area, the former distribution of palsas was about three times larger than the present one. This indicates that the formation and thawing of permafrost are not in balance; palsas are collapsing and melting more often than palsas are forming. Copyright © 2003 John Wiley & Sons, Ltd.
Matti Seppälä - One of the best experts on this subject based on the ideXlab platform.
-
36 4TH CAN. PERMAFROST CONF. (1982) An experimental study of the formation of palsas
2015Co-Authors: Matti SeppäläAbstract:A hypothesis of palsa formation has been tested experimentally in the field in northern Finland. Snow layers were deliberately removed from a peat bog surface several times during three winters. This caused the seasonally frozen layer to almost double in thickness below the surface of the experimental square, and part of this frost then survived through the summer. The cyclic development of palsas is discussed and a process of palsa formation exemplified. It was shown that the thickness of the snow-cover is the main factor controlling palsa formation in subarctic conditions. L'auteur a teste experimentalement in situ une hypothtse de formation de palses dam le Nord de la Finlande. A plusieurs reprises au cours de trois hivers successifs, on a enlevt la neige de la surface d'une tourbihe. I1 en est resulte un doublement de I'tpaisseur du gelisol sous la parcelle experimentale et une partie de celui-ci a persistk tout I'ete. Ceci permet a l'auteur de discuter le cycle de developpe-ment des palses et d'illustrer un processus de leur formation. I1 apparait que I'epaisseur du couvert nival est le principal facteur contrBlant la formation des palses en milieu subarctique. Proc. 4th Can. Permafrost Conf. (1982
-
Synthesis of studies of palsa formation underlining the importance of local environmental and physical characteristics
Quaternary Research, 2011Co-Authors: Matti SeppäläAbstract:This review presents a synthesis of four decades of palsa studies based on field experiments and observations mainly in Fennoscandia, as well as laboratory measurements. Palsas are peat-covered mounds with a permanently frozen core; in Finnish Lapland, they range from 0.5 to 7 m in height and from 2 to 150 m in diameter. These small landforms are characteristic of the southern margin of the discontinuous permafrost zone. Palsa formation requires certain environmental conditions: long-lasting air temperature below 0°C, thin snow cover, and low summer precipitation. The development and persistence of their frozen core is sensitive to the physical properties of peat. The thermal conductivity of wet and frozen peat is high, and it decreases significantly as the peat dries and thaws. This affects the development of the active layer and makes its response to climate change complex. The insulating properties of dry peat during hot and dry summers moderate the thawing of the active layer on palsas. In contrast, humid and wet weather during the summer causes deep thawing and may destroy the frozen core of palsas. Ice layers in palsas have previously been interpreted as ice segregation features but because peat is not frost-susceptible, the ice layers are now reinterpreted as resulting from ice growth at the base of a frozen core that is effectively floating in a mire.
-
Physical properties of peat and palsa formation
Cold Regions Science and Technology, 2008Co-Authors: K. Kujala, Matti Seppälä, T. HolappaAbstract:Abstract This work considers the physical properties of peat and their effects on the formation of palsas under cold environmental conditions. Peat samples were taken in winter from the surface of a palsa in Finnish Lapland and from its frozen core and the material characteristics were determined. The thermal conductivity of the peat samples, measured with a thermal needle probe, varied between 0.23 and 0.28 W/mK with natural water content and between 0.43 and 0.67 W/mK in frozen peat. The thermal conductivity of saturated peat samples ranged from 0.41 to 0.50 W/mK and after freezing from 1.48 to 1.49 W/mK. Unfrozen water content in frozen palsa sample was measured by the TDR method. Water in the studied peat freezes at temperatures of 0 to − 0.8 °C, which is considerably a higher temperature range than in high frost-susceptible soils like silts. The frost susceptibility of the peat was measured in the laboratory with a frost heave test, but no frost heave was observed, which means that these peat forming palsas have no potential to form segregated ice lenses. The formation of a palsa is based on the thermal properties of peat.
-
palsa mires in finland
2006Co-Authors: Matti SeppäläAbstract:The term palsa was originally used by the Sami people and Finns, and in their languages it means a large peat hummock with a frozen core, rising above the surface of a mire (Lundqvist 1969, Seppala 1972, Nelson & al. 1992, Gurney 2001). Palsas are characteristic to the discontinuous circumpolar permafrost zone (Seppala 1997) provided that the peat layer is thick enough. They contain a permanently frozen core of peat and/or silt, small ice crystals and thin layers of segregated ice, which can survive the heat of summers. An insulating peat layer is important for preserving the frozen core during the summer. The peat should be dry during the summer, thus having a very low thermal conductivity, and wet in autumn, when freezing starts, giving a much higher thermal conductivity. This allows the cold to penetrate so deep into the peat layers that they do not thaw during the summer. In western Finnish Lapland palsas are located north of 68o30’N latitude and in eastern Lapland north of Lake Inari (Fig. 1). Palsas are found in valleys with an insulating peat layer suffi ciently thick to preserve the frozen core. The vertical distribution of palsas in northern Finnish Lapland varied from altitudes of 180 m to 390 m a.s.l. (Luoto & Seppala 2002). On lower altitudes there is probably too much snow and on higher levels peat layers are too thin.
-
Thermokarst ponds as indicators of the former distribution of palsas in Finnish Lapland
Permafrost and Periglacial Processes, 2003Co-Authors: Miska Luoto, Matti SeppäläAbstract:Thermokarst ponds resulting from thawing of palsas were mapped in a 95 km-long transect area in northern Finland. The spatial distribution of thermokarst was related to palsa distribution and to geographical variables using GIS techniques and multivariate spatial modelling. In our 3370 km2 study area, the former distribution of palsas was about three times larger than the present one. This indicates that the formation and thawing of permafrost are not in balance; palsas are collapsing and melting more often than palsas are forming. Copyright © 2003 John Wiley & Sons, Ltd.
Masanobu Shimada - One of the best experts on this subject based on the ideXlab platform.
-
assessment of PALSAR 2 compact calibration
International Geoscience and Remote Sensing Symposium, 2018Co-Authors: R Touzi, Masanobu Shimada, Takeshi Motohka, S NedelcuAbstract:ALOS-2-PALSAR is equipped with an experimental Compact mode (CP), which can collect CP images at various incidence angles. PALSAR2-CP transmits a circular polarization and measures the received signal at the horizontal and vertical antenna polarization. Recently, it was brought out that the actual SAR technology does not permit the generation of a perfectly circular polarization and this may significantly affect CP information [1]. Since all the calibration methods assume that the transmitted polarization is perfectly circular, there is an immediate need for a new CP calibration model that is more suitable to actual CP SARs. In this study, a new CP calibration model that takes into account the non circularity of transmitted polarization, in addition to conventional SAR transmitter and receiver distortion matrices, is introduced. The CP calibration model is used as the basis for the development of an efficient method that permits the measurement of transmitted polarization axis ratio, and its variations with incidence angle. The method is validated using PALSAR2-CP data collected at various incidence angles on the Amazonian forests. It is shown that the transmitted polarization axis ratio varies with PALSAR2-CP beam, and can reach up to 1.1 dB for the CP mode (H2-8) (at about 37° incidence angle).
-
model based polarimetric sar calibration method using forest and surface scattering targets
International Geoscience and Remote Sensing Symposium, 2011Co-Authors: Masanobu ShimadaAbstract:This paper proposes a new polarimetric Synthetic Aperture Radar (PolSAR) calibration method that applies an incoherent decomposition model to the uncalibrated covariance data measured for the forest and surface, and determines the polarimetric distortion matrix (PDM). The Freeman-Durden model [1] is used to express the polarization dependent signal reflection from and penetration through the forest. Non-linear equations built for uncalibrated PolSAR data are solved iteratively. This method is applicable to the lower frequency SAR that associates with the polarization dependent signal penetration through forest canopies. Using the time series Phased Array Type L-band SAR (PALSAR) data acquired for the Amazon rainforest for around 3 years, we confirm that the proposed method succeeds the PDM estimation and that the calibrated data preserve the polarimetric performance on HH-VV orthogonality, low cross-talks, and ideal polarimetric signature for the corner reflector.
-
model based polarimetric sar calibration method using forest and surface scattering targets
IEEE Transactions on Geoscience and Remote Sensing, 2011Co-Authors: Masanobu ShimadaAbstract:This paper proposes a new polarimetric synthetic aperture radar (SAR) (PolSAR) calibration method that applies an incoherent decomposition model to the uncalibrated covariance data measured for the forest and surface and determines the polarimetric distortion matrix (PDM). The Freeman-Durden model is used to express the polarization-dependent signal reflection from and penetration through the forest. Nonlinear equations built for uncalibrated PolSAR data are solved iteratively. This method is applicable to the lower frequency SAR that associates with the polarization-dependent signal penetration through forest canopies. Using the time series Phased-Array-Type L-band SAR (PALSAR) data acquired from the Amazon rainforest for around three years, we confirm that the proposed method succeeds in the PDM estimation and that the calibrated data preserve the polarimetric performance on HH-VV orthogonality, low crosstalks, and ideal polarimetric signature for the corner reflector. This paper also investigates the signal-penetration properties of the forest associated with the L-band SAR.
-
IGARSS - Relationships between PALSAR backscattering data and forest above ground biomass in Japan
2011 IEEE International Geoscience and Remote Sensing Symposium, 2011Co-Authors: Takeshi Motohka, Masanobu Shimada, Osamu Isoguchi, Masae I. Ishihara, Satoshi N. SuzukiAbstract:We investigated the relationship between HH and HV gamma naught values by PALSAR and biomass of Japanese natural forest (44 sites) to develop an effective retrieval algorithm of forest above ground biomass by using PALSAR data. As same as previous studies, our results showed a positive logarithmic relationship between PALSAR gamma naught and in situ forest biomass. HV polarized data showed smaller RMSE and higher saturation level than HH data. The results also showed that slope corrected PALSAR data had much smaller RMSE than the non-corrected data.
-
IGARSS (3) - PALSAR CALVAL updated 2009 and change detections at the forest and the Polar regions
2009 IEEE International Geoscience and Remote Sensing Symposium, 2009Co-Authors: Masanobu Shimada, Osamu Isoguchi, Takeo TadonoAbstract:This paper describes the updated results of the PALSAR CALVAL activity conducted for last three years using the PALSAR calibration archives and the connected ground truth data. The results show that PALSAR has pretty good stabilities on the geometric and radiometric accuracies. Using PALSAR data, change detection on the forest in Indonesia and the Polar region (north pole region, Antarctica region, and Greenland region) are being in progress tracking the change of the sigma-naught and/or the phases.
Oleg Antropov - One of the best experts on this subject based on the ideXlab platform.
-
land cover and soil type mapping from spaceborne polsar data at l band with probabilistic neural network
IEEE Transactions on Geoscience and Remote Sensing, 2014Co-Authors: Oleg Antropov, Yrjo Rauste, Heikki Astola, Jaan Praks, Tuomas Hame, M HallikainenAbstract:This paper evaluates performance of fully polarimetric SAR (PolSAR) data in several land cover mapping studies in the boreal forest environment, taking advantage of the high canopy penetration capability at L-band. The studies included multiclass land cover mapping, forest-nonforest delineation, and classification of soil type under vegetation. PolSAR data used in the study were collected by the ALOS PALSAR sensor in 2006-2007 over a managed boreal forest site in Finland. A supervised classification approach using selected polarimetric features in the framework of probabilistic neural network (PNN) was adopted in the study. It has no assumptions about statistics of the polarimetric features, using nonparametric estimation of probability distribution functions instead. The PNN-based method improved classification accuracy compared with standard maximum-likelihood approach. The improvement was considerably strong for soil type mapping under vegetation, indicating notable non-Gaussian effects in the PolSAR data even at L-band. The classification performance was strongly dependent on seasonal conditions. The PolSAR feature data set was further modified to include a number of recently proposed polarimetric parameters (surface scattering fraction and scattering diversity), reducing the computational complexity at practically no loss in the classification accuracy. The best obtained accuracies of up to 82.6% in five-class land cover mapping and more than 90% in forest-nonforest mapping in wall-to-wall validation indicate suitability of PolSAR data for wide-area land cover and forest mapping.
-
Volume Scattering Modeling in PolSAR Decompositions: Study of ALOS PALSAR Data Over Boreal Forest
IEEE Transactions on Geoscience and Remote Sensing, 2011Co-Authors: Oleg Antropov, Yrjo Rauste, Tuomas HameAbstract:Model-based approaches for decomposing polarimetric backscatter data from boreal forest are discussed in this paper. Several model-based decompositions are analyzed with respect for the most accurate estimation of the volume scattering component. A novel generalized model for description of the volume contribution is proposed when observed backscatter from forest indicates that media does not follow azimuthal symmetry case. The model can be adjusted to the polarimetric synthetic aperture radar (PolSAR) data itself, taking into consideration higher sensitivity of HH against VV backscattering term to the presence of canopy at L-band. The model is general enough to allow a broad range of canopies to be modeled and is shown to comply with several earlier proposed volume scattering mechanism models. It is afterward incorporated in the Freeman-Durden three-component decomposition, yielding an improved modification. The performance of the proposed modification is evaluated using multitemporal ALOS PALSAR data acquired over Kuortane area in central Finland, representing typical mixed boreal forestland. Several decompositions are also benchmarked in order to see how they satisfy physical requirements when decomposing covariance matrix into a weighted sum of individual scattering mechanism contributions. When using experimental data, the proposed decomposition is shown to better satisfy non-negativity constraints for the covariance matrix eigenvalues at each decomposition step with less additional PolSAR data averaging needed. Discussed decompositions are also evaluated for the accuracy of initial stratification based on dominating scattering mechanism using ground reference data.
Tuomas Hame - One of the best experts on this subject based on the ideXlab platform.
-
land cover and soil type mapping from spaceborne polsar data at l band with probabilistic neural network
IEEE Transactions on Geoscience and Remote Sensing, 2014Co-Authors: Oleg Antropov, Yrjo Rauste, Heikki Astola, Jaan Praks, Tuomas Hame, M HallikainenAbstract:This paper evaluates performance of fully polarimetric SAR (PolSAR) data in several land cover mapping studies in the boreal forest environment, taking advantage of the high canopy penetration capability at L-band. The studies included multiclass land cover mapping, forest-nonforest delineation, and classification of soil type under vegetation. PolSAR data used in the study were collected by the ALOS PALSAR sensor in 2006-2007 over a managed boreal forest site in Finland. A supervised classification approach using selected polarimetric features in the framework of probabilistic neural network (PNN) was adopted in the study. It has no assumptions about statistics of the polarimetric features, using nonparametric estimation of probability distribution functions instead. The PNN-based method improved classification accuracy compared with standard maximum-likelihood approach. The improvement was considerably strong for soil type mapping under vegetation, indicating notable non-Gaussian effects in the PolSAR data even at L-band. The classification performance was strongly dependent on seasonal conditions. The PolSAR feature data set was further modified to include a number of recently proposed polarimetric parameters (surface scattering fraction and scattering diversity), reducing the computational complexity at practically no loss in the classification accuracy. The best obtained accuracies of up to 82.6% in five-class land cover mapping and more than 90% in forest-nonforest mapping in wall-to-wall validation indicate suitability of PolSAR data for wide-area land cover and forest mapping.
-
Volume Scattering Modeling in PolSAR Decompositions: Study of ALOS PALSAR Data Over Boreal Forest
IEEE Transactions on Geoscience and Remote Sensing, 2011Co-Authors: Oleg Antropov, Yrjo Rauste, Tuomas HameAbstract:Model-based approaches for decomposing polarimetric backscatter data from boreal forest are discussed in this paper. Several model-based decompositions are analyzed with respect for the most accurate estimation of the volume scattering component. A novel generalized model for description of the volume contribution is proposed when observed backscatter from forest indicates that media does not follow azimuthal symmetry case. The model can be adjusted to the polarimetric synthetic aperture radar (PolSAR) data itself, taking into consideration higher sensitivity of HH against VV backscattering term to the presence of canopy at L-band. The model is general enough to allow a broad range of canopies to be modeled and is shown to comply with several earlier proposed volume scattering mechanism models. It is afterward incorporated in the Freeman-Durden three-component decomposition, yielding an improved modification. The performance of the proposed modification is evaluated using multitemporal ALOS PALSAR data acquired over Kuortane area in central Finland, representing typical mixed boreal forestland. Several decompositions are also benchmarked in order to see how they satisfy physical requirements when decomposing covariance matrix into a weighted sum of individual scattering mechanism contributions. When using experimental data, the proposed decomposition is shown to better satisfy non-negativity constraints for the covariance matrix eigenvalues at each decomposition step with less additional PolSAR data averaging needed. Discussed decompositions are also evaluated for the accuracy of initial stratification based on dominating scattering mechanism using ground reference data.
