Mapping potential signs of gas emissions in ice of Lake Neyto, Yamal, Russia, using synthetic aperture radar and multispectral remote sensing data

International audienceRegions of anomalously low backscatter in C-band synthetic aperture radar (SAR) imagery of lake ice of Lake Neyto in northwestern Siberia have been suggested to be caused by emissions of gas (methane from hydrocarbon reservoirs) through the lake's sediments. However, to assess this connection, only analyses of data from boreholes in the vicinity of Lake Neyto and visual comparisons to medium-resolution optical imagery have been provided due to a lack of in situ observations of the lake ice itself. These observations are impeded due to accessibility and safety issues. Geospatial analyses and innovative combinations of satellite data sources are therefore proposed to advance our understanding of this phenomenon. In this study, we assess the nature of the backscatter anomalies in Sentinel-1 C-band SAR images in combination with very high resolution (VHR) WorldView-2 optical imagery. We present methods to automatically map backscatter anomaly regions from the C-band SAR data (40 m pixel spacing) and holes in lake ice from the VHR data (0.5 m pixel spacing) and examine their spatial relationships. The reliability of the SAR method is evaluated through comparison between different acquisition modes. The results show that the majority of mapped holes (71 %) in the VHR data are clearly related to anomalies in SAR imagery acquired a few days earlier, and similarities to SAR imagery acquired more than a month before are evident, supporting the hypothesis that anomalies may be related to gas emissions. Further, a significant expansion of backscatter anomaly regions in spring is documented and quantified in all analysed years 2015 to 2019. Our study suggests that the backscatter anomalies might be caused by lake ice subsidence and consequent flooding through the holes over the ice top leading to wetting and/or slushing of the snow around the holes, which might also explain outcomes of polarimetric analyses of auxiliary L-band Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar-2 (PALSAR-2) data. C-band SAR data are considered to be valuable for the identification of lakes showing similar phenomena across larger areas in the Arctic in future studies

Spatiotemporal structure of Baltic free sea level oscillations in barotropic and baroclinic conditions from hydrodynamic modelling

International audienceFree sea level oscillations in barotropic and baroclinic conditions were examined using numerical experiments based on a 3-D hydrodynamic model of the Baltic Sea. In a barotropic environment, the highest amplitudes of free sea level oscillations are observed in the northern Gulf of Bothnia, eastern Gulf of Finland, and south-western Baltic Sea. In these areas, the maximum variance appears within the frequency range corresponding to periods of 13-44 h. In a stratified environment, after the cessation of meteorological forcing, water masses relax to the equilibrium state in the form of mesoscale oscillations at the same frequencies as well as in the form of rapidly decaying low-frequency (seasonal) oscillations. The total amplitudes of free baroclinic perturbations are significantly larger than those of barotropic perturbations, reaching 15-17 cm. Contrary to barotropic, oscillations in baroclinic conditions are strongly pronounced in the deep-water areas of the Baltic Sea proper. Specific spatial patterns of amplitudes and phases of free barotropic and baroclinic sea level oscillations identified them as progressive-standing waves representing barotropic or baroclinic modes of gravity waves and topographic Rossby waves

Study of Lake Baikal ice cover from radar altimetry and in-situ observations

Comparison of ENVISAT and SARAL missions data shows that AltiKa can be successfully used for ice discrimination methodology and extension of ice conditions time series. Due to shorter wavelength and large bandwidth (480 MHz) which leads to a higher sensitivity to different surface conditions, AltiKa shows more clearly the separation between open water and various ice types. We observe significant decrease of backscatter (25–30 dB) in late spring for both ENVISAT and SARAL and discuss it in the context of ice metamorphism. There is a clear need to continue and expand our dedicated field studies of lake Baikal ice cover to better assess influence of ice structure on altimetric signal

Study of Lake Baikal ice cover from radar altimetry and in-situ observations

Comparison of ENVISAT and SARAL missions data shows that AltiKa can be successfully used for ice discrimination methodology and extension of ice conditions time series. Due to shorter wavelength and large bandwidth (480 MHz) which leads to a higher sensitivity to different surface conditions, AltiKa shows more clearly the separation between open water and various ice types. We observe significant decrease of backscatter (25–30 dB) in late spring for both ENVISAT and SARAL and discuss it in the context of ice metamorphism. There is a clear need to continue and expand our dedicated field studies of lake Baikal ice cover to better assess influence of ice structure on altimetric signal

Seasonal variability of the Western Siberia wetlands from satellite radar altimetry

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A., 2009, International Journal of Environmental Studies, V66, P447, DOI 10.1080/00207230902823578 Zakharova E.A., 2007, 2 SPAC HYDR WORKSH S Zakharova, Elena A. Kouraev, Alexei V. Remy, Frederique Zemtsov, Valeri A. Kirpotin, Sergey N. 0 ELSEVIER SCIENCE BV AMSTERDAM J HYDROLBoreal wetlands play an important role in the global water and carbon cycle but their water regime is far from being well understood. The aim of this paper is to study wetland hydrological regime over the 21 mid-size watersheds of the Western Siberia - one of the most bogged regions of the world. By using ENVISAT RA-2 radar altimetry data we analyze seasonal variability of wet zones extent, water level and storage in wetlands. We have identified three main types of wetland water regime characterized by: (1) spring inundation and following deep drainage with/without secondary peak in autumn; (2) spring inundation and low summer variation; (3) spring inundation with medium summer drainage and second autumnal peak. Our estimates show that the floodplain inundation contributes less than 8% to the total wet zones extent. Analysis of the timing of melt and freeze onset and other specific phases of hydrological regime has been done. It was found that the spring inundation lasts for almost 2 months with a latitudinal gradient of melt onset of 8 days/2. No considerable latitudinal gradient has been found for dates of full freeze onset. Our results show that seasonal amplitude of water level variation for northern part of Western Siberia from altimetry is 0.7-1.5 m for lakes and 0.2-0.5 m for bogs. This represents seasonal variation of wetland water storage of 480 mm for non-permafrost and 130 mm for permafrost-affected zones. (c) 2014 Elsevier B.V. All rights reserved