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Gonzalez P.J.,University of Leeds | Bagnardi M.,University of Leeds | Hooper A.J.,University of Leeds | Larsen Y.,Northern Research Institute Tromso Norut | And 3 more authors.
Geophysical Research Letters | Year: 2015

After 20 years of quiescence, Fogo volcano erupted in November 2014. The eruption produced fast-moving lava flows that traveled for several kilometers and destroyed two villages. This event represents the first episode of significant surface deformation imaged by the new European Space Agency's Sentinel-1 satellite in its standard acquisition mode, Terrain Observation by Progressive Scans (TOPS), which differs from that of previous synthetic aperture radar (SAR) missions. We perform a Bayesian inversion of Sentinel-1 TOPS SAR interferograms spanning the eruption and accurately account for variations in the TOPS line-of-sight vector when modeling displacements. Our results show that magma ascended beneath the Pico do Fogo cone and then moved laterally toward its southwestern flank, where the eruptive fissure opened. This study provides important insights into the inner workings of Fogo volcano and shows the potential of Sentinel-1 TOPS interferometry for geophysical (e.g., volcano monitoring) applications. © 2015. American Geophysical Union. All Rights Reserved.

Wielgolaski F.E.,University of Oslo | Nordli O.,Norwegian Meteorological Institute | Karlsen S.R.,Northern Research Institute Tromso Norut | O'Neill B.,University of Oslo
International Journal of Biometeorology | Year: 2011

First flowering was observed in some native herbaceous and woody plants in Norway at latitudes of ~58°N to nearly 71°N from 1928 to 1977. For woody plants, the timing for first bud burst was also often observed. Generally, there were highly significant correlations (0.1% level) between the timing of nearly all spring-early summer observations in plants and gridded mean monthly temperatures for the various phenophases (up to 65% of the variance was accounted for, less so for the autumn phenophases). Analyses by a low pass Gaussian smoothing technique showed early phenophases in the warm period of the early 1930s, delayed phases for most sites and species in colder periods in the early 1940s, mid-1950s, late 1960s and also towards the end of the study period in the late 1970s, all in approximately 10- to 12-year cycles. The study thus starts in a relatively early (warm) period and ends towards a late (cooler) period, resulting in mainly weak linear trends in phenophases throughout the total period. The end of the observation period in 1977 also predates the strongly increasing "earliness" in phenology of plants in most Norwegian lowland areas due to global warming. The strong altitudinal and latitudinal variations in Norway, however, do cause regional differences in trends. The study showed a tendency towards earlier spring phenophases all along the western coast from south to north in the country. On the other hand, the northeasternmost site and also the more continental sites in the southeast showed tendencies to weak trends for later phenophases during the 50 years of these field observations. © 2011 ISB.

Engen G.,Northern Research Institute Tromso Norut | Larsen Y.,Northern Research Institute Tromso Norut
IEEE Transactions on Geoscience and Remote Sensing | Year: 2011

The main operational mode of the European Space Agency's upcoming Sentinel-1 operational satellite will be the Terrain Observation by Progressive Scans (TOPS) imaging mode. This paper presents a very efficient wavenumber domain processor for the processing of TOPS mode data. In particular, a novel signal transform, called a moving band chirp $Z$-transform, is introduced in order to allow the entire azimuth aperture to be focused simultaneously without any need for temporary unaliasing, which requires upsampling, or subaperture processing. © 2011 IEEE.

Lauknes T.R.,Northern Research Institute Tromso Norut | Zebker H.A.,Stanford University | Larsen Y.,Northern Research Institute Tromso Norut
IEEE Transactions on Geoscience and Remote Sensing | Year: 2011

Satellite synthetic aperture radar interferometry (InSAR) is an invaluable tool for land displacement monitoring. Improved access to time series of satellite data has led to the development of several innovative multitemporal algorithms. Small baseline (SB) is one such time-series InSAR method, based on combining and inverting a set of unwrapped interferograms for surface displacement. Two-dimensional unwrapping of sparse data sets is a challenging task, and unwrapping errors can lead to incorrectly estimated deformation time series. It is well known that L1-norm is more robust than L 2-norm cost function minimization if the data set has a large number of outlying points. In this paper, we present an L1 -norm-based SB method using an iteratively reweighted least squares algorithm. We show that the displacement phase of both synthetic data, as well as a real data set that covers the San Francisco Bay area, is recovered more accurately than with L 2-norm solutions. © 2006 IEEE.

Karlsen S.R.,Northern Research Institute Tromso Norut | Hogda K.A.,Northern Research Institute Tromso Norut | Tolvanen A.,Finnish Forest Research Institute | Johansen B.,Northern Research Institute Tromso Norut | Elvebakk A.,University of Tromso
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

In this study we present new satellite-based maps of the growing season of northern areas. The maps show trends and mean date in onset and length of the growing season at different scales north of 50° N. For all the circumpolar area we use the GIMMS-NDVI satellite dataset for the 1982 to 2006 period, and for the Nordic countries we used the MODIS-NDVI satellite data for the 2000 to 2007 period. The circumpolar maps are not as accurate as the one covering the Nordic countries, this due to lack of ancillary environmental geo-data available that can be included in the mapping process. In particular this is a problem for the Russian part of the circumpolar north. The resulting growing season maps are useful in a broad range of ecological and climatic changes studies. Changes in the timing of the growing season are sensitive bio-indicators of climate change of northern areas, and these changes crucially affects primary industries, such as agriculture, animal husbandry and forestry, as well as the population dynamics of wild mammals and birds. The onset of growing season maps is also useful to improve pollen forecasts, and the maps can be used to improve the global change models. © 2010 SPIE.

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