349 research outputs found
ESA's Ice Sheets CCI: validation and inter-comparison of surface elevation changes derived from laser and radar altimetry over Jakobshavn Isbræ, Greenland – Round Robin results
In order to increase the understanding of the changing climate, the
European Space Agency has launched the Climate Change Initiative
(ESA CCI), a program which joins scientists and space agencies into
13 projects either affecting or affected by the concurrent
changes. This work is part of the Ice Sheets CCI and four parameters
are to be determined for the Greenland Ice Sheet (GrIS), each
resulting in a dataset made available to the public: Surface
Elevation Changes (SEC), surface velocities, grounding line
locations, and calving front locations. All CCI projects have
completed a so-called Round Robin exercise in which the scientific
community was asked to provide their best estimate of the sought
parameters as well as a feedback sheet describing their work. By
inter-comparing and validating the results, obtained from research
institutions world-wide, it is possible to develop the most optimal
method for determining each parameter. This work describes the SEC
Round Robin and the subsequent conclusions leading to the creation
of a method for determining GrIS SEC values. The participants used
either Envisat radar or ICESat laser altimetry over Jakobshavn
Isbræ drainage basin, and the submissions led to inter-comparisons
of radar vs. altimetry as well as cross-over vs. repeat-track
analyses. Due to the high accuracy of the former and the high
spatial resolution of the latter, a method, which combines the two
techniques will provide the most accurate SEC estimates. The data
supporting the final GrIS analysis stem from the radar altimeters
on-board Envisat, ERS-1 and ERS-2. The accuracy of laser data
exceeds that of radar altimetry; the Round Robin analysis has,
however, proven the latter equally capable of dealing with surface
topography thereby making such data applicable in SEC analyses
extending all the way from the interior ice sheet to margin
regions. This shows good potential for a~future inclusion of ESA
CryoSat-2 and Sentinel-3 radar data in the analysis, and thus for
obtaining reliable SEC estimates throughout the entire GrIS
COLECCIÓN CARIDAD RODRÍGUEZ PÉREZ-GALDÓS [Material gráfico]
Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte. Subdirección General de Coordinación Bibliotecaria, 201
Intercomparison of detection and quantification methods for methane emissions from the natural gas distribution network in Hamburg, Germany
In August and September 2020, three different measurement methods for quantifying methane (CH4) emissions from leaks in urban gas distribution networks were applied and compared in Hamburg, Germany: the “mobile”, “tracer release”, and “suction” methods. The mobile and tracer release methods determine emission rates to the atmosphere from measurements of CH4 mole fractions in the ambient air, and the tracer release method also includes measurement of a gaseous tracer. The suction method determines emission rates by pumping air out of the ground using soil probes that are placed above the suspected leak location. The quantitative intercomparison of the emission rates from the three methods at a small number of locations is challenging because of limitations of the different methods at different types of leak locations.
The mobile method was designed to rapidly quantify the average or total emission rate of many gas leaks in a city, but it yields a large emission rate uncertainty for individual leak locations. Emission rates determined for individual leak locations with the tracer release technique are more precise because the simultaneous measurement of the tracer released at a known rate at the emission source eliminates many of the uncertainties encountered with the mobile method. Nevertheless, care must be taken to properly collocate the tracer release and the leak emission points to avoid biases in emission rate estimates. The suction method could not be completed or applied at locations with widespread subsurface CH4 accumulation or due to safety measures. While the number of gas leak locations in this study is small, we observe a correlation between leak emission rate and subsurface accumulation. Wide accumulation places leaks into a safety category that requires immediate repair so that the suction method cannot be applied to these larger leaks in routine operation. This introduces a sampling bias for the suction method in this study towards the low-emission leaks, which do not require immediate repair measures. Given that this study is based on random sampling, such a sampling bias may also exist for the suction method outside of this study. While an investigation of the causal relationship between safety category and leak size is beyond the scope of this study, on average higher emission rates were observed from all three measurement-based quantification methods for leaks with higher safety priority compared to the leaks with lower safety concern. The leak locations where the suction method could not be applied were the biggest emitters, as confirmed by the emission rate quantifications using mobile and tracer methods and an engineering method based on the leak's diameter, pipeline overpressure, and depth at which the pipeline is buried. The corresponding sampling bias for the suction technique led to a low bias in derived emission rates in this study. It is important that future studies using the suction method account for any leaks not quantifiable with this method in order to avoid biases, especially when used to inform emission inventories.</p
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