![]() ![]() The use of these two sensors makes the most of the better spatial resolution of MODIS, but is conversely limited by only a few MODIS acquisition per day. ![]() This method exploits the different points of view of the SEVIRI instrument on board the MSG geostationary platform, and the MODIS instrument on board the polar-orbiting AQUA and TERRA platforms, for the observation of volcanic ash clouds produced during eruptions. It was developed and successfully applied to the large 2010 Eyjafjallajökull eruption by Zakšek et al. A recent method based on the parallax between volcanic ash cloud images recorded by the satellite multispectral sensors MODIS (Moderate Resolution Imaging Spectroradiometer) and SEVIRI (Spinning Enhanced Visible and InfraRed Imager) achieves a higher accuracy of ~600 m. Operational height estimates based on CO 2 absorption are available several times a day but with significantly lower accuracy : bias = −1.4 km std = 2.9 km. When available, ash cloud height measured by LiDARs or LiDAR networks are very valuable for comparison validation with other observations in integrated and synergistic approaches. The best estimates using LiDAR from a space-based platform offer an accuracy within less than 200 m, and can provide either daytime and night-time measurements of a volcanic cloud height and thickness with a revisit time of 16 days. The accuracy of the listed methods depends on the spatial resolution of the sensors and the cloud height. Table 1 briefly reviews quantitative satellite remote sensing techniques for ash/meteorological cloud height retrieval. Satellite data offers the best way to extend the ability to estimate ACTH when volcanic eruptions occur, providing the continuous global coverage required for effective monitoring. The new constellation of geostationary meteorological satellites will enable full exploitation of this technique for continuous global ACTH monitoring. The ash cloud estimate was applied to the visible channels to show the potential accuracy that will soon be achievable also in the infrared range using the next generation of multispectral imagers. The height of the volcanic cloud was tracked with a top height of ~8.5 km. Etna, Sicily, Italy, eruption of 23 November 2013. ![]() Since MVIRI does not acquire data at exactly the same time as SEVIRI, a correction procedure enables compensation for wind advection in the atmosphere. ![]() A combination of MSG SEVIRI (HRV band 1000 m nadir spatial resolution, 5 min temporal resolution) and Meteosat-7 MVIRI (VIS band, 2500 m nadir spatial resolution, 30 min temporal resolution) was implemented. An improved version of the method with a detailed discussion of height retrieval accuracy was applied to estimate ACTH from two datasets acquired by two satellites in favorable positions to fully exploit the parallax effect. A novel application is proposed of a method based on parallax data acquired from two geostationary instruments for estimating ash cloud-top height (ACTH). In the limit as the area goes to zero, the circular triangle shrinks towards the Fermat point of the given three points.The characterization of volcanic ash clouds released into the atmosphere during explosive eruptions includes cloud height as a fundamental physical parameter. Below that area, the curve degenerates to a circular triangle with "antennae", straight segments reaching from its vertices to one or more of the specified points. For smaller areas, the optimal curve will be a circular triangle with the three points as its vertices, and with circular arcs of equal radii as its sides, down to the area at which one of the three interior angles of such a triangle reaches zero. When the area is at least as large as the circumcircle of the points, the solution is any circle of that area surrounding the points. Isoperimetry Ĭircular triangles give the solution to an isoperimetric problem in which one seeks a curve of minimum length that encloses three given points and has a prescribed area. Īll circular triangles with the same interior angles as each other are equivalent to each other under Möbius transformations. ![]()
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