MetOp Analysis - Getting closer to the surface
The key characteristic about polar orbiting satellites is their high spatial resolution, since they operate on a lower flighting level (820km). However they can only provide a restricted number of imageries depending on the selected latitude. Their main mission is to provide additional high quality product to the MSG imageries. The present operating polar orbiting satellite MetOp-A carries several instruments such as IASI, GOME2 and AVHRR. These instruments operate on different spectral ranges. The products derived from these instruments will be briefly introduced.
IASI (Infrared Atmospheric Sounding Interferometer)
IASI has been developed for operational meteorological soundings with a very high level of accuracy. Based on the atmospheric emission spectra measured in the thermal infrared range temperature and humidity profiles can be derived. Therefore a Michelson interferometer which enables the spectral decomposition is being used. Moreover the information collected by IASI can be used for tracking SO2-signals or other trace gases. Below one sees a loop showing the SO2-tracking in April 2010: The x-axes denote the longitudes and the y-axis the latitudes. The grey shade in the upper left corner indicates the position of Iceland. The time lapse shows the track of the SO2 signal, whereas on 18 April a further eruption takes place.
Figure 3.9: IASI SO2 Product for 15 to 18 April 2010.
The output generated by IASI-measurements enables the discrimination among SO2-signals, ice and ash-particles. Here is brief time table showing the track of the volcanic ash cloud from the 14th of April 2010 to the 15th of April 2010.
Figure 3.10: IASI SO2, Ice and Ash product for 14 and 15 April 2010.
GOME2 (Global Ozone Monitoring Experiment-2)
GOME is an optical spectrometer which senses the Earths backscattered radiance and solar irradiance in the ultraviolet and visible part of the spectrum (240 - 790 nm) at a high spectral resolution of 0.2 - 0.4 nm. Based on this data the atmospheric ozone content as well as information about sulfur dioxide, water vapour, bromine oxide and other trace gases can be derived.
GOME-2 near-real-time products are available up to two hours after sensing (see GOME2-products).
Figure 3.11: Metop-A Gome-2 SO2 product for 15 and 16 April 2010, Ice and Ash product for 14 and 15 April 2010. Source: DLR
AVHRR(Advanced Very High Resolution Radiometer)
The Advanced Very High Resolution Radiometer (AVHRR) operates at 5 different channels simultaneously in the visible and infrared bands. As a high-resolution imager (about 1.1 km near nadir) its main purpose is to provide cloud and surface information such as cloud coverage, cloud top temperature, surface temperature over land and sea, and vegetation or snow/ice. The ability to discriminate small scale features is particularly suitable concerning the detection of volcanic ash clouds. According to Prata (see references) measurements of infrared emission in the 10-12 micrometer window can be used to detect volcanic ash during an eruption and in the days and weeks following. The major effects on the emission due to the cloud are a decrease in signal from the 11 micrometer channel with respect to the 12 micrometer channel and it is presumed that this is due to the reverse absorption effect found for liquid H2SO4 aerosols and observed for volcanic dust. This effect allows ready discrimination of volcanic cloud from water-ice clouds. In an operational environment where AVHRR imagery is available on real time, a volcanic signature can be recognized by computing an image of the IR11.0-IR12.0 temperature differences. Regions of negative IR11.0-IR12.0 are indicative of volcanic ash. It is therefore possible to use this as an operational tool to assist in the forecast of volcanic cloud hazards to air traffic. The picture below, the orange-red colors represent negative brightness temperature differences which are indicators for ash clouds. In contrast positive brightness temperature differences show water-/ice clouds.
Figure 3.12: Metop-A AVHRR 15 April 2010 1046UTC.