Volcanic Ash Monitoring: Product Guide

Transcription

1 Doc.No. Issue : : EUM/TSS/MAN/15/ v1a EUMETSAT Eumetsat-Allee 1, D Darmstadt, Germany Tel: Fax: Date : 2 June WBS/DBS : EUMETSAT The copyright of this document is the property of EUMETSAT.

2 Document Change Record Issue / Revision Date DCN. No 1 06/10/2010 Initial release. Summary of Changes 1A 28/05/2015 Change of title from Factsheet. Added the following sections: Product Description, Abbreviated algorithm, reference document, Known operation limitations. Scientific review completed. Table of Contents 1 PRODUCT DESCRIPTION PRODUCT SPECIFICATIONS Known Operational Limitations PRODUCT ILLUSTRATION BASIC STRUCTURE OF THE VOL ALGORITHM Processing mask Ash detection Ash retrieval REFERENCES AND PRODUCT ASSISTANCE... 9 Online Resources and Assistance... 9 Page 2 of 10

3 1 PRODUCT DESCRIPTION Volcanic ash plumes have been monitored with geostationary satellite images since the late 1970s. However, it is difficult to distinguish between ash plumes and thin cirrus clouds or even the underlying surface using single band infrared and visible images. Split-window techniques were introduced using channels at 11 µm and 12 µm wavelength to better detect ash plumes. This technique is also called reverse absorption and is based on the principle that the absorption of clouds, which are made of ice and water, is higher in the channel at 12 µm than the channel near 11 µm. Ash plumes have exactly the opposite characteristics. Using the geostationary MSG satellite, additional multi-spectral approaches can be tested to support and improve the split-window technique. For example, some constituents of the ash clouds such as sulfur dioxide (SO2) or sulfates can be detected in addition by channels in the 3.9 and 8.7 µm bands, i.e. MSG channels IR3.9 and IR8.7. In addition to the IR channels, the visible channels can be used during daylight. The single-scattering Albedo of water and ice clouds decreases with wavelength, while the single-scattering Albedo of an ash cloud remains constant or increases even slightly. With the ratio of the solar contribution in channel IR3.9 with the visible channel VIS0.6, ash clouds can be separated from ice or water clouds. The Volcanic Ash Detection product (VOL) is restricted to those National Meteorological Services that are members of EUMETSAT. The ash detection is based on a reversed split window technique, supported by tests in the other IR channels and two VIS channels. The execution of the ash detection algorithm covers the area of the Earth disk as seen by Meteosat. The CAP-formatted product provides e.g. an ash cloud load for an area defined by a polygon. For the detected cloud pixels, the VOL algorithm uses the following criteria to check for ash cloud pixels: Reflectance ratio IR3.9(solar component)/vis0.6 (daylight and twilight only) Brightness temperature difference of channels IR3.9 and IR10.8 (night only) Brightness temperature difference of channels IR8.7 and IR10.8 Brightness temperature difference of channels IR12.0 and IR10.8 The main two tests for ash cloud detection are the reflectance ratio test IR3.9 (solar)/vis0.6 and the brightness temperature difference test of channels IR12.0 and IR10.8. While the reflectance ratio test can almost clearly separate all ash clouds from other clouds, the brightness temperature difference test IR IR10.8 may have similar differences for thin ice and water clouds as for ash clouds. For that reason, there is a need to filter out miss-classified ash clouds/ice clouds during night, when the reflectance ratio test cannot be used. Therefore, the algorithm uses the two additional further brightness temperature difference tests. Page 3 of 9

4 2 PRODUCT SPECIFICATIONS Category Product users Input satellite data Product Distribution Product Area Product Resolution Product Distribution Frequency Product Names Product Size Volcanic Ash Advisory Centre Specification National Meteorological Services of the EUMETSAT member states. European Volcano Observatory Space Services (EVOSS) The reflectances in the VIS0.6 and the IR3.9 channels The brightness temperatures in the IR3.9, IR8.7, IR10.8 and IR12:0 channels; The solar zenith angle on pixel level; The ECMWF forecast for the determination of the expected brightness temperatures to be used in for several thresholds for the ash detection. EUMETCast (NMS restricted) on Alert Channel EUMETCast (to restricted user community) EUMETSAT Data Centre Full-earth scanning CAP Format: see general description ASCII Format: pixel (EUMETSAT Data Centre) netcdf Format: pixel CAP Format: every repeat cycle to EUMETCast and EUMETSAT Data Centre when activated netcdf Format: every repeat cycle to EUMETCast EUMETCast FES Area in CAP format: L-000-MSG2 -MPEF -VOLC EUMETCast FES Area in netcdf format: L-000-MSG2 -MPEF -VOLE Variable 2.1 Known Operational Limitations The operational status of the product has been declared as demonstrational. Very thin ash is not detected. Ice embedded in the ash cloud may obscure the ash signal. False alarms are likely. Page 4 of 9

5 3 PRODUCT ILLUSTRATION An example for the volcanic ash monitoring is given for the Karthala eruption on November 25, Karthala is situated at 11.75º S and 43.38º E in the Indian Ocean and is one of the largest active volcanoes in the world. As there has not been any significant coverage of high clouds in the area at the time of the eruption, it was possible to monitor the eruption using Metosat-8 images right from the beginning of the event. Figure 1: Animation of the Karthala eruption using the Ash RGB combination range. Figure 1 shows a still shot of the animation of this eruption using the Ash RGB combination image and the channel combination IR12.0 IR10.8, IR10.8 IR8.7 and IR10.8. To see animations and pictures of other eruptions, type Ash Plume into the Search field on the EUMETSAT web site. This Ash RGB exploits the fact that thin ash clouds tend to have a positive brightness temperature difference (BTD) between IR12.0 and IR10.8, while water/ice clouds, particularly thin clouds, have a negative BTD. Thus, thin ash clouds have a strong reddish colour, while water/ice clouds have less red; particularly thin ice clouds which appear very dark. In the case of the Karthala ash cloud shown above and in Figure 2, the BTD between IR12.0 and IR10.8 reached values of about +3 K in the early eruption phase. Other volcanic eruptions have had even higher values, in some cases +10 K. Page 5 of 9

6 An example of the derived VOL product is given in below for the 12:00 UTC repeat cycle, with the following colour coding: Blue is clear ocean. Green is clear land. White is clouds. Grey is clouds within the search area. Orange is the ash plume. The red dot on the island marks the detected hotspot of the volcano. Figure 2: Derived VOL product for the 12:00 UTC Repeat Cycle. Page 6 of 9

7 4 BASIC STRUCTURE OF THE VOL ALGORITHM The volcanic ash cloud detection algorithm is based on the work of the SAFNWC, and of Jochen Kerkmann (EUMETSAT) and Fred Prata (Norwegian Institute for Air Research). It is based on a combination of threshold tests, depending on the time of day and channel availability. For pixels where volcanic ash is detected, a volcanic ash retrieval algorithm is applied. This algorithm is based on the work of Fred Prata (Norwegian Institute for Air Research) under a EUMETSAT study contract. An outline of the VOL algorithm is provided here. For complete specifications and calculations, see the VOL Algorithm Theoretical Basis Document (ATBD). The document reference number is listed in Section Processing mask The ash detection algorithm is applied to all SEVIRI pixels in the processing area. The ash retrieval algorithm is only applied to pixels in which ash was detected. 4.2 Ash detection After reading in the image data, the related auxiliary data (latitude, longitude, solar zenith and static datasets), the processing performs the following steps for each pixel: Step 1 Step 2 Step 3 Check the solar zenith angle for day (sol_zen < 80 ), night (sol_zen > 90 ) or twilight (between 80 and 90 ), to select the thresholds and tests. Calculate the following test flags: Test1A = (T8.7 - T10.8) > a 1A + b 1A T PCS,8.7 + c 1A T PCS,10.8 Test2A = (T T10.8) > a 2A + b 2A T PCS, c 2A T PCS,10.8 Test3A = (T3.9-T10.8) > a 3A + b 3A T PCS,3.9 + c 3A T PCS,10.8 Test3B = (T3.9-T10.8) < a 3B + b 3B T PCS,3.9 + c 3B T PCS,10.8 Test4A = (Refl3.9 / Refl0.6) > a 4A with (T PCS,chan ) denoting the predicted clear sky brightness temperature in channel chan derived from the ECMWF forecast with the radiative transfer model.. Apply the following tests. Test Condition Test Logic Result Night or Twilight (sol_zen > = 80) Test 1A and Test2A and Test3A and Test3B Ash Day (sol_zen < 80) Test1A and Test2A and Test4A Ash Step 4 If the result of the tests is Ash and (T12.0 T10.8) > 0.5, the pixel is considered to be potentially ash-contaminated. Page 7 of 9

8 4.3 Ash retrieval The ash retrieval algorithm is applied to the pixels which were classified as potentially ash contaminated by the ash detection algorithm above. The inputs to the ash retrieval algorithm are as follows: The satellite zenith angle, sat_zen. The measured brightness temperature in channels IR10.8 and IR12.0: T10.8 and T12.0. An estimate of the surface skin temperature, Ts = T PCS,12.0, taken as the predicted clear sky brightness temperature in channel IR12.0. An estimate of the ash cloud top temperature, Tc, obtained from the minimum measured brightness temperature in channel IR12.0 of potentially ash contaminated pixels within the vicinity of the current pixel. After accepting these inputs into the processing cycle, the algorithm performs the following steps. Step 1 Step 2 Step 3 Determine the minimum value of T12.0 for potentially ash contaminated pixels within each block. From the satellite zenith angle and T10.8 compute a water vapour correction term. Look up the ash cloud optical depth, aod, and effective particle radius, radius in two look-up tables. Step 4 Determine the total ash mass loading, M, expressed in kg /m -2. Step 5 Derive the ash cloud top height in kilometers. Page 8 of 9

9 5 REFERENCES AND PRODUCT ASSISTANCE Type Document Name Reference Algorithm Specification Volcanic Ash Detection: ATBD EUM/MET/REP/07/0467 Survey Validation test Volcanic Information Derived from Satellite Data MSG-3 System Commissioning Product Validation Test Report F. Prata, February 2011, NILU (Norwegian Institute for Air Research) EUM/MSG/REP/12/0190 Online Resources and Assistance All of the reference documents listed above are in the EUMETSAT Technical Documents page. > Satellites > Technical Documents > Meteosat Services > 0 Meteosat Meteorological Products A training presentation for the VOL Product is here: > home > Data > Training > TrainingLibrary > index > Monitoring of Volcanic Eruptions with MSG RGB Products > Introduction to Remote Sensing Techniques for Ash and SO2 Detection > Volcanic Ash Training Module To register for data delivery from this product, go to the Data Registration page on the EUMETSAT web page: > Data > Data Delivery > Data Registration Information about the service status of EUMETSAT satellites and the data they deliver is this EUMETSAT web page: > Data > Service Status To get answers to any questions about data delivery, registration or documentation, contact the EUMETSAT User Service Help Desk: Telephone: / ops@eumetsat.int Page 9 of 9