Satellite observations of ash and SO2

SEVIRI | AATSR VIS/NIR


SEVIRI

SEVIRI ash detection analysis - continuously generated and archived
Reference documents

Prata, F. (2013) ATBD - Detecting and Retrieving Volcanic Ash from SEVIRI Measurements (1.6 MB)

Prata, A. J., and A. T. Prata (2012), Eyjafjallajökull volcanic ash concentrations determined using Spin Enhanced Visible and Infrared Imager measurements, J. Geophys. Res., 117, D00U23, doi:10.1029/2011JD016800. (19.7 MB)

AATSR VIS/NIR

The AATSR instrument is used in two ways in VAST: to retrieve the aerosol optical depth (AOD) in ash plumes, and to estimate the ash plume top height. Examples of the AATSR products and methods are shown below for an example case of an Eyjafjallajökull ash plume south of Iceland on May 15 2010.
Ash detection
Ash plumes are detected using a simple brightness temperature difference (BTD) threshold (Prata 1989). If BTD = T11 − T12 < 0, the pixel is flagged as ash. The zero threshold is not universal, and false alerts can be caused by dust or haze, but for known historic eruptions this simple method works well.

Figure 1: Ash detection. The ash mask (right) is produced from the full scene (left) by selecting the pixels with BTD<0 (Eyjafjallajökull 15/May/2010.)

AOD Retrieval
One of the main goals of VAST is to provide information on the atmospheric ash concentrations. The AATSR AOD product describes the total ash load in the atmospheric column, using the visible (VIS) and near-infrared (NIR) wavelengths. While the visible channels are not sensitive to particles larger than a few microns (which comprises most of the ash), the AATSR VIS/NIR retrieval gives information on the visible ash, required by the aviation users.

Figure 2: AATSR VIS/NIR AOD retrieval. In the false color RGB image (right) one can (barely) see the grayish ash plume extending south from Iceland, and mixing with meteorological (water) clouds. For ash over ocean the retrieved AOD values are reasonable, but for cloud contaminated areas the AOD is saturated. (Eyjafjallajökull 15/May/2010.)

ADV/ASV algorithm is used for AATSR VIS/NIR AOD retrieval at FMI; ADV is used over land and ASV over ocean. The AATSR Dual View (ADV) algorithm (Kolmonen et al. 2013) uses the stereo view of AATSR to remove the contribution of surface reflectance, while AATSR Single View (ASV) uses an ocean surface reflectance model. ADV/ASV uses a priori defined aerosol properties in a look-up-table (LUT) approach, and retrieves the optimal combined aerosol model.
Plume top height
The stereo view of the AATSR instrument is used to estimate the ash plume top heights. The AATSR correlation method (ACM) height estimate uses an area based cross correlation algorithm (e.g. Zaksek et al. 2013) to estimate the parallax between the nadir and forward views, and converts the resulting plume-top pixel shifts to a height estimate using the known satellite-Earth geometry.

Figure 3: AATSR VIS/NIR AOD retrieval. In the false color RGB image (right) one can (barely) see the grayish ash plume extending south from Iceland, and mixing with meteorological (water) clouds. For ash over ocean the retrieved AOD values are reasonable, but for cloud contaminated areas the AOD is saturated. (Eyjafjallajökull 15/May/2010.)

References

1.      Prata, A. J., Radiative transfer calculations for volcanic ash clouds, Geophys. Res. Lett., 16(11), 1293-1296, 1989.
2.      P. Kolmonen, A.-M. Sundstr ̈m, L. Sogacheva, E. Rodriguez, T.H. Virtanen and G. de Leeuw, Atmos. Meas. Tech. Discuss., 6, 4039-4075, (2013).
3.      Zakˇek, K., Hort, M., Zaletelj, J., and Langmann, B., Monitoring volcanic ash cloud top height through simultaneous retrieval of optical data from polar orbiting and geostationary satellites, Atmos. Chem. Phys., 13, 2589-2606, 2013