A. M. Sayer, G. E. Thomas, E. Carboni, R. G. Grainger, H. Huang, C. Arnold, and colleagues at the Rutherford Appleton Laboratory
False-colour composite images from the visible and near-infrared channels on AATSR reveal the spectral distinction between thick ash plumes (appearing brown-yellow in these images) and liquid water or ice clouds (appearing in greys and pale blues, with sometimes a hint of yellow). Thick snow and ice cover also appear as an intense light blue. To the right is an example scene off the southern coast of Iceland on April 17.
For an image north of Scotland on April 15, click here. For one of central Europe on April 16, click here. For a slightly larger version of the image to the right, click here.
Maps showing the locations of dense ash plumes can be obtained from infrared (IR) measurements taken by AATSR. The figure to the right shows the brightness temperature difference (BTD) between 10.8 and 12 microns from AATSR observations over northern Europe on 15/04/10. Generally, negative BTDs (yellow and red colours) are characteristic of dense ash plumes. Positive BTDs (greys) are characteristic of water or ice clouds, and different surface types. When the ash is above a cloud, the negative BTD may be masked by the signal from the cloud.
Each orbit allows AATSR to measure a swath approximately 500 km wide. The revisit time depends on the latitude of interest, and global coverage is achieved approximately every 7 days.
An archive of recent BTD images we have created from AATSR data (in PNG format) may be found here.
For cloud-free regions, we are also able to determine the aerosol optical depth (AOD), which is a measure of the effectiveness by which small particles (known as `aerosols') in the atmosphere are absorbing or scattering light. This is typically reported referenced to a wavelength in the visible part of the spectrum (here, 550 nm). Higher AODs are associated with increased amounts of aerosol (such as, in this case, the plume).
Preliminary results showing the AOD retrieved from orbits on 15/04/10 are to the right; the high AODs of 1.5 or more in the plume correspond to regions of negative BTD in the top image. Typical background AODs for `clean and clear' conditions are in the range 0.05-0.2 for Europe. Other high values are observed in the vicinity of broken cloud fields.
An archive of recent AOD images we have created from AATSR data (in PNG format) may be found here.
The Ångström exponent describes the variation of AOD with wavelength in the visible region of the spectrum. This is generally related to particle size, with large positive exponents for small particles (such as typical urban or rural continental aerosol) and small or negative exponents for larger particles (such as from volcanic ash, desert dust or maritime sea salt aerosol). Preliminary results showing the aerosol Ångström exponent from orbits on 15/04/10 are to the right; again, the ash plume is associated with negative exponents, typical for coarse volcanic ash.
An archive of recent Ångström exponent images we have created from AATSR data (in PNG format) may be found here.
The AATSR aerosol and cloud research is carried out in collaboration with the Space Science and Technology Department (SSTD) at the Rutherford Appleton Laboratory (RAL) at Harwell Science Campus, near Didcot. This is in the framework of the Oxford-RAL Aerosol and Clouds (ORAC) project.