1. Mike Pavolonis (NOAA/NESDIS/STAR)
Preliminary Assessment of Eyjafjallajokull Ash Heights Retrieved from SEVIRI and MODIS on April 20, 2010
Mike Pavolonis (NOAA/NESDIS/STAR)

2. Summary
Spaceborne lidar observations from CALIPSO were used to assess the accuracy of the infrared ash height retrievals performed using SEVIRI and MODIS on April 20, 2010 near Iceland.
Special thanks to the CALIPSO Team at NASA Langley for providing expedited L1 data.
The lidar total attenuated backscatter has a vertical resolution of 60 m, which allows for a high quality depiction of the ash cloud top.

3. April 20, 2010
04:10 UTC

4. Expedited CALIOP L1 data courtesy of NASA Langley

5. 8 - 10 km infrared cloud height retrieval off the coast of Iceland agrees well with CALIOP
The height of the optically thin ash cloud over Iceland is underestimate by the infrared retrieval (probably due to poor clear sky radiance estimate over land near coastline)
Expedited CALIOP L1 data courtesy of NASA Langley

6. April 20, 2010
13:45 UTC

7. Are these retrieved heights too low?

8. Zoom in on this area where CALIPSO had an overpass

9. Expedited CALIOP L1 data courtesy of NASA Langley

10. The CALIOP shows that this optically thin ash cloud is mainly located in the boundary layer. The infrared retrievals also indicate a very low ash cloud height.
This supports the hypothesis that the ash cloud heights on April 20, 2010 decreased rapidly with time as shown in the SEVIRI loop.
Expedited CALIOP L1 data courtesy of NASA Langley

11. Conclusions
The ash heights retrieved using infrared measurements from SEVIRI and MODIS are in good agreement with spaceborne lidar measurements (generally within km).
A more quantitative assessment will be performed when the CALIPSO cloud layers data product is released for these dates.