Horizontal and vertical structure of the Eyjafjallajökull ash cloud over the UK: a comparison of airborne lidar observations and simulations 1Department of Meteorology, University of Reading, Reading, UK
05 Nov 2012
2Met Office, Exeter, UK
Received: 20 Feb 2012 – Published in Atmos. Chem. Phys. Discuss.: 10 Apr 2012Abstract. During April and May 2010 the ash cloud from the eruption of the Icelandic
volcano Eyjafjallajökull caused widespread disruption to aviation over
northern Europe. The location and impact of the eruption led to a wealth of
observations of the ash cloud were being obtained which can be used to assess
modelling of the long range transport of ash in the troposphere. The UK FAAM
(Facility for Airborne Atmospheric Measurements) BAe-146-301 research
aircraft overflew the ash cloud on a number of days during May. The aircraft
carries a downward looking lidar which detected the ash layer through the
backscatter of the laser light. In this study ash concentrations derived from
the lidar are compared with simulations of the ash cloud made with NAME
(Numerical Atmospheric-dispersion Modelling Environment), a general purpose
atmospheric transport and dispersion model.
Revised: 22 Aug 2012 – Accepted: 27 Oct 2012 – Published: 05 Nov 2012
The simulated ash clouds are compared to the lidar data to determine how well
NAME simulates the horizontal and vertical structure of the ash clouds.
Comparison between the ash concentrations derived from the lidar and those
from NAME is used to define the fraction of ash emitted in the eruption that
is transported over long distances compared to the total emission of tephra.
In making these comparisons possible position errors in the simulated ash
clouds are identified and accounted for.
The ash layers seen by the lidar considered in this study were thin, with
typical depths of 550–750 m. The vertical structure of the ash cloud
simulated by NAME was generally consistent with the observed ash layers,
although the layers in the simulated ash clouds that are identified with
observed ash layers are about twice the depth of the observed layers. The
structure of the simulated ash clouds were sensitive to the profile of ash
emissions that was assumed. In terms of horizontal and vertical structure the
best results were obtained by assuming that the emission occurred at the top
of the eruption plume, consistent with the observed structure of eruption
plumes. However, early in the period when the intensity of the eruption was
low, assuming that the emission of ash was uniform with height gives better
guidance on the horizontal and vertical structure of the ash cloud.
Comparison of the lidar concentrations with those from NAME show that
2–5% of the total mass erupted by the volcano remained in the ash cloud
over the United Kingdom.
Citation: Grant, A. L. M., Dacre, H. F., Thomson, D. J., and Marenco, F.: Horizontal and vertical structure of the Eyjafjallajökull ash cloud over the UK: a comparison of airborne lidar observations and simulations, Atmos. Chem. Phys., 12, 10145-10159, doi:10.5194/acp-12-10145-2012, 2012.