Observations of volcanic
ash by lidar and MODIS
Robin Hogan <r.j.hogan@reading.ac.uk>
University of Reading
Last updated: 20 April 2010
Thursday
15th, 1329
Summary from
MODIS images
• Icelandic wind
from northwest
•
Further images:
www.sat.dundee.ac.uk
Friday
16th, 1234
• Volcano
obscured by
clouds
Saturday
17th, 1317
• Wind at Iceland
from the north
• Volcanic ash
heading south
behind a cold
front
Sunday
18th, 1222
• Northerly winds
weakening
Monday
19th, 1305
• New ash
entering a low
pressure system
Observations on Friday 16th April
16 April: 1044 UTC
NASA MODIS radiometer
16 April: 1224 UTC
Stationary colours in
the sea (sediment
and algae)
x Cabauw
x Chilbolton
NASA MODIS radiometer
Chilbolton Doppler lidar: 16 April
Background
aerosol particles
in the boundary
layer (0-1 km)
Descending
volcanic ash?
Mixes into turbulent
boundary layer
Vertical velocity shows
turbulence in boundary
layer and also in ash layer
Chilbolton Doppler lidar: 16 April
Background
aerosol particles
in the boundary
layer (0-1 km)
Descending
volcanic ash?
Spherical liquid
droplets have very
low depolarization
Mixes into turbulent
boundary layer
Ash is non-spherical so
strongly depolarizing
Aerosol optical depth: 16 April
Background
aerosol particles
in the boundary
layer (0-1 km)
Descending
volcanic ash?
Mixes into turbulent
boundary layer
Aerosol optical
depth at several
wavelengths
from the
Chilbolton sun
photometer,
courtesy Charles
Wrench of STFC
Chilbolton UV lidar: 16 April
Background
aerosol particles
in the boundary
layer (0-1 km)
Descending
volcanic ash?
Spherical hydrated
aerosol with minimal
depolarization
Ash is non-spherical so
strongly depolarizing
Mixes into turbulent
boundary layer
Chilbolton lidar ceilometer: 16 April
• Chilbolton has three routinely operating lidars
– 1500 micron Doppler/polarization lidar (previous slides)
– 905 nm lidar ceilometer
– 355 nm (UV) polarization lidar (previous slide)
• Can use the wavelength dependence of the scattering to
estimate particle size
– The following slides are from Ewan O’Connor and Chris Westbrook,
University of Reading...
Colour ratios for each combination
Note contrast with ordinary
boundary layer aerosol
Less than 1
Close to 1
Greater than 1
Colour ratios: 355/905
Note contrast with ordinary
boundary layer aerosol
Calculations for
different possible
refractive indices:
median diameter
greater than 800
microns
Less than 1
Colour ratios: 905/1500
Note contrast with ordinary
boundary layer aerosol
Upper bound
~2 microns
assuming not
liquid water
Calculations for
different possible
refractive indices:
median diameter
greater than 800
microns
Greater than 1
Suggests median
diameter is between
0.8 mm and 2mm
Further Note
analysis
contrast will
with ordinary
boundary
layer aerosol
narrow this
down…
Colour ratios
Less than 1
Close to 1
Upper bound
~2 microns
assuming not
liquid water
Calculations for
different possible
refractive indices:
median diameter
greater than 800
microns
Greater than 1
Just using two colours: 355/1500 nm
• Assumed ash refractive index 1.5 – 0.001i: volcanic ash is
1.7-2 microns in diameter (similar result for more absorbing
ash)
Sun photometer derived size distribution
• Courtesy of Charles Wrench, STFC
• Large-particle mode peaks at 3 microns radius: in good
agreement with lidar-derived values
Surface sulphur dioxide
http://www.airquality.co.uk/
Mixing event at
Chilbolton:
15.00, 16th Apr
Are the spikes due to
volcanic ash?
• Timing is good over London but a bit
late at other locations
• In fact, the Met Office Unified and
NAME models can both reproduce this
spike WITHOUT volcanic ash, implying
that this is an ordinary boundary layer
pollution episode!
• The amounts are much less that UK air
quality objective (1 hr average exceeds
350 mg m-3 less than 24 times per year)
No convincing sign of ash
Aerosol particles
(PM10s)
Ultraviolet EZ-lidar, Cardington
Bedfordshire, 16th April
•
•
http://www.metoffice.gov.uk/corporate/pressoffice/2010/volcano/lidar/
This plot was produced by the University of Manchester, NCAS and FGAM.
RIVM Caeli lidar, Netherlands, 16th April
Courtesy of Arnoud Apituley <Arnoud.Apituley@rivm.nl>
Volcanic ash just above
boundary-layer
• This lidar is not operated all the time but has Raman capability
• Further images here:
http://cerberus.rivm.nl/lidar/Cabauw/2010/
Cabauw EZ-lidar, Netherlands, 16th April
Courtesy of David Donovan, KNMI <donovan@knmi.nl>
Ash appears not to mix
into the boundary layer as
it did over Chilbolton…
As over Chilbolton, ash much
more depolarizing than ordinary
boundary-layer aerosol
Calipso lidar
16th April
Ash higher at leading
(southern) edge,
explaining the descending
appearance to groundbased lidar
Simultaneous MODIS image
Observations on Saturday 17th April
Chilbolton Doppler lidar: 17 April
Normal aerosol
particles in the
boundary layer:
no further sign of
volcanic ash…
•
Further images at
http://www.met.reading.ac.uk/radar/realtime/today.html
Chilbolton UV lidar, 17 April
Normal aerosol
particles in the
boundary layer:
no further sign of
volcanic ash…
Observations on Sunday 18th April
Chilbolton Doppler lidar: 18 April
Is this volcanic ash?
Doppler lidar shows that it
sits above the turbulent
boundary-layer in the
morning, which is why it is
not immediately entrained
into the boundary layer
Chilbolton UV lidar: 18th April
Depolarization implies
it is volcanic ash
Entrained into and
diluted by existing
boundary-layer
aerosol when boundary
layer grows?
Aerosol optical
depth at
several
wavelengths
from the
Chilbolton sun
photometer,
courtesy
Charles Wrench
of STFC
Sun photometer sizes, 18th April
Cabauw EZ-lidar, Netherlands, 18th April
Courtesy of David Donovan, KNMI <donovan@knmi.nl>
Similar signature
observed by UV lidar at
Cabauw
De Bilt radiosonde put
midday boundary-layer
top at ~1 km
Cabauw EZ-lidar, Netherlands, 18th April
Courtesy of David Donovan, KNMI <donovan@knmi.nl>
Another layer
coming in?
• Further images: http://www.knmi.nl/~knap/lidar_cabauw/
Observations on Monday 19th April
Chilbolton Doppler lidar, 19th April
Deeper more dilute layer of volcanic
ash above the boundary layer?
Chilbolton UV lidar, 19th April
Deeper more dilute layer of volcanic
ash above the boundary layer?
Weaker depolarizing signature