Post on 28-Mar-2015
Observations of volcanic Observations of volcanic ash by lidar and MODISash by lidar and MODIS
Robin Hogan <r.j.hogan@reading.ac.uk>University of Reading
Last updated: 20 April 2010
ThursdayThursday1515thth, 1329, 1329
Summary from MODIS images
• Icelandic wind from northwest
• Further images: www.sat.dundee.ac.uk
Volcanic ash
FridayFriday1616thth, 1234, 1234
• Volcano obscured by clouds
Dilute volcanic ash measured over southern England and the Netherlands with lidar
SaturdaySaturday1717thth, 1317, 1317
• Wind at Iceland from the north
• Volcanic ash heading south behind a cold front
Cold front
Volca
nic a
sh
No depolarizing aerosol observed over Chilbolton or Cabauw
SundaySunday1818thth, 1222, 1222
• Northerly winds weakening
Is this the ash above the cloud?
Weakening
front
Not much sign in the MODIS image, but depolarizing aerosol observed at Chilbolton and Cabauw just above the boundary layer
Monday Monday 1919thth, 1305, 1305
• New ash entering a low pressure system
Observations on Friday 16Observations on Friday 16thth April April
16 April: 1044 UTC16 April: 1044 UTC
NASA MODIS radiometer
16 April: 1224 UTC16 April: 1224 UTC
NASA MODIS radiometer
Stationary colours in the sea (sediment and algae)
x Chilbolton
x CabauwVolcanic ash
Chilbolton Doppler lidar: 16 AprilChilbolton Doppler lidar: 16 April
Descending volcanic ash?
Vertical velocity shows turbulence in boundary layer and also in ash layer
Mixes into turbulent boundary layer
Background aerosol particles in the boundary layer (0-1 km)
Chilbolton Doppler lidar: 16 AprilChilbolton Doppler lidar: 16 April
Descending volcanic ash?
Mixes into turbulent boundary layer
Spherical liquid droplets have very low depolarization Ash is non-spherical
so strongly depolarizing
Background aerosol particles in the boundary layer (0-1 km)
Aerosol optical depth: 16 AprilAerosol optical depth: 16 April
Aerosol optical depth at several wavelengths from the Chilbolton sun photometer, courtesy Charles Wrench of STFC
Descending volcanic ash?
Mixes into turbulent boundary layer
Background aerosol particles in the boundary layer (0-1 km)
Chilbolton UV lidar: 16 AprilChilbolton UV lidar: 16 April
Descending volcanic ash?
Mixes into turbulent boundary layer
Ash is non-spherical so strongly depolarizing
Background aerosol particles in the boundary layer (0-1 km)
Spherical hydrated aerosol with minimal depolarization
Chilbolton lidar ceilometer: 16 AprilChilbolton 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 combinationColour ratios for each combination
Less than 1
Greater than 1
Close to 1
Note contrast with ordinary boundary layer aerosol
Colour ratios: 355/905Colour ratios: 355/905
Less than 1
Calculations for different possible refractive indices: median diameter greater than 800
microns
Note contrast with ordinary boundary layer aerosol
Colour ratios: 905/1500Colour ratios: 905/1500
Greater than 1
Upper bound ~2 microns assuming not liquid water
Note contrast with ordinary boundary layer aerosol
Calculations for different possible refractive indices: median diameter greater than 800
microns
Colour ratiosColour ratios
Less than 1
Greater than 1
Close to 1
Note contrast with ordinary boundary layer aerosol
Suggests median diameter is between 0.8 m and 2m Further analysis will narrow this down…
Upper bound ~2 microns assuming not liquid water
Calculations for different possible refractive indices: median diameter greater than 800
microns
Just using two colours: 355/1500 nmJust 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 distributionSun 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 dioxideSurface sulphur dioxidehttp://www.airquality.co.uk/http://www.airquality.co.uk/
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 g m-3 less than 24 times per year)
Mixing event at Chilbolton:
15.00, 16th Apr
Aerosol particles Aerosol particles (PM10s)(PM10s)
No convincing sign of ash
Ultraviolet EZ-lidar, Cardington Ultraviolet EZ-lidar, Cardington Bedfordshire, 16Bedfordshire, 16thth April 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, 16RIVM Caeli lidar, Netherlands, 16thth April AprilCourtesy of Arnoud Apituley <Arnoud.Apituley@rivm.nl>Courtesy of Arnoud Apituley <Arnoud.Apituley@rivm.nl>
• This lidar is not operated all the time but has Raman capability• Further images here:
http://cerberus.rivm.nl/lidar/Cabauw/2010/
Volcanic ash just above boundary-layer
Cabauw EZ-lidar, Netherlands, 16Cabauw EZ-lidar, Netherlands, 16thth April AprilCourtesy of David Donovan, KNMI <donovan@knmi.nl>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 lidarCalipso lidar1616thth April April
Simultaneous MODIS image
Calip
so sw
ath
Boundary-layer clouds
Volcanic ash?
Ash higher at leading (southern) edge, explaining the descending appearance to ground-based lidar
Observations on Saturday 17Observations on Saturday 17thth April April
Chilbolton Doppler lidar: 17 AprilChilbolton Doppler lidar: 17 April
• Further images at http://www.met.reading.ac.uk/radar/realtime/today.html
Normal aerosol particles in the boundary layer: no further sign of volcanic ash…
Chilbolton UV lidar, 17 AprilChilbolton UV lidar, 17 April
Normal aerosol particles in the boundary layer: no further sign of volcanic ash…
Observations on Sunday 18Observations on Sunday 18thth April April
Chilbolton Doppler lidar: 18 AprilChilbolton 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
Aerosol optical depth at several wavelengths from the Chilbolton sun photometer, courtesy Charles Wrench of STFC
Chilbolton UV lidar: 18Chilbolton UV lidar: 18thth April April
Depolarization implies it is volcanic ash
Entrained into and diluted by existing boundary-layer aerosol when boundary layer grows?
Sun photometer sizes, 18Sun photometer sizes, 18thth April April
Cabauw EZ-lidar, Netherlands, 18Cabauw EZ-lidar, Netherlands, 18thth April AprilCourtesy of David Donovan, KNMI <donovan@knmi.nl>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, 18Cabauw EZ-lidar, Netherlands, 18thth April AprilCourtesy of David Donovan, KNMI <donovan@knmi.nl>Courtesy of David Donovan, KNMI <donovan@knmi.nl>
• Further images: http://www.knmi.nl/~knap/lidar_cabauw/
Another layer coming in?
Observations on Monday 19Observations on Monday 19thth April April
Chilbolton Doppler lidar, 19Chilbolton Doppler lidar, 19thth April April
Deeper more dilute layer of volcanic ash above the boundary layer?
Chilbolton UV lidar, 19Chilbolton UV lidar, 19thth April April
Deeper more dilute layer of volcanic ash above the boundary layer?
Weaker depolarizing signature