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