Observations of volcanic ash by lidar and MODIS Robin Hogan University of Reading Last updated: 20 April 2010

Thursday 15 th, 1329 Summary from MODIS images Icelandic wind from northwest Further images: Volcanic ash

Friday 16 th, 1234 Volcano obscured by clouds Dilute volcanic ash measured over southern England and the Netherlands with lidar

Saturday 17 th, 1317 Wind at Iceland from the north Volcanic ash heading south behind a cold front Cold front Volcanic ash No depolarizing aerosol observed over Chilbolton or Cabauw

Sunday 18 th, 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 19 th, 1305 New ash entering a low pressure system

Observations on Friday 16 th April

16 April: 1044 UTC NASA MODIS radiometer

16 April: 1224 UTC NASA MODIS radiometer Stationary colours in the sea (sediment and algae) x Chilbolton x Cabauw Volcanic ash

Chilbolton 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 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 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 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 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 OConnor and Chris Westbrook, University of Reading...

Colour ratios for each combination Less than 1 Greater than 1 Close to 1 Note contrast with ordinary boundary layer aerosol

Colour 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: 905/1500 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 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 nm Assumed ash refractive index 1.5 – 0.001i: volcanic ash is 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 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, 16 th Apr

Aerosol particles (PM10s) No convincing sign of ash

Ultraviolet EZ-lidar, Cardington Bedfordshire, 16 th April This plot was produced by the University of Manchester, NCAS and FGAM.

RIVM Caeli lidar, Netherlands, 16 th April Courtesy of Arnoud Apituley RIVM Caeli lidar, Netherlands, 16 th April Courtesy of Arnoud Apituley This lidar is not operated all the time but has Raman capability Further images here: Volcanic ash just above boundary-layer

Cabauw EZ-lidar, Netherlands, 16 th April Courtesy of David Donovan, KNMI Cabauw EZ-lidar, Netherlands, 16 th April Courtesy of David Donovan, KNMI 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 16 th April

Simultaneous MODIS image Calipso swath Boundary-layer clouds Volcanic ash? Ash higher at leading (southern) edge, explaining the descending appearance to ground- based lidar

Observations on Saturday 17 th April

Chilbolton Doppler lidar: 17 April Further images at Normal aerosol particles in the boundary layer: no further sign of volcanic ash…

Chilbolton UV lidar, 17 April Normal aerosol particles in the boundary layer: no further sign of volcanic ash…

Observations on Sunday 18 th 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

Aerosol optical depth at several wavelengths from the Chilbolton sun photometer, courtesy Charles Wrench of STFC Chilbolton UV lidar: 18 th April Depolarization implies it is volcanic ash Entrained into and diluted by existing boundary-layer aerosol when boundary layer grows?

Sun photometer sizes, 18 th April

Cabauw EZ-lidar, Netherlands, 18 th April Courtesy of David Donovan, KNMI Cabauw EZ-lidar, Netherlands, 18 th April Courtesy of David Donovan, KNMI Similar signature observed by UV lidar at Cabauw De Bilt radiosonde put midday boundary-layer top at ~1 km

Cabauw EZ-lidar, Netherlands, 18 th April Courtesy of David Donovan, KNMI Cabauw EZ-lidar, Netherlands, 18 th April Courtesy of David Donovan, KNMI Further images: Another layer coming in?

Observations on Monday 19 th April

Chilbolton Doppler lidar, 19 th April Deeper more dilute layer of volcanic ash above the boundary layer?

Chilbolton UV lidar, 19 th April Deeper more dilute layer of volcanic ash above the boundary layer? Weaker depolarizing signature