Observations of volcanic ash by lidar and MODIS

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 15 th, 1329 Vo lc an i c as h Summary from MODIS images • Icelandic wind from northwest • Further images: www. sat. dundee. ac. uk

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

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

Sunday 18 th, 1222 • Northerly winds weakening Is this the ash above the cloud? ing n e k Wea ont fr 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 Stationary colours in the sea (sediment and algae) ash c i an c Vol x Chilbolton NASA MODIS radiometer x Cabauw

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 m and 2 m 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. 001 i: volcanic ash is 1. 72 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, 16 th 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 (PM 10 s)

Ultraviolet EZ-lidar, Cardington Bedfordshire, 16 th 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, 16 th 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, 16 th 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 16 th April

th swa pso Cali Ash higher at leading (southern) edge, explaining the descending appearance to groundbased lidar ? ash c i can l o V er clouds Boundary-lay Simultaneous MODIS image

Observations on Saturday 17 th 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 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

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? Aerosol optical depth at several wavelengths from the Chilbolton sun photometer, courtesy Charles Wrench of STFC

Sun photometer sizes, 18 th April

Cabauw EZ-lidar, Netherlands, 18 th 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, 18 th 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 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