© Crown copyright Met Office Electromagnetic and light scattering by atmospheric particulates: How well does theory compare against observation? Anthony Baran, Met Office, Exeter, UK. LMS NAMP, Durham, July 2010

© Crown copyright Met Office Contents This presentation covers the following areas Why are atmospheric particulates important to the climate system Challenges posed by the new remote sensing era Observed sizes & shapes of atmospheric particles: Which model geometries? The single-scattering properties & Solutions Using measurements to constrain theory An application to the Met Office Unified Model: Cirrus Summary & Future Measurements

© Crown copyright Met Office Why ice crystals & dust ?

© Crown copyright Met Office Midlatitudes 20-30% Tropics 50% - 60% Hydrological cycle Radiative (reflection, transmission, emission) Uncertainties Ice Mass ± 50% SW flux ~± 30 Wm -2

© Crown copyright Met Office Wind blown mineral dust aerosol Uncertainties SW flux ~± 46 Wm -2

© Crown copyright Met Office Volcanic Eruptions Icelandic Volcano, April 2010

© Crown copyright Met Office Challenges posed by the new remote sensing era

© Crown copyright Met Office The new remote sensing problem: The A-Train Constellation Lidar 94 GHz cloud-profiling radar Solar reflection & Infrared transmission Total & polarized solar reflection

© Crown copyright Met Office Observed shapes & sizes of atmospheric particulates: Which model geometries ?

© Crown copyright Met Office Tropical Example, A. Heymsfield Fresh Anvils, Um and McFarquhar University of Manchester

© Crown copyright Met Office D m  10 µm D m ~ 4000 µm X  ~50 – ! At ~ 0.55 µm

© Crown copyright Met Office Mineral dust aerosol D m  1 µm ~ < 100 µm X  5 ~ 500

© Crown copyright Met Office Volcanic Dust (Iceland Volcano) D m  1 µm ~ several hundred µm X  5 ~ 500

© Crown copyright Met Office Macke et al., 1996 Yang and Liou 1998 Labonnote et al Baran and Labonnote Traditional single models Yang et al Nousiainen 2004 McFarquhar 2002 Asano 1980

© Crown copyright Met Office Consider an ensemble of ice crystals Try to model what is observed Ack: P Yang, B Baum and G Hong

© Crown copyright Met Office Consider an ensemble of ice crystals Number Concen Ice crystal maximum dimension Baran & Labonnote (2007)

© Crown copyright Met Office The single-scattering properties & Solutions

© Crown copyright Met Office  I inc, Q inc, U inc, V inc I sca, Q sca, U sca, V sca P 12 /P 11 describes the degree of linear polarization (DLP) -

© Crown copyright Met Office Total Optical Properties g = = g=1 g=0g= - 1  0 =k sca /(K sca + K abs )

© Crown copyright Met Office X n X ~ 50 Electromagnetic T-matrix DDA FDTD Physical Optics X ~ 200 Modified geometric optics Geometric optics X >> 200 Ray Tracing 0   ~2.0

© Crown copyright Met Office Scattering parameters: Geometric Optics

© Crown copyright Met Office Using Measurements to constrain theory

© Crown copyright Met Office Sun Aerosol/cloud layer Surface Measure transmitted radiances thro’ cloud/aerosol layer

© Crown copyright Met Office Traditional single ice crystal models do not predict the measured transmitted intensity The best fits are obtained with phase functions representing an ensemble of ice crystals Cirrus layer =0.55 µm only

© Crown copyright Met Office Mineral dust =0.55  m only max surface mean surface  aer  = 1.0 Fails at more absorbing wavelengths +

© Crown copyright Met Office What do space-based satellites show ? Satellite Sampling

© Crown copyright Met Office Tests against PARASOL total & polarized reflectance (up to 14 scattering angles) Randomised Ensemble Are the observations ‘truth’ ??? Need polarization measurements < 60 o Inappropriate phase function

© Crown copyright Met Office

An application to the Met Office Unified Model: Cirrus

© Crown copyright Met Office Consider an ensemble of ice crystals Number Concen D m Geometric Optics  0, g,  ext Directly Related to GCM IWC & T c

© Crown copyright Met Office  0 and g in IWC-T C space z=a+bT c +cq i

© Crown copyright Met Office Ensemble (q i, T c ) - old q i /D e scheme (Short-wave TOA): NO tuning Diagnostic Wm -2

© Crown copyright Met Office Outstanding issues

© Crown copyright Met Office Theory: Can we predict the observed scattered intensity using one single scattering theory ? (Holy grail) Can we develop a model ensemble that is consistent across the electromagnetic spectrum – one theory – one model ? Can we predict the ensemble from first principles -> Aggregation, hollowness & surface roughness What is the role of electric fields ? Orientation ? Does it matter ? GCM & remote sensing in terms of optical properties related to the GCM variables IWC & T c To further constrain theoretical ice crystal models high-resolution simultaneous measurements from across the electromagnetic spectrum are required (visible – far ir : sub-mm – radar) High-resolution scattering angle measurements (250) of polarization & intensity from the forthcoming GLORY mission (November 2010) will be fundamental in further constraining ice crystal ensemble models Further cirrus aircraft measurement campaigns are require to measure ice crystal or dust shapes down to < 50 µm, hollowness & surface roughness ? The particle size distribution not yet fully characterized; refractive indices of dust from optical to sub-mm frequencies