© crown copyright met office electromagnetic and light scattering by atmospheric particulates: how...

© 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


ContentsThis 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


Why ice crystals & dust ?

© Crown copyright Met Office© 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


Wind blown mineral dust aerosol

Uncertainties

SW flux ~± 46 Wm-2


Volcanic Eruptions

Icelandic Volcano, April 2010


Challenges posed by the new remote sensing era


The new remote sensing problem: The A-Train Constellation

Lidar

94 GHz cloud-profiling radar

Solar reflection & Infrared transmission

Total & polarized solar reflection


Observed shapes & sizes of atmospheric particulates: Which model geometries ?


Tropical Example, A. Heymsfield

Fresh Anvils, Um and McFarquhar 2008.University of Manchester


Dm 10 µm

Dm ~ 4000 µm

X ~50 – 23000 !

At ~ 0.55 µm


Mineral dust aerosol

Dm 1 µm

~ < 100 µm

X 5 ~ 500


Volcanic Dust (Iceland Volcano)

Dm 1 µm

~ several hundred µm X 5 ~ 500


Macke et al., 1996 Yang and Liou 1998

Labonnote et al.

2000

Baran and Labonnote 2006.

Traditional single models

Yang et al. 2003

Nousiainen 2004

McFarquhar 2002

Asano 1980


Consider an ensemble of ice crystals

Try to model what is observed

Ack: P Yang, B Baum and G Hong


Consider an ensemble of ice crystals

Number Concen

Ice crystal maximum dimension

Baran & Labonnote (2007)


The single-scattering properties & Solutions


Iinc, Qinc, Uinc, Vinc

Isca, Qsca, Usca, Vsca

V inc

U inc

Qinc

I inc

PP

PP

PP

PP

r

V sca

U sca

Qsca

I sca

4443

3433

2221

1211

2sca

00

00

00

00

4c

111

1

1( )sin 1

2P d

P12/P11 describes the degree of linear polarization (DLP)

-


Total Optical Properties1

111

(cos ) (cos )cosd P g = <cos>=

g=1 g=0 g= - 1

q)dqn()qS())q(Q)q((QK asext

0=ksca/(Ksca + Kabs)


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


Scattering parameters: Geometric Optics


Using Measurements to constrain theory


Sun

Aerosol/cloudlayer

Surface

Measure transmitted radiances thro’ cloud/aerosol layer


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


Mineral dust =0.55 m only

max surface

mean surface

aer= 1.0

Fails at more absorbing

wavelengths

+


What do space-based satellites show ?

Satellite Sampling


Tests against PARASOL total & polarized reflectance (up to 14 scattering angles)

Randomised Ensemble

Are the observations ‘truth’ ???

Need polarization measurements < 60o

Inappropriate phase function

© Crown copyright Met Office00/XXXX 48

What do single-particle levitation experiments show ?

J. Ulanowski, Univ Herts

Estimated g=0.79

Used to test fundamental scattering theory


An application to the Met Office Unified Model: Cirrus


Consider an ensemble of ice crystalsNumber Concen

Dm

Geometric Optics

0, g, ext

Directly Related to GCM IWC & Tc


0 and g in IWC-TC space

z=a+bTc+cqi


Ensemble (qi, Tc) - old qi/De scheme (Short-wave TOA): NO tuning

Diagnostic

Wm-2


Outstanding issues


•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 & Tc

•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

© crown copyright met office electromagnetic and light scattering by atmospheric particulates: how...

Documents

ice crystals dust

ensemble of ice crystalstry

g hongconsider

met office unified model

model geometries

scattering parameters

light scattering

polarization measurements