Optical and Radiative Properties of Ice Clouds

Transcription

1 Optical and Radiative Properties of Ice Clouds Lei Bi & Ping Yang Department of Atmospheric Sciences, Texas A&M University

2 Outline Ice Clouds & Science Background Development of Data Library o Select representagve ice crystal habits o Most updated refracgve index o State of the art computagonal methods PerspecGves & Summary 2

3 Comprehensive Ice Crystal Habits Exclusively nonspherical Endless ice habits Bailey and HalleL,

4 Cirrus clouds modulate outgoing far infrared radiagon Stackhouse and Stephens,

5 Spectral Signature: SensiGvity Yang et al (2003) 5

6 Ice Cloud Model Ice Cloud ScaLering Model GCMs Remote Sensing Image credit: Jess Landeros. My research effort is to develop new techniques and enhance the current modeling capabiliges to improve, refine and extend current ice scalering model.. 6

7 RefracGve Index: μm Warren & Brandt (2008) 7

8 Ice Crystal Habit: Droxtal Dominant in the uppermost portions of cirrus clouds 20-Faced Polyhedron 12 isosceles trapezoid 6 rectangular 2 hexagonal Source : Andy Heymsfield 8

9 Ice Crystal Habit: Hollow Bullet RoseLes Photo from Stephen G. Warren Yang et al (2008) 9

10 Ice Crystal Habit: Aggregates (Xie, 2011) CPI Imag (Um & McFarquhar,2009) Compact aggregate of columns (Yang et al 1998) 10

11 Ice Crystal Habits Existing Habits Incorporated Roughness Condition Surface roughness were observed for single crystals and polycrystalline ice by using an electronic microscope. Images adapted from Cross, 1968! New Habits Yang et al. (2008) Xie et al. (2011) 11

12 Essence of RadiaGve InteracGon ElectromagneGc wave! " H = # $E c $t! " E = % µ $H c $t! & E = 0! & H = 0 James Clerk Maxwell (13 June November 1879) 12

13 SoluGon of Maxwell s EquaGons & MathemaGcally Equivalents Van de Hulst (1957) 13

14 A Combination of Methods FDTD/DDA FDTD / ADDA +IGOM IGOM Yee grid (Yee, 1966) Dipoles Ray tracing Size Parameter ElectromagneGc Spectrum (Image Credit: NASA) 14

15 Discrete-Dipole-Approximation (DDA) Method ApJ (1988) Efficient parallel version codes (DDSCAT, ADDA) have been designed(draine, 2006, ;Yurkin, 2006). 15

16 Time-Domain Methods MODEL near field far field radiagon zone pargcle incident beam absorbing boundary condigon Finite difference Gme domain (FDTD): Yee grid (Yee, 1966) Pseudo spectral Gme domain (PSTD): "E z sca (r i+1 / 2, j,k, t n ) "y = F #1 {#ik y F[ E sca z (r,t n )]} 16

17 Geometric OpGcs Method Incident Wavefront External reflecgon Basic Principle Ray / Beam / Photon, Intensity, PolarizaGon, Phase Ray/Beam Tracing Snell s Law, Fresnel Formulas Fraunhofer DiffracGon 1 st order transmiled light 2 nd order transmiled light In a more rigorous sense, this method is of physics geometric opgcs hybrid (PGOH). 17

18 Ray/Beam-tracing Algorithm Incident Rays Broad Beam ConvenGonal ray tracing (a) Beam spliong technique (b) (a) Larger size parameter, larger number of rays (b) Number of beams is irrelevant to size parameter. Similar studies: Popov, 1996; Borovoi,

19 Inhomogeneous Plane Waves! i! r N r = { m 2 r! m 2 i + sin 2 " i + (m 2 r! m 2 i! sin 2 " i ) 2 + 4m 2 m 2 ir } / 2 m! t N i = cos! t {"(m 2 r " m 2 i " sin 2! i ) + (m 2 r " m 2 i " sin 2! i ) 2 + 4m 2 m 2 ir }/ 2 E exp(!kn i l)exp(ikn r ê t "r ) sin! i = msin! t sin! i = N r sin! t Effective refractive index (Yang and Liou, 2009) 19

20 Edge-Effect Question: How to understand the difference between extinction efficiency factors simulated from the PGOH and exact methods (e.g. DDA)? Hexagonal pargcle. Size parameter is

21 Mie formula: Localization Principle " 2n + 1 S 1 = # a n $ n + b n % n n(n + 1) n=1 " ( ) 2n + 1 S 2 = # a n $ n + b n % n n(n + 1) n=1 ( ) A term of the order n corresponds to a ray passing the origin at a distance (n + 1/2)" /2# Van de Hulst (1957)

22 Extinction Efficiency 4m exp{ i(m " 1)kL} # & Q ext = 2 Re$ 1" ' % (m +1) 2 " (m" 1) 2 exp( i2mkl )( d = (n + 1/2)" /2# 22

23 Q ext, edge = f e (kl ) 2 / 3 Edge effect incorporated. Q abs, edge = f a (kl ) 2 / 3 23

24 ExGncGon and AbsorpGon 24

25 Hexagonal Column 25

26 Diffraction and External Reflection (Bi et al, 2010) 26

27 Impact of the Number of pargcle s OrientaGons m = 1.3+i1.0 Increasing the number of orientagon of model pargcles! 27

28 Temperature dependent refracgve index Iwabuchi and Yang,

29 Iwabuchi and Yang,

30 T-matrix # n " " E inc (r) = a mn RgM mn (r )+ b mn RgN mn (r ) E sca (r ) = n=1 m=!n # n " " n=1 m=!n p mn M mn (r )+ q mn N mn (r) E(r ) = E inc (r) + d 3 r 'G(r, r ')(m 2!1)E (r ') " V! # "# p mn q mn $ & %& = ) ( n ' ( n '=1 m'='n '! # # " 11 T mnm ' n' 21 T mnm ' n' 12 T mnm' n ' 22 T mnm' n ' $! &# & %" # a m ' n' b m 'n ' $ & %& Mie theory: inscribed sphere IteraGon: remaining volume Extended Boundary Condition Method (Waterman, 1965) V out V 1 Accurate; Stable; Efficient for random orientagon V 0 V in s 30

31 EBCM T matrix and new T matrix Spheroid P _ 10 _ CalculaGon of EBCM is from Mishchenko s code. 31

32 Summary Single scalering property datasets have been computed for the wavelengths from 15~99 μm: Using the updated ice index reported in Warren and Brandt; Improved accuracy of scalering calculagons 9 representagve ice crystal habits PerspecGves: Temperature dependence (ice crystal habit and refracgve index) Volume integragon T matrix calculagon to further improve the efficiency and accuracy 32