Rayleigh Brillouin Scattering Spectra in atmosphere: GroundWinds Brillouin scattering investigation

Qiuhua Zheng, Ivan Dors, James Ryan

University of New Hampshire

Principle to determine the wind velocity

Wind velocity is determined by the Doppler shift between unshifted Rayleigh Brillouin scattering model and measured RBS spectrum, GroundWinds lidar is the first lidar system that provides a chance to directly measure the RBS spectra in the air.

Three regimes of Rayleigh Brillouin scattering

• –mean free path of the fluid is much larger than the incident light wavelength and the backscattering spectrum is gaussian.

• –mean free path of the fluid is much smaller than the incident light wavelength and the backscattering spectrum is three lorentzians.

• –mean free path of the fluid is of the order of the incident light wavelength and the backscattering spectrum is undetermined.

l>>λ

l<<λ

l~λ

l~λl<<λ l>>λ

TT

nnn T Δ

∂∂

+Δ∂∂

=Δ ρρρ

)) ρρ

Δ∂∂

=Δ T

nn )⇒

∝′′∝ ),(),(),( trEtrEkS ω

TT

PP PS Δ

∂∂

+Δ∂∂

=Δ ))ρρρ

nnn Δ+= 0

The scattering spectrum

Function of index refraction n

Rayleigh line shape in water

Benedek, G., et al(1971)

RBS spectrumGroundwinds NH lidar system

RBS spectrum

Raw fringe

RBS spectrum compared to Gaussian thermal broadening

Tenti S6 model

S6 model

Fitted Gaussian

Fit to Tenti S6 model

Temperature profile

Landau-Placzek ratio

0

1

2

3

4

5

6

7

8

9

10

0.1 0.15 0.2 0.25 0.3 0.35 0.4

Landau Placzek ratio

Altitude(km)

Series1 2 per. Mov. Avg. (Series1) 3 per. Mov. Avg. (Series1)

Aerosol/Molecular (AM) ratio

Summary• GroundWinds provides opportunity to measure

the RBS spectrum in air.• RBS spectrum in air has two parts: pressure and

entropy (T) fluctuations.• The better the model, the better temperature

profile, and the better the wind velocity we can measure.

• There still is room to improve Tenti’s S6 model. Extra info can be recovered by related RBS model research.

• Additional corrections are possible at low altitude when a priori Landau Placzek ratio is used

This work was supported by NOAA Grant This work was supported by NOAA Grant NA17EC1105NA17EC1105..

AcknowledgementsAcknowledgements

Molecular scattering

P

iE

scE

( )( ) ( )( )2

1111422

02

44

16

)(∑

⋅⋅+⋅⋅=

Ω l l

lsclllsc DeDeDeDe

c

e

d

d

ωεπ

ωωσ

h

The Krammers-Heisenberg formula

θ

Hydrodynamic model