Rayleigh Brillouin Scattering Spectra in atmosphere: GroundWinds Brillouin scattering investigation Qiuhua Zheng, Ivan Dors, James Ryan University of New Hampshire
Jan 31, 2016
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