Rayleigh-Brillouin Scattering Experiment with Atmospheric Lidar

Transcrição

Rayleigh-Brillouin Scattering
Experiment with Atmospheric Lidar
from a Mountain Observatory
Oliver Reitebuch, Christian Lemmerz,
Engelbert Nagel, Benjamin Witschas
Institut für Physik der Atmosphäre
Challenges for atmospheric
lineshape measurements
G a u s s + M ie
T e n ti + M ie
0 ,0 0 5
intensity [a.u.]
0 ,0 0 4
Scattering
ratio =1.01
0 ,0 0 3
0 ,0 0 2
0 ,0 0 1
0 ,0 0 0
-5 0 0 0
-4 0 0 0
-3 0 0 0
-2 0 0 0
-1 0 0 0
0
1000
 f r e q u e n c y [M H z ]
2000
3000
4000
5000
 Up to now no direct measurement of Cabannes
lineshape in the atmosphere performed; only
indirect via wind or temperature measurement
 Challenging measurement because
 difference between Gaussian and actual
lineshape is only a few % (up to 10% at ground)
=> requires to sample lineshape in the order of
100 points
 width of line is in the order 4 GHz FWHM to
6 GHz (total) at 355 nm => sampling with
50 MHz or 21 fm = 21*10-15 m
 Measured lineshape is convolution of atmospheric spectra and instrumental function
=> Instrumental function has to be known
with high accuracy
=> analysis with forward model or deconvolution is challenging for noisy measurements
 Small bandwidth aerosol contribution "contaminates spectra" significantly, because contribution
is convoluted with instrumental function
33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
Where measuring the Brillouin effect in the atmosphere?
 Measurement at a mountain observatory, because
 it is above aerosol-rich boundary layer
especially during high pressure weather
conditions in winter => no aerosol contamination
 possibility
of
horizontal
line-of-sight
measurement over distances of 20-30 km =>
horizontal averaging possible
 Mountain observatory Schneefernerhaus UFS is at
2650 m ASL and 300 m below Zugspitze summit =>
ambient pressure from 705 hPa to 730 hPa during
campaign, which should be large enough to see
Brillouin effect
 Only small Aerosol scattering disturbances will
occur  data of the German environmental agency
(UBA) shows that the mean particle density on UFS is
500 p/cm3 compared to 60000 p/cm3 in the valley.
 Additional
measurements
from
Microwave
Radiometer of horizontal temperature profile and
radiosondes from Innsbruck airport (30 km).
BRillouin Atmospheric INvestigation on Schneefernerhaus BRAINS 2009
 Use ADM-Aeolus prototype lidar - the ALADIN
airborne demonstrator A2D in horizontal pointing
geometry with wavelength of 355 nm
Zugspitze
2962 m
 Use of molecular Rayleigh channel with Fabry-Perot
interferometer to measure lineshape:
 drawback is very broad instrument function of
1.7 GHz compared to atmospheric width of 4 GHz
(FWHM) or filter function of laboratory setup at VU
Amsterdam with 0.23 GHz
 Keep Fabry-Perot interferometer constant and change
laser frequency in discrete steps over lineshape
(calibration mode of ADM-Aeolus); monitor laser
relative frequency with heterodyne unit and absolute
frequency with wavemeter
 Setup in laboratory together with H2O-Differential
Absorption Lidar DIAL of FZK/IMK (Trickl, Vogelmann)
during a period of 6 weeks in January to March 2009
 More than 1000 kg of equipment was brought to the
observatory by a cogwheel train
Horizontal LOS measurements of atmosphere and hard-target
DIAL
A2D
 Horizontal lidar measurements in nearly pure
Rayleigh atmosphere are performed, and
therefore averaging over all range gates is
possible => increase SNR (assumption that p
and T is constant is verified with MTP)
2800
2600
altitude [m]
2400
2200
2000
1800
1600
1400
0
2000
4000
6000
8000
d is ta n c e fro m S c h n e e fe rn e rh a u s [m ]
10000
 Hard Target measurements are possible over a
range of 10 km to verify the instrument function
with a Mie-type signal in addition to the internal
reference signal
Sampling of the atmospheric lineshape
and instrument function
 Atmospheric signal and internal reference for filter transmission is sampled with 240 frequency
steps with f =50 MHz (21 fm) over a frequency range of 12 GHz (5.1 pm)
 630 laser pulse returns with 60 mJ/pulse accumulated for every frequency step
 Some measurements were performed over 20 GHz with Δf = 50 MHz
atmospheric calibrationf =50Mhz - 26.01.2009 - 17.12 o´clock
500
5000000
intensity [a.u]
4000000
internal reference
layer 5
layer 6
layer 7
layer 8
layer 9
layer 10
layer 11
Chi^2/DoF
= 2672988684.57369
R^2
= 0.99856
I0
FSR
FWHM
f0
5282893.3054
13328.94598
1634.76056
6026.36317
±14693.9353
±340.00698
±7.06055
±2.25133
3000000
2000000
internal reference
400
400
intensity[a.u]
[a.u]
intensity
internal reference Airy fit
300
300
200
100
100
1000000
0
0
0
2000
4000
6000
8000
10000
12000
844740000
844,740
 frequency [MHz]
844745000
844,745
844750000
844,750
844755000
844,755
844760000
844,760
84476500
844,765
absolute
frequency [THz]
[MHz]
absolut frequency
How to compare measurements with models?
Ongoing PhD thesis by Benjamin Witschas to develop
method and analyse measurements from BRAINS
T = 253K
p = 5 1 1 ,9 hP a
y-pa ra m e ter = 0 ,2 3
F S R = 1 09 2 1 M H z
F W H M = 1 6 4 7 ,84 M H z --> R = 0 ,6 2 5 1 8
c o n v o lu tio n A iry-G a u s s
c o n v o lu tio n A iry-T e n ti
1 ,0
3)
4)
5)

The deviation between Gaussian model and Tenti
simulation is up to 5 % and has a characteristic
“fingerprint”.
0 ,8
intensity [a.u.]
2)
Develop accurate instrument function I(f) of the FabryPerot Interferometer => relevant for ADM-Aeolus
Atmospheric lineshape can be best modelled by Tenti
S6: But this is an iterative algorithm, not a function,
which can be analytically convoluted with instrument
function in order to fit to measurements
Calculate expected difference between convolution of
instrument function I(f) with Gaussian lineshape
G(f)*I(f) and lineshape from Tenti S6 T(f)*I(f)
=> "fingerprint" of Brillouin scattering
Calculate difference between measured lineshape
M(f) and Gaussian G(f)*I(f)
Compare expected difference (3) or "fingerprint" with
measured difference (4)
0 ,6
0 ,4
0 ,2
0 ,0
8 4 4 ,7 5 0
8 4 4 ,7 5 2
8 4 4 ,7 5 4
8 4 4 ,7 5 6
8 4 4 ,7 5 8
8 4 4 ,7 6 0
8 4 4 ,7 6 2
8 4 4 ,7 6 4
a b s o lu te fre q u e n c y [T H z]
0,06
deviation Gaussian
deviation Tenti
0,04
deviation
1)
0,02
0,00
-0,02
-0,04
844,735 844,740 844,745 844,750 844,755 844,760 844,765 844,770
frequency [THz]
Is fingerprint of Rayleigh-Brillouin scattering measured?
rel. deviation (fit and measurement)
844,745
rel. deviation Layer 6
mean rel. deviation Layer 6 to 9
simulation rel. deviation with Gaussian distribution
844,750
844,755
simulation
rel. deviation with
Tenit S6 distribution844,760
 The
deviations
from
measurement to Gaussian
assumption and simulation
with the Tenti S6 model to
Gaussian assumption have
the same characteristics.
0,08
0,08
0,06
0,06
0,04
0,04
0,02
0,02
0,00
0,00
-0,02
-0,02
-0,04
-0,04
-0,06
844,745
844,750
844,755
 The Tenti model describes molecular scattered
light (from gas in the kinetic
regime) quiet well.
 We have measured the
effect of Brillouin scattering
in the atmosphere the first
time!
-0,06
844,760
frequency [THz]
Summary
 For the first time the lineshape from Rayleigh-Brillouin scattering was
measured with high spectral resolution in the atmosphere
 Method to compare deviations of measurements from Gaussian lineshape and
expected from Tenti S6 model was developed including convolution with
instrument function
 Measurements confirm that the atmospheric lineshape deviates from a
Gaussian as expected
 Lineshape and deviations can be modelled with Tenti S6
 As an important secondary result an accurate description of the instrument
function of the sequential Fabry-Perot interferometer for ADM-Aeolus was
obtained:
 pure Airy-function is not sufficient to describe filter transmission
 refinement of the model was performed including plate defects similar to
an approach by McGill et al. (1997)
Next STEPS
 More than 50 spectral scans have been performed => analysis of spectra for
different conditions, e.g. influence of Mie-scattering
Excellent location for meetings

Rayleigh-Brillouin Scattering Experiment with Atmospheric Lidar

Transcrição

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