Atmospheric Profilers

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Atmospheric Profilers

• Marc Sarazin
• (European Southern Observatory)

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List of ThemesHow to find the ideal site...and
keep it good?

• Optical Propagation through Turbulence
• Mechanical and Thermal
• Index of Refraction
• Signature on ground based observations
• Correction methods
• Integral Monitoring Techniques
• Seeing Monitoring
• Scintillation Monitoring
• Profiling Techniques
• Microthermal Sensors
• Scintillation Ranging
• Modelling Techniques

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Outline

• Why do we need turbulence profiles?
• Microthermal sensing
• Sound back scattering
• Scintillation
• Mesoscale modeling

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Atmospheric Turbulence
Index of refraction of air
Assuming constant pressure and humidity, n varies
only due to temperature fluctuations, with the
same structure function
P,e (water vapor pressure) in mB, T in K, Cn2 in
m-2/3
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Turbulence Profilers
The various methods for generating atmospheric
turbulence profiles

• Full line Mauna Kea Model (Olivier, 94)
• Dots SCIDAR, Paranal
• Dashes Baloon borne microthermal sensors

Ref PARCSA Campaign, Univ. of Nice, 1992-1993
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Balloon Borne Profilers
Two Microthermal sensors, 1m apart are attached
far below the load of a standard meteorological
radiosondes

• Pros
• Provide Temperature, Humidity and Wind
• High vertical resolution (5m)
• Cons
• An ascent last one hour or more
• The balloon drifts horizontally (30 to 100 km)
• Expensive technique (1kUS/flight)

The rms of the differential temperature
fluctuations over a few seconds is computed
onboard and transmitted to the ground
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Doppler SODAR Profiler
Sound Detection And Ranging Monitoring the
backscattered acoustic energy from the
atmospheric layers
The acoustic backscattering cross-section is a
function of acoustic wavelength, absolute
temperature and temperature structure coefficient
Source http//www.remtechinc.com/sodar.htm
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Doppler SODAR Profiler

• Pros
• Provides Wind profile (design goal)
• Good vertical resolution (30m)
• Fully automated
• Cons
• Only relative Cn2 measurements no absolute
  calibration (the sound absorption by air depends
  on T,Rh profiles which are unknown)
• Limited altitude range (lt1km) when there is
  little turbulence

Source http//www.remtechinc.com/sodar.htm
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The SCIDAR
SCIntillation Detection And Ranging (J. Vernin,
1979) Analysis of the interference pattern
produced at the ground by the light of two
closeby sources diffracted by a turbulence layer
The aurocorrelation of the pupil scintillation
pattern shows a peak for the distance BC. The
separation of the double star scales the altitude
of the turbulence layer
Ref tutorial at the Imperial College Site
http//op.ph.ic.ac.uk/scidar/scidar.html/
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The SCIDAR
Practically, thousands of frames of lt1ms exposure
are combined to generate one profile every minute
Ref tutorial at the Imperial College Site
http//op.ph.ic.ac.uk/scidar/scidar.html/
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The SCIDAR
Optical Setup 2 focal plane with field stop 3
collimator 4 chromatic filter 5 conjugate pupil
plane 6 detector
Detector and pupil plane conjugate are collocated
in non generalized mode
Source A. Tokovinin, Study of the SCIDAR concept
for Adaptive Optics Applications, ESO-VLT Report
TRE-UNI-17416-0003
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The SCIDAR
The double star separation, and the telescope
diameter set the altitude range Fig auto
correlation shift X in generalized mode, with the
detector conjugated at a plane 5km below ground,
as a function of turbulence altitude. The minimum
characteristic size of the scintillation patterns
is 3.5cm. Telescope diameter1.2m
Pixel size2cm
Source A. Tokovinin, Study of the SCIDAR concept
for Adaptive Optics Applications, ESO-VLT Report
TRE-UNI-17416-0003
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The SCIDAR
Scidar Profile, seeing 1
Scidar Profile, seeing 2
Ref tutorial at the Imperial College Site
http//op.ph.ic.ac.uk/scidar/scidar.html/
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The SCIDAR and the Models
MM5, a mesoscale model is available as freeware.
It is used at the Mauna Kea Weather Center
(http//hokukea.soest.hawaii.edu/forecast/mko/) to
produce vertical profiles of the
turbulence. Comparison of MM5 profiles above
Mauna Kea Observatory with in situ SCIDAR
observations
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The SCIDAR and the MASS
A single star profiler the MASS Multi Aperture
Scintillation Sensor
A portable instrument for site surveys with a
reduced altitude resolution (1km instead of 200m)
A. Tokovinin, V. Kornilov Measuring turbulence
profiles from scintillation of single stars, IAU
Site 2000 Workshop, Marrakech, Nov. 2000