July 2001 Zanjan, Iran 1

Atmospheric Profilers

Marc Sarazin(European Southern Observatory)

July 2001 Zanjan, Iran 2

List of Themes

How 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

Outline

• Why do we need turbulence profiles?

• Microthermal sensing

• Sound back scattering

• Scintillation

• Mesoscale modeling

July 2001 Zanjan, Iran 4

Index of refraction of air

Assuming constant pressure and humidity, n varies only due to temperature fluctuations, with the same structure function

matCT

PC Tn 5.01080 2

2

262

T

eP

Tn 48101052.71

106.771 23

6

Atmospheric Turbulence

P,e (water vapor pressure) in mB, T in K, Cn2 in m-2/3

July 2001 Zanjan, Iran 5

Turbulence Profilers

Ref: PARCSA Campaign, Univ. of Nice, 1992-1993

The various methods for generating atmospheric turbulence profiles

•Full line: Mauna Kea Model (Olivier, 94)

•Dots: SCIDAR, Paranal

•Dashes: Baloon borne microthermal sensors

July 2001 Zanjan, Iran 6

Balloon Borne Profilers

Two Microthermal sensors, 1m apart are attached far below the load of a standard meteorological

radiosondesPros:

•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)

3

222()( rCrrTrT T

The rms of the differential temperature fluctuations over a few seconds is computed onboard and transmitted to the ground

July 2001 Zanjan, Iran 7

Doppler SODAR Profiler

Source: http://www.remtechinc.com/sodar.htm

Sound Detection And RangingMonitoring the backscattered acoustic

energy from the atmospheric layers

2

2310075.0T

CT

The acoustic backscattering cross-section is a function of acoustic wavelength, absolute temperature and temperature structure coefficient

July 2001 Zanjan, Iran 8

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 (<1km) when there is little turbulence

Source: http://www.remtechinc.com/sodar.htm

July 2001 Zanjan, Iran 9

The SCIDAR

Ref: tutorial at the Imperial College Site: http://op.ph.ic.ac.uk/scidar/scidar.html/

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

July 2001 Zanjan, Iran 10

The SCIDAR

Practically, thousands of frames of <1ms 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/

July 2001 Zanjan, Iran 11

The SCIDAR

Source: A. Tokovinin, Study of the SCIDAR concept for Adaptive Optics Applications, ESO-VLT Report TRE-UNI-17416-0003

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

July 2001 Zanjan, Iran 12

The SCIDAR

Source: A. Tokovinin, Study of the SCIDAR concept for Adaptive Optics Applications, ESO-VLT Report TRE-UNI-17416-0003

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 diameter=1.2m Pixel size=2cm

July 2001 Zanjan, Iran 13

The SCIDAR

Ref: tutorial at the Imperial College Site: http://op.ph.ic.ac.uk/scidar/scidar.html/

Scidar Profile, seeing 1” Scidar Profile, seeing 2”

July 2001 Zanjan, Iran 14

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

July 2001 Zanjan, Iran 15

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