Download 59 - Keele Astrophysics Group

Revealing the onset of convection in terrestial planet atmospheres
free convection observed by the TMT Site Testing
Sebastian Els 1 & Konstantinos Vogiatzis 2
IAU Symposium 239
Convection in Astrophysics
Poster 239-59
Prague, 21-25 August 2006
1
AURA-NIO, CTIO, Casilla 603, La Serena, Chile, [email protected]
2
AURA-NIO, NOAO, 950 North Cherry Avenue, Tucson, AZ 85719, USA, [email protected]
Abstract The site testing program for the Thirty-Meter-Telescope (TMT) is monitoring the seeing conditions above several mountains.
These mountains represent very different terrains and topographical conditions. However, it is found that the ground layer seeing is strongly
influenced by the onset of convection under radiation dominated conditions. Observations and computational fluid dynamics (CFD) modelling
of seeing conditions above these sites is presented. As the appearance of excellent seeing conditions is supressed under certain atmospheric
conditions this allows to define a quantitative threshold based on observables for the onset of convection.
The TMT site testing instrumentation
The site testing program for TMT monitors five mountains for their atmospheric properties. On
each of these mountains a robotic set of instruments is deployed [1]. The instrumentation consists
of
- DIMM (Differential Image Motion Monitor [4]):
> seeing through entire atmosphere
- MASS (Multi Aperture Scintillation Sensor [5]):
> Cn2(z) at z =0.5, 1.0, 2.0, 4.0, 8.0, 16.0 km above ground
> seeing above 500 m
- Meteorological station z=2m above the ground:
> T, atm. P, rel. humidity, wind speed (ws) and direction, net radiation, ground heat flux
- Sonic anemometers next to the telescope at 7m altitude
> 3D windspeed
Additional instrumentation is deployed temporaly on each site: SODAR, IRMA.
Furthermore 30m towers are currently being erected to measure in detail the first 30m above
ground.
Low wind
e dyne therm
L
e dyne therm
Unstable atmosphere
z/L < -1
Measuring the seeing conditions of the lowest 500m of the atmosphere
From DIMM and MASS seeing data the seeing component of the first 500 m is computed by
GL=S
Where T is the temperature, =0.4 the von Karmann constant, g the acceleration of gravity and  atmospheric
density.
The sensible heat flux Q H =H /c p  with H as the difference of the radiative flux and ground heat flux.
Using the wind speed ws(z) measurements at z=2m and assuming for the surface roughness r=0.005 m
(corresponding to ~5cm features) the friction velocity can be estimated by u= ws  z /ln  z / r 
Left: The night of April 06. 2006 taken at Cerro Armazones. The
dots show the seeing of the groundlayer (GL) from z=7m (telescope
level) to 500m above ground. The Monin-Obukhov Length L is
shown as dashed line. The strong increase of the GL seeing between
1:30 UT and 2:00 UT coincides with a drop of -L below 10m. Under
these circumstances the condition for free convection z/L < -1 is
fullfilled very close to the ground. As the wind picks up again, the
dynamic layer grows and the the formation of plumes ranging high
above the ground is supressed. The seeing returns to its average
value.
Neutral atmosphere
0 > z/L > -1
e dyne therm
e dyne therm

Atmospheric stability
The stability conditions in the atmosphere are described by the Monin-Obukhov length L [1] which indicates the
height at which the turbulent energy production rate e due to dynamic effects is equal to turbulent engery
production due to thermal (buoyancy) effects
resulting in the stability conditions at height z
- unstable
z / L−1 free convection
3
−u T  c p 
- neutral
−1z / L0
L=
 g Q H 
- stable
z / L0
The TMT site testing equipment on Cerro Armazones.
On top of the 7m white tower a 35cm telescope with a MASSDIMM device is installed. The smaller tower to the left is the
meteostation, recording atmospheric parameters 2m above the
ground. Below: The arrows sketch a neutral atmosphere (-1< z/L
< 0) at which the dynamic layer extends well above the telescope
level. Above: Here an unstable atmosphere (-1> z/L) is outlined.
The condition for free convection is fullfilled very close to the
ground.
L
−S
5/3 3/5
M
Occasional strong increases of this seeing were found to be correlated with drops of windspeed.
As pointed out in [3] such observations show that Computational Fluid Dynamics (CFD) properly predicts the
effects of convection. In the following we will demonstrate under what atmospheric conditions free convection
starts to occur and affects the seeing of the lowest atmospheric layer in the example case of Cerro Armazones in
northern Chile.
Even though undesired, such effects are likely to occur on every potential and existing observatory site.
Sensible heat flux
Medium strong wind
5/3
D
Right: 1.5 yrs of data from Cerro Armazones.
The quotient of H, the difference of the radiative flux from
the ground and the ground heat flux, and the wind speed ws
shown. The increase of seeing from the very best values to
about average is apparent beyond 30 J/m3 (cyan line).
Towards H/ws=0 mechanical turbulence also increases the
seeing.
Zone of free convection (z/L<-1)
Sensible heat flux
Summary and Outlook
* The negative influence of free convection on the GL seeing has been clearly
measured.
* Adaptive Optics, but also other observing techniques, can perform best under
stable seeing conditions. By having quantitative indicators for the stability of the
current atmosphere the observer can take a more sophisticated “guess” as to what
expect and how to plan the next hour of observation. Using H/ws is an easy to
measure indicator (no knowledge of u needed) and is linked directly to L.
* Future measurements of temperatures and wind speed at different heights
above the ground will help to understand the detailed physics further.
References
[1] Skidmore, Schoeck, Tokovinin et al. (2004), SPIE, 5489-154
[2] Monin & Obukhov (1954) Trudy geofiz inst AN SSSR 24(151): 163
[3] Els & Vogiatzis (2006), SPIE, 6267-55
[4] Tokovinin (2002) PASP, 114, 1156
[5] Kornilov, Tokovinin, Vozyakova O., et al., (2003), SPIE, 4839, 837-845
Right: The GL seeing vs. Monin Obukhov Length L
measurements only in the domain where H/ws is
larger than 30 J/m3. In this domain L is always found
to be less than 10m, showing that the H/ws value is
also a good indicator for free convection.
Acknowledgment
The authors gratefully acknowledge the support of the TMT partner institutions. They are the
Association of Canadian Universities for Research in Astronomy (ACURA), the Association of
Universities for Research in Astronomy (AURA), the California Institute of Technology and the
University of California. This work was supported, as well, by the Canada Foundation for
Innovation, the Gordon and Betty Moore Foundation, the National Optical Astronomy
Observatory, which is operated by AURA under cooperative agreement with the National Science
Foundation, the Ontario Ministry of Research and Innovation, and the National Research Council
of Canada.