[asac] [almanews] ALMA Memo 363 Released

[Carolyn White]

ALMA Memo #363

Velocity of the Effective Turbulence Layer at Chajnantor Estimated From
183 GHz Measurements

Guillermo Delgado and Lars-Ake Nyman

The cross-correlation of the precipitable water vapour (PWV) variation
of two water vapour radiometers placed at the ends of a 300-m baseline
along the E-W direction was used to measure the time lag between the
structures seen at each radiometer. With a fixed separation between the
two radiometers, and observing with parallel beams, we can determine the
effective speed of the turbulence layer.


A linear relation between the turbulence layer speed and the ground
wind speed was found indicating that the turbulence speed is roughly twice
as high as the ground measured wind speed.

The temporal phase structure function was calculated, showing that the
turbulence screen goes through a daily cycle associated with the
insolation, with a well-defined layer structure at the extremes of the
cycle.

The cross-correlation method allows the reliable measurement of the
effective turbulence speed for about 90% of the day, apparently associated
with the wind direction and the time when the turbulence layer is high up
(the definition of “high up” is still uncertain). This implies that
with a variation of this method, by moving the beams in the sky, we can
continuously measure the height of the turbulence layer during most of the
day, providing a very useful tool to assess the dynamics of the turbulence
layer.

View a pdf version of the file at URL:

     http://www.alma.nrao.edu/memos/html-memos/alma363/memo363.pdf


Download a ps version of the file at URL:

         http://www.alma.nrao.edu/memos/html-memos/alma363/memo363.ps




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<P><EM> Last modified: April , 2001</EM></P>
<P><ADDRESS><A HREF="mailto:alma-memos at nrao.edu">alma-memos at nrao.edu</A></ADDRESS></P></BODY>

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