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ALMA Memo # 529

Velocity and Structure Function of Phase Screen aloft Chajnantor
Hideharu Ishizaki and Seiichi Sakamoto


We analyzed the phase data from simultaneous measurements with the two sets of radio seeing monitors aligned in series at the ALMA site.

During the one month period in summer from 1999 November 11 to December 12, the median value of the spatial phase structure function exponent deduced from the 300 m and 600 m baseline data was 0.59.

The exponent independently extracted from the temporal phase structure function at the 300 m baseline was 0.62, supporting the above value obtained from the spatial phase structure function. These median values lie midway of the values for thin screen (1/3) and thick screen (5/6). No evidence for diurnal variation of the temporal phase structure function exponent was found in our data on the contrary to the previous report.

The phase screen velocity calculated from the lag time of the maximum correlation of the two sets of phase data had good correlation with the surface wind velocity at 8 m except for higher values by about 1.2. This indicates that the screen height deduced using the surface wind velocity as the screen velocity needs to be corrected upward. Provided the logarithmic wind profile and plausible roughness length (z_0 ~ 0.05), the measured small ratio is understood as a reflection of small height difference of the flows, namely very low height to the phase screen (~ 100 m).

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Last modified: 2005-06-24