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ALMA Memo # 542

Height and Velocity of the Turbulence Layer at Chajnantor Estimated From Radiometric Measurements

Juan Pablo Pérez Beaupuits, Roberto C. Rivera, Lars-Ake Nyman


The height of the turbulence layer above Chajnantor was determined using two Water Vapour Radiometers (WVR) located along a 300m East-West baseline. The radiometers measure the fluctuations in the precipitable water vapour (PWV). The fluctuations are similar in both WVR and thus a cross-correlation of the signal of both radiometers make possible to determine the transit time from the correlation time lag. With a known transit time, and with both beams parallel along the East-West baseline, it is possible to obtain the velocity of the turbulence layer, projected along the baseline. Using the projected turbulence velocity together with a second transit time found with the beams crossed at a given height, we obtained the height of the turbulence layer. Data were taken during a two-month period in December 2003 and January 2004. The most reliable estimates of the velocity and height of the turbulence layer occurred between 14 and 22 UT, which corresponds to about 38% of a whole day. During this period the median height of the turbulence layer is about 600 m. About 70% of the analyzed time the turbulence layer was below 1000 m. During night-time, the estimate of the height becomes less accurate because the wind speeds are lower, the wind direction changes from East-West towards North-South, and the amount of PWV seems to be very low, which all together is reflected in weaker signals in the radiometers and less amount of strong cross-correlations.

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Last modified: 2005-11-13