ALMA Memo No. 497 ANALYSIS OF WIND DATA GATHERED AT CHAJNANTOR

th Floor, Tucson, AZ 85721 2004 May 7 Abstract A general description and statistics of the wind speeds and directions registered at the ALMA site during 2001 and 2002 are presented. Measurements from different wind directions within 22.5o (in azimuth), topographic sectors and two markedly different daily periods were obtained. Spectra of the wind turbulence are presented for three wind speed conditions. During the diurnal period the convective turbulence and mean wind speed determine the shape and magnitude of the wind spectra, whereas in the nocturnal period the effects of the mechanical turbulence, such as the wind shear and surface roughness together with the mean wind speed, become relevant. No significant differences were found between the spectra obtained from three topographic sectors in the diurnal period, whereas in the nocturnal period a statistical test showed a significant difference for a particular topographic sector and for a given wind speed range, reflecting changes in the wind spectrum structure due to the local topography. General models of average spectra were found for three mean wind speeds, in the diurnal and nocturnal periods. Relations between the models and the mean wind speed were found. These relations makes it possible to estimate the spectral behaviour of the wind at the ALMA site for different mean wind speeds, which will be useful in the study of the wind loading on the antenna structure and pointing. Based on the results we can also conclude that the performance of the ALMA antenna regarding wind load is expected to be better for both day time and nigh time periods.

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