# ALMA MEMO #500

Wind power spectrum near Chajnantor

## S. Sakamoto (NAOJ),
H. Ishizaki (NAOJ),
K. Kohno (IoA, U. Tokyo)

2004-05-20

Keywords: site testing, wind, gust, pointing

We present wind power spectral density up to ~ 1 Hz measured near Cerro Chajnantor. The power spectral density of variable component is represented by its value around 0.1 Hz. Median values of the wind power spectral density are smaller than those predicted by the Simiu model with a roughness length of 0.05 m in most cases. The 90-percentile values of the wind power spectral density are approximately three times larger than the corresponding median values. The wind power spectral density is not a simple function of mean wind velocity, and sometimes shows significant suppression after sunset. There exists no significant change of the wind power spectral density and roughness length that depend on the wind direction. Agreement of wind velocity measured at 8.0 m and 3.0 m above the surface within a few 10% difference supports small roughness length (<< 0.5). The van der Hoven spectrum extracted from long-term weather station data implies that the weather system in this area is very stable and change of mean wind velocity (i.e., duration of gusty conditions) is dominated by the diurnal cycle.

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