[asac][almanews] ALMA Memo 500 Released

[Stacy Oliver]

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 0.05m 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. 

View a pdf version of ALMA Memo #500. 
http://www.alma.nrao.edu/memos/html-memos/alma500/memo500.pdf

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