[asac] [almanews] ALMA Memo No. 333 Released

[Carolyn White]

ALMA Memo No. 333

52 YEARS OF CLIMATOLOGICAL DATA FOR THE CHAJNANTOR AREA 

Ricardo Bustos, Guillermo Delgado, Lars-Ake Nyman, and Simon Radford

November 2000

We present 52 years of weather data for the Chajnantor area. This data has
been prepared by the National Center for Environmental Prediction (NCEP)
and the National Center for Atmospheric Research (NCAR) from historical
weather observations and state of the art climatological models. The basic
idea of the NCEP/NCAR reanalysis project is to provide reliable historical
weather data from 1948 to the present. The data is referenced to a
2.5ºx2.5º global grid. One of these grid points is located
in Salar de Chalviri (Bolivia), about 60 km to the north of Chajnantor at
a height of approximately 4,000 m.

The reanalysis data produced for this grid point are estimated from
radiosondes, weather stations, satellites, and other sources scattered in
a large area including Chile, Peru, Bolivia and Argentina. In order to
find out if this data can be used to study the long term weather pattern
on Chajnantor, at a height of 5,050 m and 60 km away, we compared 5 years
of ground temperature and precipitable water vapour (PWV) measured at
Chajnantor between 1995 and 2000. The PWV at Chajnantor is calculated from
opacity measurements by a 225 GHz tipping radiometer.

The comparison of the NCEP/NCAR reanalysis data corrected for the altitude
of Chajnantor and the actual data measured during the last five years is
very good. We therefore believe that the reanalysis data can be used to
study the long-term climatological variations on Chajnantor.

The data presented for the 52-year period shows that the Chajnantor area
is indeed a place with very low PWV contents, with clear seasonal
indications of higher humidity during the austral summer. Climatological
cycles can be seen, but the 52 years of data show that anomalies larger
that the ones that we have experienced during the last years are not to be
expected.


View a PDF version of ALMA Memo #333 at
   http://www.alma.nrao.edu/memos/html-memos/alma333/memo333.pdf

Download postscript version of ALMA Memo #333 from
   http://www.alma.nrao.edu/memos/html-memos/alma333/memo333.ps
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