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ALMA Memo #333

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 2.5x2.5 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.

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Last modified: November 13, 2000