Horizontal Temperature variations at Chajnantor Abstract
Return to Memo list
ALMA Memo # 541
Alison Stirling, Angel Otarola, Roberto Rivera, Juan Bravo
2006-03-26
In August 2005 an observing campaign was conducted to measure the horizontal variability in the temperature profile above the Chajnantor site. The temperature profile is known to affect pointing and phase corrections, as well as amplitude calibrations, and so knowledge of the likely variation in temperature is essential for planning ancillary meteorological equipment for the site.
The campaign concentrated on analysing the atmosphere in two locations of the extended array configuration. In these two sites, radiosonde balloons were launched at regular intervals and high frequency surface measurements were taken using a meteorological mast. The results of the study have shown that the temperature profile in the first 100m above the ground is strongly controlled by surface heating and cooling, and that variation in the altitude of the terrain can introduce horizontal temperature variations of up to 5 K over the site.
We have analysed the likely impact of these temperature variations on pointing and phase corrections, looking at the errors introduced by assuming that the temperature profile from one location can be used to estimate the pointing and phase corrections at the second location. We find that pointing errors introduced by using a temperature profile from a different part of the Chajnantor site are of order 0.3" at an elevation of 60 degrees. Path errors introduced as a result of using the distant temperature profile are of order 2%. These errors are similar in magnitude if an idealised temperature profile is used, in which a constant lapse rate is assumed, in conjunction with the measured surface temperature at that location.
In addition we have measured the parameters required for future atmospheric modelling studies of the site, for example the net radiation (incoming minus outgoing, shortwave and longwave) in August peaks at 460-495 W m at midday, and the surface albedo is 0.6. The surface sensible and latent heat fluxes peak at ~ 300 W m^-2 40 W m^-2, respectively, and the roughness length is measured to be ~1 cm. In the presence of antennas, this is expected to increase to 10 cm in the extended configuration, and 160 cm in the compact configuration, increasing the mechanically induced turbulence at the site.
View a pdf version of ALMA Memo #541.
