Thirty Meter Telescope Site Testing X: Precipitable Water Vapor

Abstract

The results of the characterization of precipitable water vapor in the atmospheric column carried out in the context of identifying potential sites for the deployment of the Thirty Meter Telescope (TMT) are presented. Prior to starting the dedicated field campaign to look for a suitable site for the TMT, candidate sites were selected based on a climatology report utilizing satellite data that considered water vapor as one of the study variables. These candidate sites are all of tropical or subtropical location at geographic areas dominated by high-pressure systems. The results of the detailed on-site study, spanning a period of 4 yr, from early 2004 until the end of 2007, confirmed the global mean statistics provided in the previous reports based on satellite data, and also confirmed that all the candidate sites are exceptionally good for astronomy research. At the locations of these sites, the atmospheric conditions are such that the higher the elevation of the site, the drier it gets. However, the data analysis shows that during winter, San Pedro Mártir, a site about 230 m lower in elevation than Armazones, is drier than the Armazones site. This finding is attributed to the fact that Earth's atmosphere is largely unsaturated, leaving room for regional variability; it is useful in illustrating the relevance of in situ atmospheric studies for understanding the global and seasonal variability of potential sites for astronomy research. The results also show that winter and spring are the driest seasons at all of the tested sites, with Mauna Kea (in the northern hemisphere) and Tolonchar (in the southern hemisphere) being the tested sites with the lowest precipitable water vapor in the atmospheric column and the highest atmospheric transmission in the near and mid-infrared bands. This is the tenth article in a series discussing the TMT site-testing project.

Worldwide site comparison for submillimetre astronomy

“…A radiometer at 225 GHz is operating at the CSO 5 on Mauna Kea. PWV quartiles between 1997 and 2001 can be found in Otárola et al (2010) and Radford (2011), 1.08 mm (25%), 1.82 mm (50%), and 3.32 mm (75%). The difference with the satellite measurements is around 15-20% and can again be caused by variations between the periods 1997-2001 and 2008-2010.…”

Atmospheric extinction properties above Mauna Kea from the Nearby SuperNova Factory spectro-photometric data set

“…For these reasons, such aerosol measurements at Mauna Loa have only been carried out during daytime, and usable data requires reasonably good weather conditions (no clouds). (Otárola et al 2010) from the atmospheric optical depth at 225 GHz data by the 225 GHz tipping radiometer at the Caltech Submillimeter Observatory (Peterson et al 2003). site is almost 1000 m higher than MLO, and the fact that the AERONET observations are carried out during day time make a point to point comparison difficult. Nevertheless, the general features of both distributions still can be compared with each other: the mean SN Ångström exponent (1.3 ± 1.4) is compatible with the one measured by AERONET (1.2 ± 0.9).…”

“…In order to check the consistency of our approach, we compare our water intensity measurement to the PWV amount at Mauna Kea. For that purpose, we used the optical depth at 225 GHz data from the Caltech Submillimeter Observatory (CSO, Peterson et al 2003) and the empirical formula from Otárola et al (2010) to compute the PWV at Mauna Kea during the SN observation campaign (cf. Fig.…”

Astroclimatic Characterization of Vallecitos: A Candidate Site for the Cherenkov Telescope Array at San Pedro Mártir

“…The results of the study by Otárola et al (2010), spanning a period of 4 years from early 2004 until the end of 2007, confirmed that the OAN site is dry and exceptionally good for astronomy research. The data analysis shows that during winter, even though the SPM site is about 230 m lower in elevation than Armazones, it is drier than the Armazones site.…”