Atmospheric characterization for sites of interest in millimetric/submillimetric astronomy.

Abstract

Water vapor is the main source of atmospheric opacity for mm/sub-mm astronomy, henee several studies seek to effectively characterize it for site testing puiposes. Reanalysis databases are quickly becoming popular as an alternative to on-site measurements due to easy accessibility and the versatility of the data they provide. In the framework of validating the usage of reanalysis data as a site-testing oriented tool, a statistical compaxison of atmospheric water vapor valúes obtainable from the MERRA-2 database is performed, with ground-based microwave radiometer measurements taken at two astronómica! sites in Chile: Llano de Chajnantor, Atacama, and Cerro Paranal, Antofagasta. MERRA-2 data was interpolated both vertically (across pressure levels) and geographically (latitude-longitude). For each site, different plots are generated: a direct temporal variation plot, to visually compare the data variation over time between both sources; a PWV vs. PWV plot, fitting a linear fit through robust linear regression and calculating both Pearson (r) and Spearman (p) correlation coefficients to look for correlations between both data sources; a histogram showing the distribution of the differences between MERRA-2 data and the water vapor measurements (defined as AP17V = PWVmerra-2 — PWl4ite), along with its standard deviation (<r), mean (p), and median valúes, aiming to better appreciate the similarities of the data sources over time; and a CDF plot to compare both data distributions disregarding timestamps. Finally, mm/sub-mm transmittance curves are created through the am atmospheric modeling software which uses ozone and temperatura data along with the verified water vapor data for both sites studied, as well as three other sites of interest for the next-generation Event Horizon Telescope: Las Campanas Observatory near La Serena, Chile; Valle Nevado, located near Santiago, Chile; and the General Bernardo O’Higgins base, located in the Antárctica. The interpolated MERRA-2 PWV valúes are highly correlated with the groundbased PWV valúes with a Pearson coefíicient greater than 0.9 and a Spearman coefficient higher than 0.9. Their dependence is however, not linear, but PWVapex = ni • PWVmerra-2, with m being higher than 0.9 in both cases. The difference histograms show an almost zero-centered distribution for Llano de Chajnantor, with a p valué of -0.02 and a median valué of —0.007. On the other hand, in Cerro Paranal, the difference histogram is slightly offset towards positive valúes, with a ¡i valué of 0.171 and a median valué of 0.256. This offset is most likely due to the strong winds present in the site’s location, cióse to the Pacific Ocean. Additionally, the transmittance curves show different performances depending on the site studied. Results obtained at Valle Nevado site suggest promising atmospheric conditions for astronomic observation in the mm/sub-mm range. The results obtained show that the atmospheric water vapor estimation using MERRA-2 data can be used for site-testing of new sites by evaluating the mm/submm transmittance profile through vertical pressure correction and averaging the closest grid points to the site. This new method opens the door for future sitetesting studies using MERRA-2 and potentially other reanalysis databases (i.e. ERA5) as a reliable source of Information.