Local Monitoring of Atmospheric Transparency from the NASA MERRA-2 Global Assimilation System

Ground-based astronomy has to correct astronomical observations from the impact of the atmospheric transparency and its variability. The current objective of several observatories is to achieve a sub-percent-level monitoring of atmospheric transmission. A promising approach has been to combine internal calibration of the observations with various external meteorological data sources, upon availability and depending on quality. In this paper we investigate the use of the NASA Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) which is a general circulation model (GCM) and data assimilation system that renders freely available for any given site, at any time, all the parameters constraining atmospheric transmission. This paper demonstrates the extraction of the relevant atmospheric parameters for optical astronomy at two sites: Mauna Kea in Hawaii and Cerro Tololo International Observatory in Chile. The temporal variability for the past eight years (annual, overnight, and hourly) as well as the spatial gradients of ozone, precipitable water vapor, and aerosol optical depth are presented and their respective impacts on the atmospheric transparency are analyzed.