Water Vapor Isotopic Measurements on the Chajnantor Plateau, Chile and Implications for Subtropical Humidity Dynamics

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This is a project to collect and analyze measurements of water vapor isotope ratios and ozone concentrations at the site of the Atacama Large Millimeter Array (ALMA) observatory on the Chajnantor Plateau in Chile (23S latitude, elevation 5100m). The measurements will be used to study the processes which determine the humidity of subtropical air masses, which are known to be more moist than region of the tropical upper troposphere from which they descend into the subtropics. The site is ideal for making these measurements because it is one of the few places where air from the tropical upper tropopause can descend all the way to the Earth's surface without substantial modification during descent. It is also an extremely convenient site given the research infrastructure already in place for ALMA.The measurements address four questions: 1) What is the co-variability of water vapor and isotopic composition on daily, intraseasonal, seasonal, and interannual time scales? 2) How do upper tropospheric processes set the aridity of the Southern Hemisphere relative humidity minimum? 3) To what extent is subtropical humidity influenced by instrusions of stratospheric air? 4) What processes moisten the subtropical RH minimum to specific humidities above those encountered in the tropical upper troposphere? Ozone measurements are used to address question 3, as high ozone levels can identify intrusions of stratospheric air. The measurements will be complemented by a suite of diagonstic activities to identify water vapor transport pathways that account for the observed isotope ratios, specific humidity, and ozone concentration.The work has broader impacts due to the importance of water vapor in the atmosphere, both through its role in the hydrological cycle and through its radiative effect as a greenhouse gas. The measurements and their analysis will provide increased understanding of the pathways through which water vapor circulates in the atmosphere, and the processes which determine atmospheric relative humidity in dry regions, which may ultimately improve our ability to forecast weather and anticipate climate change. In addition, the project will educate and train a graduate student at a university with a substantial Hispanic and Native American student population.