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In situ measurements of water vapor isotopic composition from Mauna Loa, Hawaii, are merged with soundings from Hilo to show an inverse relationship between the estimated inversion strength (EIS) and isotopically derived measures of lower-tropospheric mixing. Remote sensing estimates of cloud fraction, cloud liquid water path, and cloud top pressure were all found to be higher (lower) under low (high) EIS. Inverse modeling of the isotopic data corresponding to terciles of EIS conditions provide quantitative constraints on the last-saturation temperatures and mixing fractions that govern the humidity above the trade inversion. The mixing fraction of water vapor transported from the boundary layer to Mauna Loa decreases with respect to EIS at a rate of about 3%K-1, corresponding to a mixing ratio decrease of 0.6gkg(-1)K(-1). A last-saturation temperature of 240K can match all observations. This approach can be applied in other settings and may be used to test models of low-cloud climate feedbacks.

Plain Language Summary Water vapor plays a central role in Earth's climate, especially in the ways that it relates to the formation of clouds. The potential for cloud abundance in the lower atmosphere to change as the climate warms is one of the most important unresolved questions in climate science and depends on the ways that water vapor is mixed in the lower atmosphere. This mixing process is hard to assess from observations, but this study shows a new way to use tiny variations in the chemistry of water to constrain this mixing. This technique can be applied to observations or climate model output and may help reduce the uncertainty in projections of future warming.