Pairing Measurements of the Water Vapor Isotope Ratio with Humidity to Deduce Atmospheric Moistening and Dehydration in the Tropical Midtroposphere

Measurements of the isotope ratio of water vapor (expressed as the δ value) allow processes that control the humidity in the tropics to be identified. Isotopic information is useful because the change in δ relative to the water vapor mixing ratio (q) is different for different processes. The theoretical framework for interpreting paired q–δ data is established and based on a set of simple models that account for mixing and a range of condensation conditions. A general condensation model is derived that accounts for cloud precipitation efficiency and postcondensation exchange. Using data from the Tropospheric Emission Spectrometer (TES), aspects of subtropical hydrology are characterized by the match between theoretical curves and observed displacement in q–δ space. The subtropics are best described as the balance between drying associated with (mostly horizontal) transport of dry air from high latitudes and moistening by clouds with low precipitation efficiency. In the western Pacific moistening involves the import of air into which raindrops have evaporated and is identified by “super-Rayleigh” isotopic distillation. In the dry subtropics, the observations are consistent with the condensation–advection explanation for the humidity minimum but also reflect details of the cloud processes and moistening by high humidity filaments of tropical origin. In spite of limitations of the TES data, the success of the analysis highlights the value of using isotopic data in analysis of tropospheric moisture budgets and the role water isotopic ratio measurements can play in identifying mechanisms associated with large-scale changes in atmospheric humidity.