Radon measurements in the lower tropical stratosphere: Evidence for rapid vertical transport and dehydration of tropospheric air

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Abstract
During the tropical experiment of NASA's Stratosphere‐Troposphere Exchange Program (STEP), in situ radon and other trace constituent measurements were made aboard a NASA ER‐2 high‐altitude research aircraft to investigate the mechanisms of irreversible transfers from the troposphere into the tropical stratosphere. Observations made in and downwind of the cirrus shields of three large tropical cyclones and downwind of the cirrus anvil of a large cumulonimbus cloud cluster showed several clear instances of elevated radon activity occurring simultaneously with low total water mixing ratios. These observations are unambiguous evidence of an effective dehydration process, capable of reducing total water vapor mixing ratios to less than 2.5 ppmv, occurring in conjunction with troposphere‐to‐stratosphere transport and indicate that rapid localized convection, rather than slow regional mean motions, was responsible for the observed transports and associated with the accompanying dehydration. Radon activities measured in regions of active or recent troposphere‐to‐stratosphere transport were consistent with the 17 pCi/scm mean value needed to support the observed abundance of stratospheric 210Pb.