RADON MEASUREMENTS IN THE LOWER TROPICAL STRATOSPHERE - EVIDENCE FOR RAPID VERTICAL TRANSPORT AND DEHYDRATION OF TROPOSPHERIC AIR

During the tropical experiment of NASA's Stratosphere-Troposphere Exch ange Program (STEP), in situ radon and other trace constituent measure ments were made aboard a NASA ER-2 high-altitude research aircraft to investigate the mechanisms of irreversible transfers from the troposph ere 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 w ith low total water mixing ratios. These observations are unambiguous evidence of an effective dehydration process, capable of reducing tota l water vapor mixing ratios to less than 2.5 ppmv, occurring in conjun ction with troposphere-to-stratosphere transport and indicate that rap id localized convection, rather than slow regional mean motions, was r esponsible for the observed transports and associated with the accompa nying dehydration. Radon activities measured in regions of active or r ecent troposphere-to-stratosphere transport were consistent with the 1 7 pCi/scm mean value needed to support the observed abundance of strat ospheric Pb-210.