Isotopic composition of stratospheric water vapor: Implications for transport

We develop a series of models of transport in the upper tropical tropospher e in order to explain the observed abundance and isotopic composition of st ratospheric water vapor. We start with the Rayleigh fractionation process a nd add the effects of mixing and recirculation of stratospheric air through the upper troposphere. We compare our measurements with model calculations for a range of input parameters and find that the observations are best ex plained by a model that mixes vapor from roughly 11 km (carried aloft eithe r as condensate or through radiative heating and uplift) with air that has been dehydrated (in a large convective system) to a mixing ratio substantia lly below the saturation mixing ratio of the mean tropical tropopause. The result is that while most of the moisture comes from convective outflow nea r ii km, most of the air in the upper troposphere consists of dehydrated ai r from convective systems with cloud top temperatures below that of the mea n tropical tropopause. We also find that the water vapor mixing ratio in th e stratosphere is determined not only by the temperature of the tropical tr opopause but also by the relative importance of radiative heating, recircul ation of stratospheric air, and deep convection in supplying air to the upp er troposphere. Our results show that water vapor isotope ratios are a powe rful diagnostic tool for testing the results of general circulation models.