| Factors governing the efficiency of convective moistening in the tropical upper troposphere between 15°S and 15°N are investigated using observational satellite data together with a trajectory model. In particular, the relative roles of ambient temperature, convective ice water content, ice cloud effective radius, and ambient relative humidity with respect to ice are examined. Results indicate that humidity downstream from convection is dominated by temperature influences. The correlation between water vapor content and coldest previous temperature along the trajectory accounts for about half of water vapor variance. Cloud ice water content and ice cloud effective radius exert much less influence than temperature, but appear to be important secondary controls downstream. Ice is observed to both hydrate and dehydrate the upper troposphere, primarily as a function of ambient relative humidity; increases in water vapor mixing ratio are typically larger in areas with low ambient relative humidity; while convective detrainment tends to dehydrate regions of high ambient relative humidity. The limits of convective influence on water vapor in the upper troposphere are estimated to be about 36 hours and 600 km removed from convection, consistent with the lifetimes and travel distances of detrainment cirrus. Observations of water vapor are compared with the output of a simple model that includes only temperature influences on water vapor. The model and the observations are generally in good qualitative agreement, but the model is found to overpredict the occurrence of high humidities at all levels of detrainment. Potential applications and implications of these results are discussed. |
