An analysis of the regulation of tropical tropospheric water vapor

[1] We use a simple trajectory model to investigate the mechanisms that regulate midand upper-tropospheric humidity. Our model advects water passively and contains no microphysics other than the requirement that water vapor is immediately removed so as to prevent the relative humidity from ever exceeding 100%. We demonstrate that our model accurately reproduces H2O measurements made by the Atmospheric Infrared Sounder onboard NASA’s Aqua satellite. Our results show that, given the large-scale circulation of the troposphere, detailed microphysics need not be included in order to accurately simulate H2O. We have also identified three preferred regions where air parcels in the mid and upper troposphere experience their final dehydration. The first is in the equatorial upper troposphere and is associated with convective outflow at the top of the tropical Hadley circulation. Final dehydration of air that detrains at potential temperature q above 340 K ( 10 km) predominantly occurs here. The other two regions are found at lower altitudes in the midlatitudes of both hemispheres and are associated with dehydration during isentropic excursions to midlatitudes. Final dehydration of air that detrains at q below 340 K predominantly occurs here. Finally, we analyze the water budget of the dry eastern Pacific subtropics and find that dehydration in both the equatorial upper troposphere and the midlatitudes contribute to the dryness there.