83rd Annual

Wednesday, 12 February 2003: 9:15 AM
Bimodal distribution of tropical upper tropospheric humidity
Brian Mapes, NOAA/CIRES/CDC, Boulder, CO; and C. Zhang and B. Soden
Bimodality in upper-tropospheric water vapor distributions is shown to be a common feature in the tropics, implying sharp gradients between dry and moist regions in space and time. A method of testing for and quantifying bimodality is introduced. Using this method, the bimodality of water vapor is surveyed in both satellite and in situ observations, as well as a hierarchy of model simulations.

Bimodality implies that the homogenization time for moisture, whether physical (mixing) or mathematical (averaging), is long compared to a radiative-subsidence drying time after an injection of moisture by convection. This radiative drying time varies with altitude, and is shortest (about a day) near 11 km. It is shown that the local bimodality found in point measurements and cloud-resolving model simulations disappears with very modest averaging (~1 day, ~250 km). However, bimodality reappears on the global scale, where climatically dry and moist regions are separated so widely that synoptic and large-scale mixing and smearing times exceed the drying time.

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