| Abstract: Probability distribution functions of
tropospheric water vapour in the tropics are shown to be commonly
bimodal. This bimodality implies sharp gradients between dry and moist
regimes in space and time. A method of testing for and quantifying
bimodality is introduced. Using this method, the bimodality of water
vapour is surveyed in satellite and in situ observations, as well
as in global model re-analysis data and simulations. The bimodality
suggests that the radiative drying time after an injection of moisture
by convection is short (1-2 days) compared to a homogenizing time,
whether physical (mixing) or mathematical (averaging). It is shown that
the local bimodality found in cloud-model simulations and in situ
point measurements disappears with modest time averaging (18 h and 200
km), but then reappears on the global-scale, where dry and moist regions
are separated so widely that synoptic- and large-scale mixing times
exceed the drying time-scale. Large discrepancies exist in the ability
to reproduce the global-scale bimodality by global model re-analysis and
simulations. |