Bimodality in tropical water vapour

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91 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)2847-2866
Number of pages20
JournalQuarterly Journal of the Royal Meteorological Society
Issue number594 PART A
StatePublished - Oct 2003


  • Humidity
  • Mixing
  • Probability distribution function

ASJC Scopus subject areas

  • Atmospheric Science


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