Water's two height scales: The moist adiabat and the radiative troposphere

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The temperature structure of the tropical troposphere resembles a moist adiabat, with a lapse-rate transition toward dry adiabatic where water becomes scarce at an altitude Hma - 8 km (350 hPa). Infrared emission by water vapour cools a deeper layer, extending up to Hrad - 14 km (160 hPa). Five consequences of these unequal heights are reviewed. 1. Upper-tropospheric relative humidity is often low, highly variable, and bimodal, due to the rapidity of drying by radiative subsidence. 2. Large-scale divergent circulations (e.g. equatorial u wind) exhibit a two-celled vertical structure, with an elevated convergence layer near 8-10 km in the rising branch. 3. The dominant deep convective heating process changes from latent heating at low levels to eddy heat-flux convergence in the upper troposphere. This requires a substantial updraught-environment temperature difference, which leads to large entrainment near Hma, yielding stratiform anvil clouds which also contribute radiative heating. 4. The rising branches of deep (-Hrad) vertical circulations export more heat than they import as moisture, so that large-scale tropical dynamics can be characterized by a 'gross moist stability'. 5. Divergent motions with a vertical wavelength -8 km, corresponding to Kelvin or gravity wave speeds of -15 m s-1, are excited by simple (e.g. uniform) heating profiles extending through the lapse-rate change near Hma.

Original languageEnglish (US)
Pages (from-to)2353-2366
Number of pages14
JournalQuarterly Journal of the Royal Meteorological Society
Issue number577
StatePublished - Oct 1 2001
Externally publishedYes


  • Gross moist stability
  • Radiative-convective equilibrium
  • Tropical atmosphere
  • Vertical structure

ASJC Scopus subject areas

  • Atmospheric Science


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