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Mapes, B. E., 2001: Water's two height scales: the moist adiabat and the radiative troposphere. Quart. J. Roy. Met. Soc., 127, 2353-2366.


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 deep 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 V 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.