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

Abstract

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 H ma ∼ 8 km (350 hPa). Infrared emission by water vapour cools a deeper layer, extending up to H rad ∼ 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 H ma , yielding stratiform anvil clouds which also contribute radiative heating. 4. The rising branches of deep (∼ H rad ) 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 H ma .