Brian E. Mapes
R/CDC 325 S. Broadway, Boulder, Colorado, 80303 USA
brian.mapes@noaa.gov
NOAA-CIRES Climate Diagnostics Center (CDC)
QJRMS, 127, 2353-2366 (2001).
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 vapor 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 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 updraft-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 ~8km, 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.