  Precipitation generates small-scale turbulent air flows the energy of which
ultimately dissipates to heat. The power of this process has previously been
estimated to be around 2-4 W m-2 in the tropics: a value comparable in
magnitude to the dynamic power of the global circulation. Here we suggest that
this previous power estimate is approximately double the true figure. Our
result reflects a revised evaluation of the mean precipitation path length Hp.
We investigate the dependence of Hp on surface temperature,relative
humidity,temperature lapse rate and degree of condensation in the ascending
air. We find that the degree of condensation,defined as the relative change of
the saturated water vapor mixing ratio in the region of condensation, is a
major factor determining Hp. We estimate from theory that the mean large-scale
rate of frictional dissipation associated with total precipitation in the
tropics lies between 1 and 2 W m-2 and show that our estimate is supported by
empirical evidence. We show that under terrestrial conditions frictional
dissipation constitutes a minor fraction of the dynamic power of
condensation-induced atmospheric circulation,which is estimated to be at least
2.5 times larger. However,because Hp increases with surface temperature Ts, the
rate of frictional dissipation would exceed that of condensation-induced
dynamics, and thus block major circulation, at Ts >~320 K in a moist adiabatic
atmosphere.
