The key physical parameters governing frictional dissipation in a precipitating atmosphere

TL;DR

This paper revises the estimate of frictional dissipation power in a precipitating atmosphere, showing it is about half previous estimates and depends on key physical parameters like the mean precipitation path length and degree of condensation.

Contribution

It introduces a revised evaluation of the mean precipitation path length and its dependence on atmospheric parameters, refining the estimate of frictional dissipation power in tropical atmospheres.

Findings

Frictional dissipation power is estimated between 1 and 2 W/m².

Degree of condensation significantly influences the precipitation path length.

Frictional dissipation is minor compared to circulation power but can dominate at high surface temperatures.

Abstract

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.