How Falling Raindrops Energize Clouds through Frictional Heating

Raindrops fall at constant terminal velocity due to a balance between gravitational and frictional forces. Dissipation of energy is associated with this friction that results in heating of the atmosphere, but the nature and vertical profile of this dissipation have not been studied in detail. First, we will discuss the origin of dissipated energy and show explicitly that it arises from the conversion of hydrometeor potential energy during settling. It is suggested that the heating imparted to the atmosphere by dissipation allows the air to recover most of the energy previously expended in lofting hydrometeors. Next, we will present results from a cloud resolving model simulation of tropical, aggregated convection in which the frictional heating term has been newly included. Maximum heating from hydrometeor friction reaches ~10 K hr^-1 which is consistent with an order of magnitude scaling of the term. Substantial changes are seen when this simulation is compared to one in which the hydrometeor frictional heating term has been excluded. Specifically, inclusion of the hydrometeor frictional heating term in the simulation results in increased cloud cover, decreased convective mass flux, a drier mean state, and higher stability throughout much of the troposphere. In shallow clouds, where the magnitude of this term is generally less than in deep clouds, the frictional heating destabilizes the cloudy layer and stabilize the sub-cloud layer. The sensitivity of the simulations to raindrop frictional heating warrants more regular inclusion of the term in cloud microphysics schemes.