The Physics of Falling Raindrops in Diverse Planetary Atmospheres

TL;DR

This paper develops a physics-based framework to characterize raindrop properties such as shape, velocity, and evaporation in any planetary atmosphere, providing insights into precipitation processes across diverse worlds.

Contribution

It introduces a universal approach to calculate raindrop characteristics and a new dimensionless number to quantify their vertical transport capabilities, independent of growth mechanisms.

Findings

Raindrop size range is tightly constrained by atmospheric properties.

A new dimensionless number captures raindrop's heat and mass transport ability.

Results impact understanding of precipitation, storm dynamics, and planetary climate.

Abstract

The evolution of a single raindrop falling below a cloud is governed by fluid dynamics and thermodynamics fundamentally transferable to planetary atmospheres beyond modern Earth's. Here, we show how three properties that characterize falling raindrops -- raindrop shape, terminal velocity, and evaporation rate -- can be calculated as a function of raindrop size in any planetary atmosphere. We demonstrate that these simple, interrelated characteristics tightly bound the possible size range of raindrops in a given atmosphere, independently of poorly understood growth mechanisms. Starting from the equations governing raindrop falling and evaporation, we demonstrate that raindrop ability to vertically transport latent heat and condensible mass can be well captured by a new dimensionless number. Our results have implications for precipitation efficiency, convective storm dynamics, and rainfall rates, which are properties of interest for understanding planetary radiative balance and (in the case of terrestrial planets) rainfall-driven surface erosion.