The Physics of Falling Raindrops in Diverse Planetary Atmospheres

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.

submitted to JGR: Planets; 40 pages, 8 figures, 3 tables, 4 appendices; supporting information with 9 pages, 8 figures, 1 table; associated code at https://github.com/kaitlyn-loftus/rainprops

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• Hydrologic Cycle Weakening in Hothouse Climates
• Multiple moist climate equilibrium states on arid rocky M-dwarf planets: A last-saturation tracer analysis
• Capillary processes in extraterrestrial contexts
• Nonuniform Water Distribution in Jupiter's Mid Latitudes: Influence of Precipitation and Planetary Rotation
• Water droplet dynamics and evaporation in airtanker firefighting