Evaporation and condensation of water mist/cloud droplets with thermal radiation

Abstract The role of thermal radiation in water droplet evaporation and condensation is investigated theoretically. The primary droplet size regime considered is that between non-continuum gas effects and gas–liquid velocity-slip effects, nominally 10 −1 –10 2  μm, with mist or clouds (nominally 20-μm diameter) being the primary application of interest. Principles for extending the results to larger droplets are also discussed, even to the radiative–evaporative balance of the oceans. The importance of distributed (volumetric, in-depth) absorption in the droplet, conduction, and advection induced by surface regression are analyzed. It is found that advection within the liquid phase induced by surface regression can be neglected for most conditions and that conduction usually establishes a spatially isothermal droplet. Comparison with experimental results for a laser-irradiated droplet suggests that, due to the isothermal condition, these assumptions are reasonable for predicting evaporation rate. Moreover, it is shown that infrared absorption and emission can be treated as surface phenomena; in-depth effects can be neglected. Extending the analysis to cloud droplets shows that radiation, whether with colder upper layers of the atmosphere, warmer lower layers/ground, or solar radiation, can have a significant influence on cloud droplet growth and stability. For typical ambient conditions the thermal radiation effect near the edge of a cloud can be as strong as the effect of a 0.1% super- or sub-saturation, enough to modify significantly the Kohler curves governing equilibrium droplet size distribution and enough to provide the missing mechanism of warm rain droplet growth between the upper limit of non-radiating, diffusional condensation growth (∼30 μm) and the lower limit of turbulent-inertial coalescence growth (∼80 μm), the so-called condensation–coalescence “bottleneck”.