A Closer Look at the Evolution of Supercooled Cloud Droplet Temperature and Lifetime in Different Environmental Conditions with Implications for Ice Nucleation in the Evaporating Regions of Clouds

This study investigates the evolution of temperature and lifetime of evaporating, supercooled cloud droplets considering initial droplet radius (r₀) and temperature (T_r0), and environmental relative humidity (RH), temperature (T_∞), and pressure (P). The time (t_ss) required by droplets to reach a lower steady-state temperature (T_ss) after sudden introduction into a new subsaturated environment, the magnitude of ∆T = T_∞ - T_ss, and droplet survival time (t_st) at T_ss are calculated. The temperature difference (∆T) is found to increase with T_∞, and decrease with RH and P. ∆T was typically 1–5 K lower than T_∞, with highest values (~10.3 K) for very low RH, low P, and T_∞ closer to 0ºC. Results show that t_ss is <0.5 s over the range of initial droplet and environmental conditions considered. Larger droplets (r₀ = 30–50 mm) can survive at T_ss for about 5 s to over 10 min, depending on the subsaturation of the environment. For higher RH and larger droplets, droplet lifetimes can increase by more than 100 s compared to those with droplet cooling ignored. T_ss of the evaporating droplets can be approximated by the environmental thermodynamic wet-bulb temperature. Radiation was found to play a minor role in influencing droplet temperatures, except for larger droplets in environments close to saturation. The implications for ice nucleation in cloud-top generating cells and near cloud edges are discussed. Using T_ss instead of T_∞ in widely used parameterization schemes could lead to enhanced number concentrations of activated ice-nucleating particles (INPs), by a typical factor of 2–30, with the greatest increases (≥100) coincident with low RH, low P, and T_∞ closer to 0ºC.