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 (Tr0), and environmental relative humidity (RH), temperature (T∞) and pressure (P). The time (tss) required by droplets to reach a lower steady-state temperature (Tss) after sudden introduction into a new subsaturated environment, the magnitude of ΔT = T∞ − Tss, and droplet survival time (tst) at Tss are calculated. ΔT is found to increase with T∞, and decrease with RH and P. ΔT was typically 1-5K lower than T∞, with highest values (~10.3K) for very low RH, low P, and T∞ closer to 0°C. Results show that tss is < 0.5s over the range of initial droplet and environmental conditions considered. Larger droplets (r0 = 30 to 50 μm) can survive at Tss for about 15s to over 3 minutes, depending on the subsaturation of the environment. For higher RH and larger droplets, droplet lifetimes can increase by more than 100s compared to those with droplet cooling ignored. Tss 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 Tss 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 0°C.