Can evaporative cooling of water droplets play a role in enhancing ice formation at moderately supercooled cloud boundaries?

Cloud droplet temperature is an important parameter influencing cloud microphysical and radiative processes. The supercooled droplet temperature and lifetime impact cloud ice and precipitation formation via homogeneous freezing and activation of ice-nucleating particles through contact and immersion freezing. While most observational and modeling studies often assume droplet temperature to be almost equal to the ambient temperature (Ta), this assumption may not always be valid, particularly when droplets experience strong relative humidity (RH) gradients at cloud boundaries.

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), ambient temperature (Ta), 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 = Ta - Tss, and droplet survival time (tst) at Tss are calculated. The temperature difference (ΔT) is found to increase with Ta, and decrease with RH and P. ΔT values are typically 1–5 K lower than Ta, with highest values (~10.3 K) for very low RH, low P, and Ta closer to 0°C. Results show that tss is < 0.5 s over the range of initial droplet and environmental conditions considered. 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 Ta 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 Ta closer to 0°C. The findings corroborate the hypothesized mechanism of potential enhancement of ice nucleation at cloud boundaries, such as cloud-top generating cells and for ambient temperatures close to 0°C. The importance of using accurate droplet temperatures to improve existing primary ice nucleation parameterization schemes, especially in sub-saturated environments, is highlighted.

The impacts of droplet evaporative cooling on droplet lifetimes are compared with Maxwellian pure diffusion-limited evaporation approach under similar conditions. For higher RH and larger droplets, droplet lifetimes can increase by more than 100 s compared to those with droplet cooling ignored. Larger droplets (r0 ~ 30–50 µm) can survive at Tss for about 5 s to over 10 min, depending on the subsaturation of the environment. The impacts of droplet evaporative cooling on evolution of drop size distributions, using high-resolution direct numerical simulations of moderately supercooled mixed-phase cloud boundaries, are discussed.