Investigation of Supercooled Cloud Droplet Temperatures and Lifetimes in Evaporating Regions of Clouds with Implications for Ice Nucleation at Cloud Boundaries
Cloud droplet temperature plays a critical role in influencing fundamental cloud microphysical and radiative processes. These processes include the determination of the diffusional growth and decay rates of cloud droplets, and the specification of droplet refractive indices, which in turn impact the radiative properties of clouds. Additionally, the supercooled droplet temperature and lifetime affect cloud ice and precipitation formation via homogeneous freezing and activation of ice-nucleating particles (INPs) through contact and immersion freezing. While most observational and modeling studies often assume droplet temperature to be spatially uniform and equal to the ambient temperature (T∞), this assumption may not always be valid, particularly at evaporating regions of the clouds such as cloud tops and edges, where the droplets experience strong relative humidity (RH) gradients.
This research aims to address the current knowledge gap in the literature by discussing two numerical frameworks designed to investigate the evolution of an evaporating, supercooled cloud droplet temperature and radius in subsaturated environments. The first approach, a simpler and idealized approximation, solves for the coupled, time-dependent heat and mass transfer between an isolated cloud water droplet and its environment, assuming constant ambient relative humidity (RH), temperature (T∞), and pressure (P). The results demonstrate that after sudden introduction into a new subsaturated environment, the time required by droplets to reach a lower steady-state temperature is < 0.5 s for the range of initial droplet and environmental conditions considered. The reduction in droplet temperature, ΔT is found to increase with T∞, and decrease with RH and P. ΔT is typically ~ 1-5 K lower than T∞ , with highest value ~10.3 K for very low RH, low P, and T∞ closer to 0ºC. Larger droplets (initial radii ~ 30 to 50 μm) can survive at for about 15 s to over 3 minutes, 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. Temperatures 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 second approach relaxes the idealized assumption of a constant ambient environment and adopts a more sophisticated technique to present a first-of-its-kind quantitative investigation of evaporating droplet temperatures and lifetimes, by also including internal thermal gradients within the droplet as well as resolving thermal and vapor density gradients in the surrounding spatiotemporally varying ambient air domain. Using an advanced numerical model, this framework employs the finite-element method to solve the Navier-Stokes and continuity equations, coupled with heat and vapor diffusion, with appropriate boundary conditions. Results from this study demonstrate the evolution of internal thermal gradients within evaporating droplets and show a higher subsaturation-dependent decrease in temperature of the droplet as well as the envelope of air surrounding the droplet due to droplet evaporation. For an ambient environment specified far from the droplet, with T∞ = -5°C, RH∞ = 10%, 40%, and 70%, the decreases in droplet temperatures due to evaporative cooling are ~ 24, 11, and 5° C, respectively. Compared to the previous approach, the evaporatively cooled droplets survive longer in this framework. Finally, the implications of the evaporative cooling and increased lifetimes of supercooled cloud droplets on ice particle formation near cloud edges, such as cloud-top generating cells, are discussed. The findings corroborate the hypothesized mechanism of potential enhancement of primary ice nucleation at evaporating regions of the cloud, especially for ambient temperatures close to 0°C, and may also partially help understand discrepancies between number concentrations of observed ice crystals and estimated activated INPs, especially at relatively higher sub-0°C temperatures.
