Evolution of Cloud Droplet Temperature and Radius in Spatiotemporally Varying Subsaturated Environments with Implications for Ice Nucleation at Cloud Edges

Cloud droplet temperature is a critical factor 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 spatially uniform and equal to the ambient temperature (T), this assumption may not always be valid, particularly when droplets experience strong relative humidity (RH) gradients at cloud edges. Here, we present a first-of-its-kind quantitative investigation of evaporating droplet temperatures and lifetimes, considering internal thermal gradients within the droplet as well as resolving thermal and vapor density gradients in the surrounding ambient air. Our approach employs solving the Navier-Stokes and continuity equations, coupled with heat and vapor transport, using an advanced numerical model. This study advances our understanding of cloud droplet evaporation by investigating the spatiotemporal evolution of droplet temperature, radius, and its environment under various subsaturated environmental conditions, including ambient pressure (P), T, and RH, and initial droplet characteristics. The results discuss the evolution of internal thermal gradients within evaporating droplets to assess the potential for enhanced ice nucleation at the interface. For droplets in an environment with initial T = -5°C and P = 500 hPa, the decrease in droplet temperatures due to evaporative cooling is ~ 24, 11, and 5°C for initial ambient RH of 10%, 40%, and 70% respectively. The impacts of droplet evaporative cooling on droplet lifetimes and their immediate environment are compared with previous studies conducted under similar conditions. 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. Finally, the importance of improving existing primary ice nucleation parameterization schemes, especially in subsaturated environments, accounting for the decrease in droplet temperature and increase in droplet lifetime due to evaporative cooling are highlighted.