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

Abstract. Ice formation mechanisms in generating cells near stratiform cloud-tops, where mixing and entrainment occurs in the presence of supercooled water droplets, remain poorly understood. Supercooled cloud droplet temperature and lifetime may impact heterogeneous ice nucleation through contact and immersion freezing; however, modeling studies normally assume droplet temperature to be spatially uniform and equal to the ambient temperature. Here, we present a first-of-its-kind quantitative investigation of the temperature and lifetime of evaporating droplets, considering internal thermal gradients within the droplet as well as thermal and vapor density gradients in the surrounding air. Our approach employs solving the Navier-Stokes and continuity equations, coupled with heat and vapor transport, using an advanced numerical model. For typical ranges of cloud droplet sizes and environmental conditions, the droplet internal thermal gradients dissipate quickly (≤ 0.3 s) when droplets are introduced to new subsaturated environments. However, the magnitude of droplet cooling is much greater than estimated from past studies of droplet evaporation, especially for drier environments. For example, for an environment with pressure of 500 hPa, and ambient temperature far from the droplet of -5 °C, the droplet temperature reduction can be as high as 24, 11, and 5 °C for initial ambient relative humidities of 10 %, 40 %, and 70 % respectively. Droplet lifetimes are found to be tens of seconds longer compared to previous estimates due to weaker evaporation rates because of lower droplet surface temperatures. Using these new end-of-lifetime droplet temperatures, the enhancement in activation of ice-nucleating particles predicted by current ice nucleation parameterization schemes is discussed.