Different responses of cold-air outbreak clouds to aerosol and ice production depending on cloud temperature

Abstract

Aerosol–cloud interactions and ice production processes are important factors that influence mixed-phase cold-air outbreak (CAO) clouds and their contribution to cloud-phase feedback. Recent case studies of CAO events suggest that increases in ice-nucleating particle (INP) concentrations cause a reduction in cloud total water content and albedo at the top of the atmosphere. However, no study has compared the sensitivities of CAO clouds to these processes under different environmental conditions. Here, we use a high-resolution nested model to quantify and compare the responses of cloud microphysics and dynamics to cloud droplet number concentration (Nd), INP concentration, and efficiency of the Hallett–Mossop (HM) secondary ice production process in two CAO events over the Labrador Sea, representing intense (cold, March) and weaker (warmer, October) mixed-phase conditions. Our results show that variations in INP concentrations strongly influence both cases, while changing Nd and the HM process efficiency affects only the warmer October case. With a higher INP concentration, cloud cover and albedo at the top of the atmosphere increase in the cold March case, while the opposite responses are found in the warm October case. We suggest that the CAO cloud response to the parameters is different in ice-dominated and liquid-dominated regimes and that the determination of the regime is strongly controlled by the cloud temperature and the characteristics of ambient INP, which both control the glaciation of clouds. This study provides an instructive perspective to understand how these cloud microphysics affect CAO clouds under different environmental conditions and serves as an important basis for future exploration of the cloud microphysics parameter space.