An improved understanding of the lifecycle of mixed-phase stratiform clouds through observations and simulation.

摘要

This work explores links between ice crystal nucleation in the immersion mode and the lifecycle of mixed-phase stratiform clouds. A background discussion is included on the general properties of mixed-phase clouds, their influence on climate, observational techniques and numerical studies used to better understand them. Also, an overview of ice nucleation principles and Arctic aerosol characteristics is provided. An observational analysis including thousands of half hour cases of single layer mixed-phase clouds measured using remote sensors at Barrow, Alaska and Eureka, Canada is reviewed. An overview of the techniques used in this effort is provided, including information on the instruments and all implemented retrieval algorithms. These observations show distinct differences between cloud properties at the two locations, as well as clear seasonal patterns in cloud macro and microphysical properties. This dataset is compared with those obtained in previous studies, and implications of the measurements on numerical simulation and cloud detection using other observational platforms are discussed.;Utilizing results from these observations, as well as those from the work of others, a hypothesis on ice nucleation in these clouds via immersion freezing is formed, in which the concentration of soluble aerosol mass within liquid droplets results in a freezing point depression. Subsequent growth of these droplets dilutes the concentration of soluble mass, and the droplet can freeze. In order to test this hypothesis, an advanced numerical model is utilized. Simulation results show that immersion freezing does contribute significantly to ice production within mixed-phase clouds. Additionally, the soluble mass fraction assumed for the aerosol particles impacts simulated clouds via the effect discussed above. However, unlike suggested in the presented hypothesis, nucleation through the immersion mode was not limited to the regions above updrafts in the completed simulations. Instead, a combination of soluble mass fraction and temperature variations resulted in immersion freezing occurring throughout the top of the simulated cloud layer. A application of information on Arctic aerosols and ice nucleus measurements leads to a realistic simulation which maintained a mixed-phase cloud for over 16 hours. Finally, discussions on model uncertainties, future work and a summary are provided.