An intricate balance of liquid and ice: The properties, processes, and significance of Arctic stratiform mixed-phase clouds

Clouds are exceedingly important in global climate, particularly in the Arctic where temperatures are cold, the atmosphere is dry, and most components of this seasonally frozen environment are sensitive to changes in temperature. Within Arctic clouds themselves, there is an intricate balance of condensed liquid and ice which is largely responsible for the cloud microphysical composition and persistence. Moreover, the condensed phase balance determines the manner in which these clouds interact with atmospheric radiation and the hydrologic cycle. Despite the important roles of clouds in Arctic climate, models at all scales struggle to simulate realistic Arctic clouds, primarily because of the confounding balance of liquid and ice phases.;To address this deficiency, a new set of ground-based remote-sensor observations is analyzed to develop a detailed characterization of the macrophysical, microphysical, and dynamical properties of Arctic mixed-phase clouds. Methods have been developed and/or adapted for retrieving properties such as the vertical distribution of cloud phase, the cloud ice and liquid microphysical properties, the cloud macrophysical properties, and the vertical air velocity from an observation suite containing cloud radar, lidar, microwave radiometer, infrared radiometer, and radiosondes. Retrieval methods are applied to more than a year of observations at Arctic Ocean and Arctic coastal sites, resulting in a baseline Arctic cloud data set for use in characterizing mixed-phase cloud properties and processes.;Mixed-phase clouds are found to occur more than 40% of the time in the Western Arctic, predominantly in the form of low-level stratiform layers wherein ice crystals form in, and fall from, a persistent layer of supercooled liquid. These clouds are most frequent in the spring and fall transition seasons, but also occur in significant fractions during the cold winter months. While the temperature of these clouds is most often between -25 and -5°C, supercooled liquid is observed down to -40°C. Cloud depth, including both liquid and ice, is typically between 0.5 and 3 km with the falling ice often reaching the surface. Condensed cloud mass is usually dominated by liquid water, although the partitioning of cloud phase is loosely dependent on temperature. Mixed-phase clouds at the more southerly coastal location contain more condensed water in both phases than similar clouds over the perennial ice pack to the north. Observations indicate that the cloud ice growth and vertical distribution are strongly dependent on the presence of cloud liquid. Updrafts associated with in-cloud circulations simultaneously produce both liquid and ice phases. This interplay of cloud microphysics and dynamics is developed into a conceptual model for the formation and dissipation of cloud condensate in Arctic coastal, autumn mixed-phase clouds.;The research presented here has lead to significant advances in our understanding of Arctic stratiform mixed-phase clouds. Observations over a complete annual cycle have provided a first look at the annual variability of mixed-phase cloud occurrence, macrophysical properties, and microphysical composition. In addition, it has been shown that temperature alone is a poor predictor of the condensed phase partitioning and that parameters such as vertical velocity play a significant role in the life cycle of both liquid and ice phases. In spite of these advances, further study of Arctic mixed-phase clouds is warranted to address deficiencies in our understanding of spatial variability, cloud forcing mechanisms, and the processes through which both liquid and ice particles form and grow.