Simulating Mixed‐Phase Open Cellular Clouds Observed During COMBLE: Evaluation of Parameterized Turbulence Closure.

Marine cold‐air outbreaks, or CAOs, are airmass transformations whereby relatively cold boundary layer (BL) air is transported over relatively warm water. To more deeply understand BL and mixed‐phase cloud properties during CAO conditions, the Cold‐Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) took place from late 2019 into early 2020. During COMBLE, the U.S. Department of Energy's first Atmospheric Radiation Measurement Mobile Facility (AMF1) was deployed to Andenes, Norway, far downstream (∼1,000 km) from the Arctic pack ice. This study examines the two most intense CAOs sampled at the AMF1 site. The observed BL structures are open cellular with high (∼3–5 km) and cold (−30 to −50 ° ${}^{\circ}$C) cloud tops, and they often have pockets of high liquid water paths (LWPs; up to ∼1,000 g m−2 ${\mathrm{m}}^{-2}$) associated with strong updrafts and enhanced turbulence. We use a high‐resolution mesoscale model to explore how well four turbulence closure methods represent open cellular clouds. After applying a radar simulator to model outputs for direct evaluation, cloud top properties agree well with AMF1 observations (within ∼10%), but radar reflectivity and LWP agreement is more variable. Results suggest that the turbulent Prandtl number may play an important role for the simulated BL and cloud properties. All simulations produce enhanced precipitation rates that are well‐correlated with a cloud transition. Finally, the eddy‐diffusivity/mass‐flux approach produces the deepest cloud layer and therefore the largest and most coherent cellular structures. We recommend the use of a non‐local turbulence closure approach to better capture turbulent processes in intense CAOs. Plain Language Summary: Over the high latitude oceans, shallow clouds containing both liquid and frozen hydrometeors, or mixed‐phase clouds, are frequently present. Moreover, they are important to the climate system due to their role in the radiation and moisture budgets. As a result of their microphysical makeup, they are especially challenging to simulate accurately for many numerical models across a range of spatial scales. To better understand these clouds during an intense outbreak of cold air from the Arctic, we utilize measurements from a recent field campaign called the Cold‐Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE). We complement the COMBLE observations with high‐resolution numerical modeling to reveal more information about the cloud structures. We find that the simulated cloud properties, including morphology and abundance of liquid water at subfreezing temperatures, are dependent upon the method used to represent vertical turbulent exchanges between the ocean and atmosphere. Key Points: Cloud properties are well‐simulated compared to one satellite data set, and disagree more than they agree with ground‐based instrumentsEddy diffusivity‐mass flux approach produces the deepest clouds and largest cell sizesPrecipitation processes likely initiate cloud transition from closed to open cells [ABSTRACT FROM AUTHOR]