Arctic mixed-phase clouds from the micro- to the mesoscale: insights from high-resolution modeling

Clouds play a key role in the local Arctic radiative budget and the hydrological cycle. In particular, cloud presence, vertical and horizontal extent, micro- and macrophysical characteristics, temperature, and thermodynamic phase are relevant properties determining the cloud's interaction with its environment. Especially Arctic low-level clouds have been found to strongly interact with atmospheric radiation and to have a net warming effect on the Earth surface during most of the year through the emission of longwave radiation. Low-level clouds reside within the boundary layer and are often mixed-phase, i.e. they contain a mixture of cloud ice and liquid water. Even though the coexistence of ice and liquid is thermodynamically unstable, low-level Arctic mixed-phase clouds (MPCs) have been found to be particularly persistent, a phenomenon that has puzzled the scientific community for decades. Yet, a profound understanding of the processes controlling Arctic MPC persistence, microphysics, and dynamics is so far missing. In order to contribute to the current understanding of processes controlling Arctic low-level MPC properties, semi-idealized cloud-resolving model simulations are explored. At high horizontal (120-200 m) and vertical (25-50 m within the atmospheric boundary layer) resolutions, cloud driven circulations are accurately resolved, while cloud microphysical properties are parameterized. All simulations are based on MPC observations obtained during the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign in March 2013. In the first part of this work, cloud properties over varying surface conditions (i.e. open ocean and sea ice) are investigated. As sea ice has been retreating in recent years, altered cloud dynamics in response to surface forcing provide valuable insights of future cloud occurrence in the Arctic. In addition, with retreating sea ice the Arctic Ocean becomes more accessible for shipping, such that Arctic clouds are likely to be exposed to higher levels of aerosols. In our simulations, a stratiform MPC forms, which features additional convective structures over the open ocean. Over both surfaces, aerosol perturbations (cloud condensation nuclei and ice-nucleating particles; CCN and INPs, respectively) induce a temporary increase in cloud liquid and ice water content. The MPC over the open ocean shows a faster and stronger response to sudden aerosol perturbations, while the cloud properties over sea ice adjust more slowly to increased levels of aerosols. However, the MPC over sea ice remains perturbed throughout the simulated period (20 h), while the liquid water path of the stratocumulus cloud over the open ocean relaxes back to its unperturbed state. Precipitation and ice formation effectively buffer any cloud-aerosol interactions on timescales beyond half a day and the stratocumulus cloud re-organizes into its original structure. Generally, stratocumulus dynamics in the mixed-phase regime remain relatively unexplored. To investigate cloud morphology changes and regime transitions at high latitudes, simulations of stratocumulus clouds independent of external drivers have been performed. We show that similar to liquid clouds at low latitudes, mixed-phase stratocumulus tend to organize into cellular patterns. The spatial scales of these patterns increase through precipitation enhancement, which can be induced through a reduction in available CCN or an increase in the cloud ice phase. A high (> 50 %) ice to liquid water content ratio as expected for example from increased background INP concentrations, can cause locally intense rain events, which induce subcloud evaporative cooling, cold pool formation and lateral cloud cell growth. This change in spatial organization has implications for the radiative balance at the Earth's surface. Arctic stratocumulus are also substantially impacted by processes acting on the large-scale, such as air mass advection. The advection of warm and moist air masses into the Arctic is a common phenomenon which alters the properties of the whole lower troposphere. In the final chapter, the impact of warm and moist air masses intruding the Arctic at different heights is explored. Boundary layer advection lifts and weakens the temperature inversion, induces a rise of the cloud layer, and leads to a stratocumulus-cumulus transition. The decreased cloud fraction substantially cools the Earth's surface in the absence of incoming shortwave radiation. On the contrary, air mass advection above the inversion layer lowers and reinforces the temperature inversion and cloud cover remains at 100 %. Employing a cloud-resolving modeling approach, this work identifies drivers of Arctic mixed-phase stratocumulus microphysical properties and demonstrates the interplay of cloud microphysics and dynamics. Mixed-phase stratocumulus organization represents one example of precipitation enhancement through aerosol-cloud interactions at the microscale, which drive cloud morphology changes on the mesoscale