Large-Eddy Simulation of a Stratocumulus-Topped Arctic Boundary Layer over Sea Ice

Arctic low-level stratocumulus is the main contributor to the intra-seasonal variability of surface energy balance which in turn infuences the existence and evolution of the clouds. Although stratocumuli may extend over hundreds of meters vertically, phenomena on scales below few tens of meters, like cloud-top entrainment or mixing within the atmospheric boundary layer, drive their development. The interaction of radiation, microphysics, and turbulence represents not only a major challenge for climate and weather modelling but also for large-eddy simulation. Using highly-resolved large-eddy and direct numerical simulation, recent studies (e. g. Matheou and Chung, 2014; Kopec et al., 2016; de Lozar and Mellado, 2017) for the first time yield a detailed view of the cloud-driving processes. These studies focus on steady and homogeneous cases while transitional cases and heterogeneous surfaces remain elusive. Studies with comparable resolutions are non-existent for the Arctic where observational data is lacking, most clouds are mixed-phase, and the characteristic eddies are smaller.

We investigate low-level mixed-phase stratocumulus in Arctic summer over sea ice in a weakly stable boundary layer capped by a strong temperature inversion. Initializing and benchmarking our modified version of WRF-LES with data from the recent measurement campaigns ACLOUD and PASCAL (Wendisch et al., 2017) grants insight to the cloud-driving processes beyond what can be learned from observational data or large-eddy simulation alone. Performing a resolution-convergence study enables us to analyze cloud-driving non- or under-resolved microphysical, radiative and turbulent processes and their interaction as well as the interaction of stratocumulus and the surface. Our set-up is benchmarked against the well-investigated DYCOMS-II RF01 scenario for a corresponding scenario at lower latitude. A resolution-convergence study for this case reveals that the first-moments of e. g. liquid potential temperature, cloud water mixing ratio and boundary layer height are independent of the horizontal resolution only below 10 m - less than what many other studies of low-level stratocumulus employ. Due to smaller characteristic eddies, we apply a resolution of 3.5 m in the stably stratified Arctic scenario to ensure a sufficient representation of those eddies and their effects.

References

• De Lozar and Mellado (2017), J. Atmos. Sci., vol74(3), 751-765.
• Kopec et al. (2016), Q.J.R. Meteorol. Soc., vol142(701), 3222-3233.
• Matheou and Chung (2014), J. Atmos. Sci., vol71(12), 4439-4460.
• Wendisch et al. (2017), EOS, vol98(8), 22-26.