Abstract

Key Points: 10 • We improve a long-standing stratocumulus (Sc) dim bias in a high-resolution Mul-11 tiscale Modeling Framework. 12 • Incorporating intra-CRM hypervisocity hedges against the numerics of its momen-13 tum solver, reducing entrainment vicinity. 14 • Further adding sedimentation boosts Sc brightness close to observed, opening path 15 to more faithful low cloud feedback analysis. Abstract 17 High-Resolution Multi-scale Modeling Frameworks (HR)-global climate models that 18 embed separate, convection-resolving models with high enough resolution to resolve bound-19 ary layer eddies-have exciting potential for investigating low cloud feedback dynam-20 ics due to reduced parameterization and ability for multidecadal throughput on mod-21 ern computing hardware. However low clouds in past HR have suffered a stubborn prob-22 lem of over-entrainment due to an uncontrolled source of mixing across the marine sub-23 tropical inversion manifesting as stratocumulus dim biases in present-day climate, lim-24 iting their scientific utility. We report new results showing that this over-entrainment 25 can be partly offset by using hyperviscosity and cloud droplet sedimentation. Hypervis-26 cosity damps small-scale momentum fluctuations associated with the formulation of the 27 momentum solver of the embedded LES. By considering the sedimentation process ad-28 jacent to default one-moment microphysics in HR, condensed phase particles can be re-29 moved from the entrainment zone, which further reduces entrainment efficiency. The re-30 sult is an HR that is able to produce more low clouds with a higher liquid water path 31 and a reduced stratocumulus dim bias. Associated improvements in the explicitly sim-32 ulated sub-cloud eddy spectrum are observed. We report these sensitivities in multi-week 33 tests and then explore their operational potential alongside microphysical retuning in 34 decadal simulations at operational 1.5 degree exterior resolution. The result is a new HR 35 having desired improvements in the baseline present-day low cloud climatology, and a 36 reduced global mean bias and root mean squared error of absorbed shortwave radiation. 37 We suggest it should be promising for examining low cloud feedbacks with minimal ap-38 proximation. 39 Plain Language Summary 40 Stratocumulus clouds cover a large fraction of the globe but are very challenging 41 to reproduce in computer simulations of Earth's atmosphere because of their unique com-42 plexity. Previous studies find the model produces too few Stratocumulus clouds as we 43 increase the model resolution, which, in theory, should improve the simulation of impor-44 tant motions for the clouds. This is because the clouds are exposed to more conditions 45 that make them evaporate away. On Earth, stratocumulus clouds reflect a lot of sun-46 light. In the computer model of Earth, too much sunlight reaches the surface because 47 of too few stratocumulus clouds, which makes it warmer. This study tests two methods 48 to thicken Stratocumulus clouds in the computer model Earth. The first method smooths 49 out some winds, which helps reduce the exposure of clouds to the conditions that make 50 them evaporate. The second method moves water droplets in the cloud away from the 51 conditions that would otherwise make them evaporate. In long simulations, combining 52 these methods helps the model produce thicker stratocumulus clouds with more water. 53