Improving stratocumulus cloud turbulence and entrainment parametrizations in OpenIFS

Abstract A series of modifications to the stratocumulus cloud turbulence and entrainment parametrizations in the Open Integrated Forecasting System (OpenIFS) model are performed in a Lagrangian single‐column model framework to determine whether they are able to improve the representation of the stratocumulus‐topped boundary layer and its transition to cloud break‐up. Two very different regimes are studied and compared with large‐eddy simulations: a subtropical marine stratocumulus‐to‐cumulus transition case driven by a warming sea‐surface temperature, and an Arctic air‐mass transformation case where warm and moist air is advected over sea ice during wintertime. The addition of nonlocal mixing driven by stratocumulus cloud‐top cooling for stable and dry convective boundary‐layer types is found to sustain the cloud deck for longer in the Arctic case, while it dissipates around one day earlier in the original OpenIFS. The assumption that cloud‐top‐driven mixing reaches the surface, forming a coupled boundary layer, is relaxed. The resulting eddy diffusivity profile is altered such that mixing at cloud top may be enhanced relative to cloud base, promoting decoupling. Decoupling in the subtropical boundary layer is further promoted when the cloud‐top turbulent velocity scale is based on the integral of the in‐cloud buoyancy flux, accounting for the turbulence generated by latent heat release within the cloud, which increases cloud‐top entrainment further relative to cloud base. Some long‐standing model issues persist, such as underestimation of the subtropical boundary‐layer deepening rate, which leads to a cold and moist bias in the boundary layer. In the Arctic, the boundary layer is generally too well mixed and warm at the surface. The degree of decoupling is also underestimated in both cases. However, the modifications discussed here lead to some improvement in OpenIFS.