From five metres to one kilometre: How does the W-Layer shape the stratocumulus-topped boundary layer?

The recently discovered “W-Layer” reveals an unexplored perspective in the modelling of the stratocumulus-topped boundary layer (STBL). The clear-sky radiative heating in the capping inversion, which constitutes the W-Layer, was previously unresolved in numerical models, either because of insufficient vertical resolution or the lack of an appropriate radiation model which represents the clear-sky effects.

To study the impact of the W-Layer on the STBL, we introduce a “band” radiation model to direct numerical simulations (DNSs) which resolve metre-scale features. The band model partly retains the spectral dependence of the clear-sky absorption by partitioning the longwave spectrum into two water vapour absorption bands, one CO₂ absorption band and a window region. The band model is computationally efficient and can reproduce the warming characteristics in the W-Layer.

We run a set of DNSs at Reynolds number Re = 5000, which corresponds to a vertical resolution of 2.2 m at the cloud top. We find that the W-Layer causes a direct effect of warming, which is most prominent in the cloudy region, leading to a slight drop in liquid water content. Meanwhile, an indirect effect of the W-Layer suppresses entrainment and the STBL growth rate by enhancing the buoyancy jump across the capping inversion. Correspondingly, the turbulence and convection intensity is reduced.