Observations of turbulence properties in coupled and decoupled stratocumulus-topped marine boundary layers

Wide-spread presence, persistence and high albedo of marine stratocumulus clouds makes them important for the energy balance of the planet, hence also in model-based climate predictions. Typically, stratocumulus clouds occupy upper few hundred meters of the atmospheric boundary layer which is commonly referred as stratocumulus-topped boundary layer (STBL). The transport of moisture from the ocean surface maintains the cloud against entrainment drying from the free toposphere. When the STBL grows in depth, the drivers of the circulation weaken or the subcloud layer stabilizes, then the mixing of air volumes across the entire STBL depth may become impossible to sustain - the boundary layer decouples.

Within the present study, the stratification and turbulence properties in coupled and decoupled marine STBL are compared using high resolution in situ measurements performed by the helicopter-borne platform ACTOS in the region of the Eastern North Atlantic. Particular attention is given to small-scale turbulence.

The coupled STBL was characterized by a comparable latent heat flux at the surface and in the cloud top region, and substantially smaller sensible heat flux in the entire depth. Turbulence kinetic energy (TKE) was efficiently generated by buoyancy in the cloud and at the surface, and dissipated with comparable rate across the entire depth. Structure functions and power spectra of velocity fluctuations in the inertial range were reasonably consistent with the predictions of Kolmogorov theory. The turbulence was close to isotropic.

In the decoupled STBL, decoupling was most obvious in humidity profiles. Heat fluxes and buoyant TKE production at the surface were similar to the coupled case. Around the transition level, latent heat flux decreased to zero and TKE was consumed by weak static stability. In the cloud top region, heat fluxes almost vanished and buoyancy production was significantly smaller than for the coupled case. TKE dissipation rate inside the decoupled STBL varied between its sublayers. Structure functions and power spectra in the inertial range deviated from Kolmogorov scaling. This was more pronounced in the cloud and subcloud layer in comparison to the surface mixed layer. The turbulence was more anisotropic than in the coupled STBL, with horizontal fluctuations dominating. The degree of anisotropy was largest in the cloud and subcloud layer of the decoupled STBL.

Integral length scales were of the order of 100 m in both cases which is smaller than the depth of the coupled STBL or of the sublayers of the decoupled STBL. It may be speculated that turbulence produced in the cloud or close to the surface is redistributed across the entire coupled STBL but rather only inside the sublayers where it was generated in the case of the decoupled STBL.