Coupled and decoupled stratocumulus-topped boundary layers: turbulence properties

• Main Authors: Nowak, Jakub L., Siebert, Holger, Szodry, Kai-Erik, Malinowski, Szymon P.
• Format: Text
• Language: English
• Published: 2021
• Subjects: North Atlantic
• Online Access: https://doi.org/10.5194/acp-2021-214; https://acp.copernicus.org/preprints/acp-2021-214/

We compare turbulence properties in two cases of marine stratocumulus-topped boundary layer, coupled (CP) and decoupled (DCP), using high resolution in situ measurements performed by the helicopter-borne platform ACTOS in the region of Eastern North Atlantic. Thermodynamically well-mixed CP was characterized by large latent heat flux at the surface and in cloud top region, and substantially smaller sensible heat flux. 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 inertial range were reasonably consistent with the predictions of Kolmogorov theory. The turbulence was close to isotropic. In the DCP, decoupling was most obvious in humidity profiles. Heat fluxes and buoyant TKE production at the surface were similar to the CP. Around the transition level, latent heat flux decreased to zero and TKE was consumed by weak stability. In the cloud top region heat fluxes almost vanished and buoyancy production was significantly smaller than for the CP. TKE dissipation rate inside the DCP differed between its sublayers. Structure functions and power spectra in 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 CP, with horizontal fluctuations dominating. The degree of anisotropy was largest in the cloud and subcloud layer of the DCP. Integral lengthscales, of the order of 100 m in both cases, indicate turbulent eddies smaller than the depth of the CP or of the sublayers of the DCP. We hypothesize that turbulence produced in the cloud or close to the surface is redistributed across the entire CP but rather only inside the relevant sublayers in the DCP. Scattered cumulus convection may play a role in transport between those sublayers.