Coupled and decoupled stratocumulus-topped boundary layers: turbulence properties

Abstract

Abstract. 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.

Aircraft Observations of Turbulence in Cloudy and Cloud‐Free Boundary Layers Over the Western North Atlantic Ocean From ACTIVATE and Implications for the Earth System Model Evaluation and Development

“…Most recently, Dodson and Small Griswold (2021) and Nowak et al. (2021) investigated turbulence measured in both level legs and vertical profiles.…”

Non-equilibrium dissipation scaling in atmospheric turbulence

“…For the analysis, the signal was first decomposed into the mean and fluctuating parts w(t) = w(t) + w (t) using a moving average of 50 s. The same window size was used in Ref. [9]. Time series corresponding to the horizontal leg at height 992 (inside the stratocumulus cloud) and the calculated moving average are presented in the upper panel in figure 1.…”

“…We focus on the stratocumulus-topped boundary layer (STBL) data collected during the ACORES (Azores stratoCumulus measurements Of Radiation, turbulEnce and aeroSols) campaign in the Eastern North Atlantic [8]. The detailed study of turbulence statistics for this campaign was performed in [9], however, the temporal tendencies were not discussed therein.…”

Estimating the turbulent kinetic energy dissipation rate from one-dimensional velocity measurements in time

“…In the case of atmospheric flows, in situ measurements made via airborne (Malinowski et al, 2013;Siebert et al, 2006Siebert et al, , 2013Muschinski et al, 2004;Frehlich et al, 2004;Nowak et al, 2021;Dodson and Small Griswold, 2021, e.g. ) as well as ground-based (Chamecki and Dias, 2004;O'Connor et al, 2010;Risius et al, 2015;Siebert et al, 2015, e.g.…”