Abstract
Abstract. We compare turbulence properties in coupled and decoupled marine stratocumulus-topped boundary layers (STBLs) using high-resolution in situ measurements performed by the helicopter-borne Airborne Cloud Turbulence Observation System (ACTOS) platform in the region of the eastern North Atlantic. The thermodynamically well-mixed 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 stability. In the cloud-top region, heat fluxes almost vanished and buoyancy production was significantly smaller than for the coupled case. The 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, of the order of 100 m in both cases, indicate turbulent eddies smaller than the depth of the coupled STBL or of the sublayers of the decoupled STBL. We hypothesize 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. Scattered cumulus convection, developed below the stratocumulus base, may play a role in transport between those sublayers.
Highlights
Low-level stratocumulus clouds cover around 20 % of the Earth’s surface in annual mean, more than any other cloud type
The primary mechanism driving the circulation inside the stratocumulus-topped boundary layer (STBL) is longwave radiative cooling at the cloud top which produces convective instability
The climatology of the marine boundary layer was inferred by Rémillard et al (2012) based on the long-term groundbased measurements of the CAP-MBL (Clouds, Aerosol, and Precipitation in the Marine Boundary Layer) project (Wood et al, 2015) utilizing the Atmospheric Radiation Measurement (ARM) facility established right next to the Graciosa airport
Summary
Low-level stratocumulus clouds cover around 20 % of the Earth’s surface in annual mean, more than any other cloud type They occupy the upper few hundred meters of the planetary boundary layer, preferentially in the conditions of large-scale subsidence, strong lower-tropospheric stability and moisture supply from the surface (Wood, 2012). STBL decoupling is the factor which strongly influences further evolution of cloud pattern and boundary layer structure It constitutes an intermediate stage of transition from overcast stratocumulus into shallow cumulus convection over subtropical oceans as the air masses are advected by the trade winds towards the Equator (Albrecht et al, 1995; Bretherton and Wyant, 1997; De Roode et al, 2016; Zheng et al, 2020). The results of the comparison are summarized and discussed in the last section
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