Coupled and decoupled stratocumulus-topped boundary layers:
turbulence properties

Nowak, Jakub L.; Siebert, Holger; Szodry, Kai-Erik; Malinowski, Szymon P.

doi:https://doi.org/10.5194/acp-2021-214

Preprints

https://doi.org/10.5194/acp-2021-214

Preprints

12 Mar 2021

Review status: this preprint is currently under review for the journal ACP.

Coupled and decoupled stratocumulus-topped boundary layers: turbulence properties

Jakub L. Nowak¹, Holger Siebert², Kai-Erik Szodry², and Szymon P. Malinowski¹ Jakub L. Nowak et al.

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¹Institute of Geophysics, Faculty of Physics, University of Warsaw, Pasteura 5, 02-293 Warsaw, Poland
²Leibniz Institute for Tropospheric Research, Permoserstr. 15, 04318 Leipzig, Germany

Received: 10 Mar 2021 – Accepted for review: 11 Mar 2021 – Discussion started: 12 Mar 2021

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.

Jakub L. Nowak et al.

Status: open (until 15 May 2021)

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RC1:
'Comment on acp-2021-214', Anonymous Referee #1, 30 Mar 2021 reply

General comments

This paper presents a description of airborne measurements of turbulence during the ACORES campaign, distinguishing between two cases: 1) a fully coupled cloud-topped marine boundary and 2) a boundary layer that is partially decoupled from surface fluxes. The technical quality of the analysis appears to be excellent and, with a couple of exceptions noted below, is thoroughly explained. As such, I believe this paper makes a significant contribution to our empirical understanding of turbulence in the marine boundary layer. While it is longer than most manuscripts that I review, I’m not sure that it can be substantially shortened without omitting important information.

The focus of this study lies somewhat outside my own areas of greatest experience, so I’m not able to confidently assess the relationship between this contribution and prior work in this same area. That said, the reference list is extensive, and the authors appear to be thorough in drawing connections to earlier work.

Overall, my recommendation is that it be published after considering the suggestions for revisions below.

Specific comments

Lines 43–45: Could a reduction in cloud-top LW cooling due to an overrunning cloud layer at somewhat higher altitude also contribute to decoupling?

Line 106: LEGs are described as being 10 km long, but the time intervals shown on Fig. 2 seem too short at the nominal flight speed of 20 m/sec. I would prefer to see lengths and altitudes of the LEGs included in a table. Among other things, this is relevant to the question of flux sampling error (see comment further down).

The helicopter used weighs somewhere around 2000 kg and imparts substantial downward momentum and turbulent kinetic energy to the environment directly below it. In fact, rotor downwash speeds a short distance below the helicopter are probably around 30 m/sec, and the area of influence expands considerably with distance below the aircraft (albeit with reduced velocities). With that in mind, I would have liked to see more discussion, including any relevant references, in support of the assumption that a 20 m/sec forward speed is sufficient to avoid any influence by the rotorwash on the ACTOS package suspended 150 m below the helicopter, taking into account as well that the package probably trails behind the helicopter by some distance during forward flight.

I believe there should be explicit discussion of sampling error, and its relationship to flight leg length, in connection with the turbulent flux measurements. One newly published paper that seems relevant is Petty, G. W.: Sampling error in aircraft flux measurements based on a high-resolution large eddy simulation of the marine boundary layer, Atmos. Meas. Tech., 14, 1959–1976, https://doi.org/10.5194/amt-14-1959-2021, 2021.

Note for the authors or ACP copy editor:

The quality of the English writing is excellent. The only real indication that the paper was not written by a native speaker is the frequent omission of the articles “a”, “an”, and “the” in sentences where they would normally be expected.

Reply

Citation: https://doi.org/10.5194/acp-2021-214-RC1
- AC1: 'Reply to the Anonymous Referee #1', Jakub Nowak, 12 Apr 2021 reply
  
  We are grateful to the Referee #1 for the insightful comments and suggestions on ourmanuscript. We respond to them in detail in the attached pdf file.
  
  Reply
  
  Citation: https://doi.org/10.5194/acp-2021-214-AC1

Jakub L. Nowak et al.

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Short summary

Turbulence properties in two cases of marine stratocumulus-topped boundary layer have been compared using high resolution helicopter-borne in situ measurements. In the coupled one, small-scale turbulence was close to isotropic and reasonably followed inertial range scaling according to Kolmogorov theory. In the decoupled one, turbulence was more anisotropic and the scaling deviated from theory. This was more pronounced in the cloud and subcloud layer in comparison to the surface mixed layer.


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