Turbulent structure of the Arctic boundary layer in early summer driven by stability, wind shear and cloud-top radiative cooling: ACLOUD airborne observations

Abstract

Abstract. Clouds are assumed to play an important role in the Arctic amplification process. This motivated a detailed investigation of cloud processes, including radiative and turbulent fluxes. Data from the aircraft campaign ACLOUD were analyzed with a focus on the mean and turbulent structure of the cloudy boundary layer over the Fram Strait marginal sea ice zone in late spring and early summer 2017. Vertical profiles of turbulence moments are presented from contrasting atmospheric boundary layers (ABLs) from 4 d. They differ by the magnitude of wind speed, boundary-layer height, stability, the strength of the cloud-top radiative cooling and the number of cloud layers. Turbulence statistics up to third-order moments are presented, which were obtained from horizontal-level flights and from slanted profiles. It is shown that both of these flight patterns complement each other and form a data set that resolves the vertical structure of the ABL turbulence well. The comparison of the 4 d shows that especially during weak wind, even in shallow Arctic ABLs with mixing ratios below 3 g kg−1, cloud-top cooling can serve as a main source of turbulent kinetic energy (TKE). Well-mixed ABLs are generated where TKE is increased and vertical velocity variance shows pronounced maxima in the cloud layer. Negative vertical velocity skewness points then to upside-down convection. Turbulent heat fluxes are directed upward in the cloud layer as a result of cold downdrafts. In two cases with single-layer stratocumulus, turbulent transport of heat flux and of temperature variance are both negative in the cloud layer, suggesting an important role of large eddies. In contrast, in a case with weak cloud-top cooling, these quantities are positive in the ABL due to the heating from the surface. Based on observations and results of a mixed-layer model it is shown that the maxima of turbulent fluxes are, however, smaller than the jump of the net terrestrial radiation flux across the upper part of a cloud due to the (i) shallowness of the mixed layer and (ii) the presence of a downward entrainment heat flux. The mixed-layer model also shows that the buoyancy production of TKE is substantially smaller in stratocumulus over the Arctic sea ice compared to subtropics due to a smaller surface moisture flux and smaller decrease in specific humidity (or even humidity inversions) right above the cloud top. In a case of strong wind, wind shear shapes the ABL turbulent structure, especially over rough sea ice, despite the presence of a strong cloud-top cooling. In the presence of mid-level clouds, cloud-top radiative cooling and thus also TKE in the lowermost cloud layer are strongly reduced, and the ABL turbulent structure becomes governed by stability, i.e., by the surface–air temperature difference and wind speed. A comparison of slightly unstable and weakly stable cases shows a strong reduction of TKE due to increased stability even though the absolute value of wind speed was similar. In summary, the presented study documents vertical profiles of the ABL turbulence with a high resolution in a wide range of conditions. It can serve as a basis for turbulence closure evaluation and process studies in Arctic clouds.

Stability of the Arctic Winter Atmospheric Boundary Layer over Sea Ice in CMIP6 Models

“…Part of this variation is likely driven by differences in the mean-state wind shear amongst the models. We would expect an increase in wind-driven atmospheric turbulent heat fluxes with stronger wind shear (Chechin et al, 2023), and as a result, a stronger suppression of low-level stability. This relationship is seen (not shown) in the NP station data, for which there is a positive correlation (significant at 90% confidence) between the mean-state wind shear at a particular station, and the reduction in stability with wind speed recorded at that station.…”

“…Higher near-surface wind speeds are associated with increased near-surface air temperature and a reduction in the strength of near-surface temperature inversions over Arctic sea-ice (Chechin et al, 2019;Wiel et al, 2017). Additionally, wind shear is an important control on low-level atmospheric turbulent heat fluxes in the polar night (Chechin et al, 2023).…”

Elucidating the boundary layer turbulence dissipation rate using high-resolution measurements from a radar wind profiler network over the Tibetan Plateau

“…The evolution of turbulence in the PBL has been previously recognized to be closely associated with atmospheric stability (Chechin et al, 2023;Chen et al, 2013;Lai et al, 2021;Muhsin et al, 2016). Therefore, we take the gradient Richardson number (𝑅𝑖) as a variable to characterize atmospheric stability and the formation of turbulence over the TP.…”

“…A vast range of previous studies have attempted to figure out the mechanisms behind the observed turbulence, but most of them are based on radiosonde measurements or model simulation or reanalysis data (e.g., Banerjee et al, 2018;Che and Zhao, 2021;Wang et al, 2023a). A myriad of driving mechanisms is proposed to account for the PBL development over the TP, such as surface thermal and dynamic forcing, atmosphere stability, among others (Chechin et al, 2023;Chen et al, 2016;Lai et al, 2021;Wang et al, 2023a;Wang and Zhang, 2022). It has been demonstrated that the buoyancy term contribution on the southern slope of the TP is significantly larger than that on the southeastern edge of the TP (Wang et al, 2015).…”

Thermodynamic and kinematic drivers of atmospheric boundary layer stability in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC)

“…Measurements of near-surface wind speed can be used to infer mechanical production of turbulence (Banta, 2008). Radiatively influenced processes impacting the Arctic ABL include the generation of buoyant turbulence through surface energy fluxes emitted from open-water regions such as leads (Lüpkes et al, 2008); cold-air advection, especially over thin ice (Vihma et al, 2005); enhanced downwelling longwave radiation from low-level clouds (Wang et al, 2001); or turbulent mixing within the clouds and below cloud base due to cloud-top radiative cooling (Tjernström et al, 2004;Chechin et al, 2023). Measurements of the surface radiation budget and cloud characteristics support an understanding of the possibility for radiatively generated turbulence in the ABL.…”