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

Abstract. Clouds are supposed to play an important role for the Arctic amplification process. This motivated a detailed investigation of cloud processes including radiative and turbulent fluxes. Data of 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/early summer 2017. Vertical profiles of turbulence moments are presented, which belong to contrasting atmospheric boundary layers differing by the magnitude of wind speed, boundary-layer height, stability and by the strength of the cloud-top radiative cooling. 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. It is shown that especially during weak wind, even in shallow and relatively dry Arctic ABLs, cloud-top cooling can serve as a main source of turbulent kinetic energy. 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 reach also maxima in the cloud layer as a result of cold downdrafts. Turbulent transport of heat flux and of temperature variance are both negative in the cloud layer suggesting an important role of large eddies caused by the cloud top cooling. In strong wind, wind shear is shaping the ABL turbulent structure, especially over rough sea ice. 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. 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.