Kelvin–Helmholtz Waves within the Rising Turbulent Layer during the Morning Transition at Dome C, Antarctica: Regimes and Patterns

Abstract

Understanding processes of wave-turbulence interactions in stably stratified shear flows is a challenging problem in atmospheric physics and hydrography. This observational study emphasizes the importance of carefully considering Kelvin–Helmholtz (KH) waves and their interaction with vortical turbulence for an adequate description and parametrization of the stable boundary layer. This study investigates two phenomena, traditionally discussed separately, occurring during the morning transition from stable to unstable stratification at Dome C, Antarctica. Using high-resolution sodar echograms, the simultaneous occurrence of (i) the morning rise of the inversion turbulent layer, and (ii) KH waves within this layer was observed. Regular braid-like structures associated with KH shear instabilities within the elevated turbulent layer were observed in more than 70% of days over a four-month summer period. Two regimes of the morning evolution (when KH waves occurred) are identified, depending on the presence or absence of turbulence during the preceding night. The weather and turbulent conditions favoring the occurrence of these regimes are analyzed corresponding the morphology of the wind and temperature fields retrieved from an instrumented 45-m tower with six measurement levels. Two distinct patterns of KH waves are observed: (1) quasi-periodical (with periods ≈ 8−15 min) trains containing 5−10 braids, (2) about continuous series lasting 20–70 min containing 20−80 braids. A composite shape of KH waves is determined. The periodicity of these waves is estimated to be between 20 and 70 s, and their wavelength is estimated roughly to be 100−400 m. The vertical thickness of individual braids at the wave crests ranges between 5 and 25 m. The total depth of a rising turbulent layer containing these waves varies between 15 and 120 m, and the ratio of the wavelength to the depth of the wave layer varies from 3 to 12 with a mean value ≈ 8. Numerical simulations face challenges in reproducing turbulent wavelike structures, particularly KHBs. Further investigation is needed to unveil the precise mechanisms driving the morning elevation and growth of KH waves along with the developing convective layer.