Atmospheric turbulence under very strong static stability and extremely low temperatures at Concordia station (Dome C, Antarctica) is investigated using measurements collected in a long-term experiment carried out in 2012, 2014 and 2015. The spatial and temporal structure of turbulence between a few meters above the surface and 200 m is examined using measurements from a high-resolution sodar with a vertical resolution better than 2 m. In situ measurements from a weather station, an ultrasonic anemometer/thermometer, a fourcomponent net radiometer, radiosoundings, and temperature and wind-speed sensors at six levels on a 45-m tower complement the sodar observations. Enhanced thermal turbulence extending up to several tens of meters is frequently observed despite (i) the existence of strong static stability at very low temperatures, (ii) the absence of the diurnal cycle of solar heating and (iii) the absence of orographic features. It is shown that it is necessary to distinguish clearly between the entire stable boundary layer (SBL, traditionally associated with a layer of temperature inversion) and the stable surface-based turbulent layer (SBTL), which can differ signicantly in depth - with HSBL persistently much greater than HSBTL, often by more than one order of magnitude. The SBTL depth varies from a few to several tens of metres and consistently occupies only the lowest 3–15% of the temperature inversion layer. The winter average SBTL depths are estimated to be approximately 23 m, 20 m, and 25 m, while the corresponding inversion-layer depths are 380 m, 350 m, and 400 m in 2012, 2014, and 2015, respectively. These measurements provide a unique database of experimentally characterized SBTL, covering different aspects of its morphology. The inuence of wind speed, temperature, longwave radiation, Richardson number and Brunt–Väisälä frequency on the SBTL depth is examined. Both repetition and variability in the behaviour of these parameters are analysed. The interannual Submission Completed https://ams.confex.com/ams/25BLT/blt/papers/confirmation.cgi?username=461699&EntryTable=Pa... 1 di 6 12/02/2025, 20:02 variability of the SBTL depth is primarily associated with that of wind speed. Visual inspection of more than 15000 h of sodar records is used to classify the SBTL for synoptically undisturbed conditions into several types, characterized not only by its depth and intensity, but also by its internal structure. Here, we distinguish three main SBTL types, even though this is a very subjective oversimplication: 1) very shallow depth (HSBTL < 15 m) with no visible regularity in the internal structure (sometimes with sporadic elevated bursts and sublayers); 2) shallow depth (HSBTL = 15–70 m) with a uniform chaotic internal structure and no visible regularity; 3) shallow depth (HSBTL = 20–70 m) with a wavelike internal structure showing braid-like ne-scale structures lasting several hours. Weather conditions (including the shape of vertical temperature and wind speed prole) associated to these SBTL types are identied and characterized. For type 1, temperature proles show a logarithmic dependence on height with a clear convex exponential shape. Both the uniform and the wavy types 2 and 3, which do not differ markedly in their temperature proles, exhibit ‘convex–concave–convex’ proles with two inexion points and maximum gradients in the upper part of the layer. Moreover, in regions with enhanced turbulence, the Richardson number varies considerably around a value of 0.25, and sometimes markedly exceeds it. The main peculiarity of the spatial and temporal pattern of thermal turbulence captured by sodar is the clear wave activity in the SBTL which occurs during a signicant portion of the time (28–35 %, with slight variations across different years), showing different kinds of internal gravity waves – mainly shear–generated and sometimes buoyancy driven. In many cases, wave patterns are characterized by regular trains (with periodicity of 8–15 min and vertical amplitude of oscillations ranging from a few to a few tens of meters) of Kelvin–Helmholtz billows with periods of 20–60 s, lasting from several minutes to several hours. The principal characteristics of these wave structures (shape, spatial and temporal scales), which are of particular interest for the more realistic representation of the SBTL, are estimated. These results allow to conclude that wave processes in the SBTL can play an important role in the transfer and exchange of energy through the shallow stable boundary layer. The obtained results constitute a large experimental database which can be used to develop and validate both numerical models and theoretical approaches, leading to a deeper understanding of turbulence features under different stable stratication regimes.
The Stable Surface-Based Turbulent Layer at Dome C, Antarctica from Sodar and In Situ Observations over Three Polar Winters
Petenko I.
Primo
;G. Casasanta;R. Sozzi;A. P. Viola;A. Cecilia;A. Conidi;
2025
Abstract
Atmospheric turbulence under very strong static stability and extremely low temperatures at Concordia station (Dome C, Antarctica) is investigated using measurements collected in a long-term experiment carried out in 2012, 2014 and 2015. The spatial and temporal structure of turbulence between a few meters above the surface and 200 m is examined using measurements from a high-resolution sodar with a vertical resolution better than 2 m. In situ measurements from a weather station, an ultrasonic anemometer/thermometer, a fourcomponent net radiometer, radiosoundings, and temperature and wind-speed sensors at six levels on a 45-m tower complement the sodar observations. Enhanced thermal turbulence extending up to several tens of meters is frequently observed despite (i) the existence of strong static stability at very low temperatures, (ii) the absence of the diurnal cycle of solar heating and (iii) the absence of orographic features. It is shown that it is necessary to distinguish clearly between the entire stable boundary layer (SBL, traditionally associated with a layer of temperature inversion) and the stable surface-based turbulent layer (SBTL), which can differ signicantly in depth - with HSBL persistently much greater than HSBTL, often by more than one order of magnitude. The SBTL depth varies from a few to several tens of metres and consistently occupies only the lowest 3–15% of the temperature inversion layer. The winter average SBTL depths are estimated to be approximately 23 m, 20 m, and 25 m, while the corresponding inversion-layer depths are 380 m, 350 m, and 400 m in 2012, 2014, and 2015, respectively. These measurements provide a unique database of experimentally characterized SBTL, covering different aspects of its morphology. The inuence of wind speed, temperature, longwave radiation, Richardson number and Brunt–Väisälä frequency on the SBTL depth is examined. Both repetition and variability in the behaviour of these parameters are analysed. The interannual Submission Completed https://ams.confex.com/ams/25BLT/blt/papers/confirmation.cgi?username=461699&EntryTable=Pa... 1 di 6 12/02/2025, 20:02 variability of the SBTL depth is primarily associated with that of wind speed. Visual inspection of more than 15000 h of sodar records is used to classify the SBTL for synoptically undisturbed conditions into several types, characterized not only by its depth and intensity, but also by its internal structure. Here, we distinguish three main SBTL types, even though this is a very subjective oversimplication: 1) very shallow depth (HSBTL < 15 m) with no visible regularity in the internal structure (sometimes with sporadic elevated bursts and sublayers); 2) shallow depth (HSBTL = 15–70 m) with a uniform chaotic internal structure and no visible regularity; 3) shallow depth (HSBTL = 20–70 m) with a wavelike internal structure showing braid-like ne-scale structures lasting several hours. Weather conditions (including the shape of vertical temperature and wind speed prole) associated to these SBTL types are identied and characterized. For type 1, temperature proles show a logarithmic dependence on height with a clear convex exponential shape. Both the uniform and the wavy types 2 and 3, which do not differ markedly in their temperature proles, exhibit ‘convex–concave–convex’ proles with two inexion points and maximum gradients in the upper part of the layer. Moreover, in regions with enhanced turbulence, the Richardson number varies considerably around a value of 0.25, and sometimes markedly exceeds it. The main peculiarity of the spatial and temporal pattern of thermal turbulence captured by sodar is the clear wave activity in the SBTL which occurs during a signicant portion of the time (28–35 %, with slight variations across different years), showing different kinds of internal gravity waves – mainly shear–generated and sometimes buoyancy driven. In many cases, wave patterns are characterized by regular trains (with periodicity of 8–15 min and vertical amplitude of oscillations ranging from a few to a few tens of meters) of Kelvin–Helmholtz billows with periods of 20–60 s, lasting from several minutes to several hours. The principal characteristics of these wave structures (shape, spatial and temporal scales), which are of particular interest for the more realistic representation of the SBTL, are estimated. These results allow to conclude that wave processes in the SBTL can play an important role in the transfer and exchange of energy through the shallow stable boundary layer. The obtained results constitute a large experimental database which can be used to develop and validate both numerical models and theoretical approaches, leading to a deeper understanding of turbulence features under different stable stratication regimes.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
