A study of gravity waves in the planetary boundary layer by acoustic sounding

In the period January-February 2014, observations were made at the Concordia station, Dome C, Antarctica to study atmospheric turbulence in the boundary layer using a high-resolution sodar. The turbulence structure was observed beginning from the lowest height of about 2 m, with a vertical resolution of less than 2 m. Typical patterns of the diurnal evolution of the spatio-temporal structure of turbulence detected by the sodar are analyzed. Here, we focus on the wavelike processes observed within the transition period from stable to unstable stratification occurring in the morning hours. Thanks to the high-resolution sodar measurements during the development of the convection near the surface, clear undulations were detected in the overlying turbulent layer for a significant part of the time. The wavelike pattern exhibits a regular braid structure, with undulations associated with internal gravity waves attributed to Kelvin-Helmholtz shear instability. The main spatial and temporal scales of the wavelike structures were determined, with predominant periodicity of the observed wavy patterns estimated to be 40-50 s. The horizontal scales roughly estimated using Taylor's frozen turbulence hypothesis are about 250-350 m.

Section: Introductionmentioning

confidence: 83%

Section: Wavy Structures During the Morning Evolution Of The Ablmentioning

confidence: 99%

Wavelike Structures in the Turbulent Layer During the Morning Development of Convection at Dome C, Antarctica

Petenko

Argentini

Casasanta

et al. 2016

“…In addition there are observations made in conditions of low geostrophic wind and zero surface fluxes when gravity-wave interaction with 'boundary-layer' flow may be important. These show (or imply) that turbulent activity is mostly confined to elevated layers associated with gravity-wave breaking with negligible momentum exchange with the surface (e.g., Beran et ul., 1973 and references therein;Eymand and Weill, 1979;Einaudi and Finnigan, 1980;Finnigan and Einaudi 1980). Many of these gravity-wave phenomena may be the result of forcing due to uneven terrain, or to the presence of thunderstorms, frontal systems or sea-breeze circulations in the region.…”

Section: Observations In the Nocturnal Boundary Layermentioning

confidence: 99%

Observations in the nocturnal boundary layer

Garratt

1982

Low-latitude observations of the stably-stratified planetary boundary layer (SRL) above rough terrain are compared to observations of the mid-latitude SBL mainly through the depth h and its dependence upon surface fluxes. This involves the quantity h/L and the similarity prediction h = y(u*L/yy .Mid-latitude observations are consistent with model calculations for nighttime-averaged quantities and their deviations, as functions of latitude and surface roughness, from the equilibrium values found at large i. The above applies to horizontaIly-homogeneous terrain.Low-Iatitude observations of$ and h/L are signi~cantly smaller than mid-latitude values, apparently the result of katabatic flows at the site and not the differences in latitude. This is consistent with model calculations for non-zero slope terrain.

“…The presence of internal gravity waves in the atmospheric boundary layer (ABL) has frequently been reported, e.g., Caughey and Readings (1975), Eymard and Weill (1979) and King et al (1987). Gravity waves play an important role in the energy cascade and in mixing processes, for example, in the disperson of pollutants under stable conditions.…”

Section: Introductionmentioning

confidence: 99%

On the characteristics of eddies in the stable atmospheric boundary layer

Rees

1991

A discussion of the cross-spectral properties of eddies in the lowest 40m of the nocturnal boundary layer is presented. The study involves the analysis of meteorological data collected by the British Antarctic Survey at Halley Station, Antarctica, during the austral winter of 1986. Cross-spectral analysis is used to determine whether the nature of the observed eddies is primarily turbulent or whether their structure is characteristic of ,qoherent internal gravity waves. It is found that the crossspectral phases indicate the presence of turbulent eddies only when the local gradient Richardson number (Ri) is less than the critical value of l/4. Trapped modes were only observed when an offshore wind prevailed, indicating that topographic effects are responsible for their generation. The relative phases of velocity and temperature were often observed to change with height. This can be explained by examining the underlying meteorological conditions. On several occasions, regions of counter-gradient fluxes were detected. A physical explanation of this phenomenon is proposed.