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SUMMARY:Turbulent vorticity and helicity in stratified atmospheric boundar
 y layer - Chkhetiani\, O\, Koprov\, V\, Koprov\, B (Obukhov Institute of A
 tmospheric Physics\, Russian Academy of Sciences)
DTSTART:20120723T111000Z
DTEND:20120723T113000Z
UID:TALK39018@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:To measure the spatial derivative of velocity v\, it is necess
 ary to possess the sensors\, which size much less than internal scale of t
 urbulence. In a surface layer it is estimated by size of an order 1 mm. Th
 e concept of the acoustical method of vorticity measurements and the first
  results of its realization are obtained in IAP [Bovsheverov et al. 1971].
  \nHelicity (a scalar product of the velocity v and the vorticity) is one 
 of the important characteristics of large-scale atmospheric motions [Etlin
 g 1985\, Moffat\, Tsinober 1992\; Kurgansky 2002\, Chkhetiani 2001]. \nDir
 ect experiments aimed at the measurement of turbulent helicity are extreme
 ly rare. They have been carried out under laboratory conditions in turbule
 nce beyond a grid [Kholmyansky et al. 2001].\nFirst helicity measurements 
 in atmospheric boundary layer were made in IAP Zvenigorod station in 2004 
 [Koprov et al. 2005]. Experimental estimates of the spectrum of the turbul
 ent helicity in the atmospheric boundary layer give the spectrum slope of 
 about -5/3. Proceeding from the helicity and energy spectra\, we obtain fo
 r dissipation ? ? 0.0003 m/s3\, ? ? 0.003 m2/s3 and ? ? 0.0005 m/s3\, ? ? 
 0.001m2/s3.\nHelicity components in day conditions shows considerably big 
 intermittency than circulation. Average value for Hz has made 0.2 m/s2. Th
 e correlation factor between product factors in a day series at moderately
  unstable stratification has made 0.344. Similar indicators for Hx: averag
 e value of 0.46 m/s2\, factor of correlation 0.215. In an evening series a
 verage values of both measured helicity components had the same sign\, as 
 in the afternoon\, but on 1-2 order smaller values. \nProbability density 
 functions (PDF)  for circulations Zz\, Zx\, vertical velocity w and temper
 ature T have been calculated at unstable and stable stratification. Asymme
 try of  Zz changes a sign at change of a sign on parameter of stratificati
 on whereas asymmetry for Zx is small and keeps a sign at any stratificatio
 n. PDF of helicity components and complex triple two-point correlations of
  velocity and vorticity show strong non-gaussian character. Spectrum slope
  for these correlations is close to f -1. This fact corresponds to the "2/
 15" law for the helicity cascade [Chkhetiani 1996\, 2010]. \nThe data obta
 ined for both the values of turbulent helicity in the atmospheric boundary
  layer and helicity spectra indicate the existence\, at least in the regio
 n of the scales that have been considered\, of parallel cas-cades of the e
 nergy and helicity. The importance of the determination of actual helicity
  cascades in natural systems is also stimulated by the fact that numerical
  calculations of the Navier-Stokes equations manifest certain effects of n
 onzero helicity on the energy transfer over the spectrum. This emphasizes 
 the role of helicity in the formation of large-scale structures.\n
LOCATION:Seminar Room 1\, Newton Institute
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