Turbulent Helicity in the Atmospheric Boundary Layerстатья
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Дата последнего поиска статьи во внешних источниках: 11 апреля 2019 г.
Аннотация:We consider the assumption postulated by Deusebio and Lindborg (J Fluid Mech
755:654–671, 2014) that the helicity injected into the Ekman boundary layer undergoes
a cascade, with preservation of its sign (right- or alternatively left-handedness), which is
a signature of the system rotation, from large to small scales, down to the Kolmogorov
microscale of turbulence. At the same time, recent direct field measurements of turbulent
helicity in the steppe region of southern Russia near Tsimlyansk Reservoir show the opposite
sign of helicity from that expected. A possible explanation for this phenomenon may be the
joint action of different scales of atmospheric flows within the boundary layer, including
the sea-breeze circulation over the test site. In this regard, we consider a superposition of
the classic Ekman spiral solution and Prandtl’s jet-like slope-wind profile to describe the
planetary boundary-layer wind structure. The latter solution mimics a hydrostatic shallow
breeze circulation over a non-uniformly heated surface. A 180°-wide sector on the hodograph
plane exists, within which the relative orientation of the Ekman and Prandtl velocity profiles
favours the left rotation with height of the resulting wind velocity vector in the lowermost
part of the boundary layer. This explains the negative (left-handed) helicity cascade toward
small-scale turbulent motions, which agrees with the direct field measurements of turbulent
helicity in Tsimlyansk. A simple turbulent relaxation model is proposed that explains the
measured positive values of the relatively minor contribution to turbulent helicity from the
vertical components of velocity and vorticity.