Technique and Results of Measurements of Turbulent Helicityin Stratified Surface Layer
In August 2014, measurements of a turbulent velocity rotor, a turbulent temperature gradient, a turbulent helicity, and a turbulent potential vortex were performed at the IFA testing site in Tsymlyansk under various stratification conditions. The measurements were carried out using the technique, first used in the Tsimlyansk expedition in 2012 [1]. The measurement setup consisted of four three-component acoustic anemometers-thermometers Gill Windmaster, placed at the vertices of the rectangular tetrahedron with a base scale of 0.7 m (in contrast to the experiment in 2012, when the base scale was 5 m). The measurement setup was placed at the top of the mast, the height of which assumed values of 3.5, 5, 13.5 and 25 m, and was equipped with a pivoting device. The temperature profile in the 10–600 m layer was continuously recorded by the E.A. Kadygrov microwave profiler [2]. The series of density of instantaneous helicity $He = u_{i}^{''}\omega _{i}^{''} = u_{1}^{''}\omega _{1}^{''} + u_{2}^{''}\omega _{2}^{''} + u_{3}^{''}\omega _{3}^{''}$ and average values of the whole of this sum and its terms for 12 day and 10 night 2-hour intervals were calculated. Estimates were made of the dimensionless helicity. The helicity value averaged over 12 day realizations is of the order of 0.2 m/s<sup>2</sup>, and the average cosine is close to 0.08 ± 0.03. At night, helicity is estimated by magnitude 0.07 ± 0.03 m/s<sup>2</sup>, and the cosine is close to 0.025 ± 0.03. For the 12 day and 10 night two-hour intervals mentioned, the covariance and correlation matrices of the components of temperature, velocity rotor, velocity and temperature gradient were calculated. The off-diagonal terms of the covariance matrix exceed in absolute value the diagonal terms by several times. Similar characteristics of a potential vortex were estimated in the incompressibility approximation. A systematic error due to the spatial averaging of the measured quantities is discussed.