Technique and Results of Measurements of Turbulent Helicity in a Stratified Surface Layer

Abstract

In August 2014, measurements of the turbulent velocity rotor, turbulent temperature gradient, turbulent helicity, and turbulent potential vortex were performed at the Obukhov Institute of Atmospheric Physics testing ground in Tsimlyansk under different stratification conditions. The measurements were carried out using the technique first used in the Tsimlyansk expedition in 2012 [1]. The measuring facility consisted of four three-component acoustic Gill Windmaster anemometers–thermometers placed at the vertices of a 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 measuring facility was placed on top of a mast with an adjustable height of 3.5, 5, 13.5, and 25 m and was equipped with a rotator. The temperature profile in the 10–600 m layer was continuously recorded by the Kadygrov microwave profiler [2]. The series of density of instantaneous helicity He = u_i'ω'_i = u₁'ω₁' + u₂'ω'₂ + u₃'ω'₃ and average values of the total and its summands were calculated for 12 daytime and 10 daytime 2-hour intervals. The helicity value averaged over 12 day realizations is about 0.2 m/s², and the average cosine is close to 0.08 ± 0.03. At night, the helicity is estimated as 0.07 ± 0.03 m/s², and the cosine is close to 0.025 ± 0.03. For the abovementioned 12 daytime and 10 daytime 2-hour intervals, the covariance and correlation matrices of temperature components, velocity rotor, velocity, and temperature gradient are calculated. The off-diagonal terms of the covariance matrix exceed by absolute values the diagonal terms several times. Similar characteristics of a potential vortex were estimated in the incompressibility approximation. The systematic error due to spatial averaging of the measured quantities is discussed.