Statistical Characteristics of Thermal Convection Structures based on Acoustic Sounding Data

The thermal convection structures (TCS) and their characteristics manifestations in the atmospheric boundary layer were investigated using the data from acoustic Doppler sodar LATAN-3M. A longwave LATAN-3M sodar with a vertical resolution of 20 m in 2007 and 10 m in 2016, 2018, 2019, a pulse emission interval of 5 s in 2007 and 3 s in 2016, 2018, 2019, an altitude range of 400–600 m in 2007 and 350 m in 2016, 2018, 2019, and a basic carrier frequency of 2 kHz in 2007 and 3 kHz in 2016, 2018, 2019 had measured the profiles of the wind velocity components which were used for calculating the scale of TCS. Experimental data were being obtained during the field campaigns organized by the A.M. Obukhov Institute of Atmospheric Physics RAS in Rostov region and over semi-arid zones of the Caspian lowland in the eastern part of Kalmykia Republic, Russia.

The wind was weak and the convection was well-developed in the case studies over July of years 2007, 2016, 2018, 2019. A moving rectangular filter was used for averaging the original data of the horizontal and vertical wind-velocity components. The averaging interval had been empirically chosen and, in this case, amounted to 10 min. At such values, the spatiotemporal velocity-field structure was adequately reproduced.

The original method of acoustic sounding data treatment for extracting TCS has been developed and put to an evaluation test. The episodes of the vertical velocities above limit values at which TCS aroused hypothetically were considered. As the threshold, a few alternatives were used: 0.3 m/s, 0.6 m/s and 1.2 m/s. The duration of vertical velocity excess over the threshold, the maximum velocity within this interval and the horizontal scale were calculated. It is assumed that TCS move forward with some averaged velocity during any relatively small time step. In this case, the spatial distribution of velocity field and its time variations have been reproduced suitably.

The statistical distribution was close to Rayleigh distribution:

p(U) = (2U/U₀²)*exp ((U_m²-U ²)/U₀²),

where U₀² = (<U ²>-U_m²), <U ²> is the root-mean-square vertical velocity of TCS, and U_m – the threshold for vertical velocity. This closeness can facilitate the understanding of the processes in the so-called “grey-zone” of numerical simulation and be implemented in the parameterization, forecast of TCS. Note that Rayleigh distribution is applied to the statistics of the intense moist convective vortices and also of the height of the ocean waves.

This work was supported by Russian Foundation for Basic Research (projects No.19-05-50110, No.19-05-01008, No.17-05-41121), and by fundamental research program of Russian Academy of Science (program No.1).