LIDAR STUDY OF WIND TURBULENCE, LOW LEVEL JET STREAMS, AND ATMOSPHERIC INTERNAL WAVES IN THE BOUNDARY LAYER OF ATMOSPHERE

The results of lidar study of wind turbulence, low level jet streams, and internal atmospheric waves in the stable boundary layer of atmosphere on the coast of Lake Baikal are presented. Few events of the atmospheric internal waves (AIWs) were registered during the experimental campaign. All the registered AIWs were observed in the presence of low level jet streams. Two dimensional time–height patterns of the wind turbulence dissipation rate during AIW events were obtained as well.


INTRODUCTION
Wind turbulence, low level jet streams, and atmospheric internal waves (AIWs) were studied on the coast of the Lake Baikal in summer campaigns of 2015 and 2016 with the use of a Stream Line lidar (Halo Photonics). The lidar was mounted 340 m away from the coast of the lake, on the territory of the Baikal Astrophysical Observatory of Institute of Solar-Terrestrial Physics, Siberian Branch, Russian Academy of Sciences (51°50'47.17"N, 104°53'31.21"E); the lidar altitude above the lake level was 180 m. The analysis of the lidar data processing results shows several AIW events occurred in the presence of jet streams. Two jet streams were observed simultaneously during some AIW events. Using the initial data of these measurements (arrays of radial velocities and signalto-noise ratios (SNR)) we also retrieved the timeheight patterns of the turbulence kinetic energy dissipation rate and calculated the relative error of assessment of the dissipation rate from the lidar data.

METHODOLOGY
Stream Line lidar measurements were carried out under the following parameters: elevation angle To retrieve the vertical profiles of the wind speed U, the wind direction angle V θ , and the vertical component of the wind velocity , the method of filtered sine wave fitting was used [1,2]. This method allows estimation of the mean wind velocity with acceptable accuracy from very noisy arrays of radial velocities containing up to 50% of bad estimates . Here is the number of current scan. As shown in [3] for =3000, the minimal signal to noise ratio is 0.005 (-23 dB), under which the wind speed estimation error does not exceed 0.1 m/s. n a N The turbulence kinetic energy dissipation rate ε is one of key parameters of the wind turbulence. The turbulence structure in its inertial interval is completely determined by this parameter and obeys the Kolmogorov law [4]. To assess the turbulence energy dissipation rate ε , we used the method of transverse (azimuth) structure function of the radial velocity measured by the lidar during conical scanning [5]. Therefore, a high resolution in the azimuth angle was important during the measurements. At the same time, the tangential speed of the volume sounded movement   Fig. 1 clearly show the presence of two jet streams with speed maxima at altitudes of about 250 and 750 m, respectively (hereinafter, the altitudes are given relative to the lidar location point). The directions of these jet streams are almost perpendicular to each other (the bottom jet stream was directed from north to south through mountains, and the top jet stream was directed from east to west, i.e., from the lake side) [3]. The atmospheric conditions promoted generation of an AIW. Near-harmonic oscillations of and are clearly seen in Fig. 1 from 14:20 to 15:00. The data on the turbulence energy dissipation rate in Fig. 1 show that values from a wide range, attaining a maximum of about 0.006 m 2 /s 3 before the generation of a jet stream. Relatively strong wind turbulence is also observed under the jet stream. The turbulence is quite weak inside the jet stream, including during an AIW event.    Fig. 1).The turbulence energy dissipation rate obviously can be retrieved from lidar measurements with an error less than 30% at m 2 /s 3 . The SNR should be no less than 0.075 in this case. The turbulence spatial structure should include an inertial interval with the upper boundary of no less than a half of the longitudinal size of the sounded volume ( = 30 m for the Stream Line lidar). 6 5 10

CONCLUSIONS
The results of lidar study of wind turbulence, low level jet streams, and internal atmospheric waves in the boundary layer of atmosphere on the coast of Lake Baikal are presented.
For estimation of the mean wind velocity the modified method of filtered sine wave fitting [2,3] was used. The method modified allows a 1.5-fold increase in the altitude of retrieval of the mean wind velocity profiles from lidar data as compared to the traditional sine wave fitting.
The method of azimuth structure function [5] was applied for estimation of the turbulent energy dissipation rate from radial velocities measured by the lidar with the use of conical scanning by probing beam around the vertical axis. The validity of this method depends on the magnitude of the dissipation rate measured and the lidar signal to noise ratio. For minimal possible values of the dissipation rate, the method is valid if the signal to noise ratio of lidar echo signal exceeds 0.24.
Few events of the internal atmospheric waves were registered during the experimental campaigns. All the registered AIWs were observed in the presence of low level jet streams. The periods and amplitudes of wave variations of wind velocity during AIWs and parameters of jet streams were determined.
Two dimensional time-height patterns of the wind turbulence dissipation rate during AIW events were obtained as well. It is shown that the wind turbulence in the area occupied by jet streams is very weak. Possibly, it is absent at all, and wind velocity variations in the area of jet streams are caused by mesoscale atmospheric processes. In the process of dissipation of AIWs the wind turbulence strength increases.