This paper describes a pulsed coherent Doppler lidar, recently created at the Wave Propagation Laboratory of the Institute of Atmospheric Optics SB RAS (WPL lidar). The WPL lidar was tested in atmospheric experiments. Comparison of the results of joint measurements of the radial velocity by the WPL lidar and Stream Line lidar showed their satisfactory agreement. From measurement of radial velocities by the WPL lidar at various azimuth angles (that is, with the use of the conical scanning), the height profiles of wind velocity and wind direction angle in the atmospheric boundary layer have been retrieved.
Data are presented about the variations in the average power of lidar echo signals with coherent and incoherent receiving over wide range of variations of the refractive turbulence strength in the atmosphere. It is established that in the case of incoherent receiving the backscatter amplification coefficient first increases with increase of the turbulence strength, then saturates at a certain level in the regime of strong turbulence on the probing path and decreases with further refractive turbulence strength increase. The maximum value of the amplification coefficient achievable at a given distance depends on the refractive turbulence strength at the moment of measurement. The time course of the backscattering amplification coefficient is in a good agreement with the temporal changes of the structural constant of turbulent fluctuations of the air refractive index, determined from independent measurements.
The task was to create a compact turbulent lidar, working on the backscatter enhancement effect. Initially, it was planned to make a lidar with the spacing of the receiving channels using two small 50 mm Mersenne telescopes collected from off-axis parabolic mirrors. Despite the measures taken, the system did not pass the stability test for prolonged operation. Then we used an optical layout in which the receiving channels were combined. The converted device turned out to be workable, but its sensitivity was slightly lower than that of the channel spacing scheme. Lidar “BSE-4” was successfully used in field experiments in 2017 (Tomsk, Lake Baikal).
The turbulent lidar is a micro pulse dual-channel aerosol laser locator, operating on the backscattering enhancement effect (BSE). This effect forms a peak on the axis of the laser beam, therefore, when the intensity of atmospheric turbulence changes in one of the receiving channels of the lidar, a proportional change in the echo signal occurs. The second channel is necessary for normalization of the first one in order to exclude the influence of transmitter power fluctuations and scattering properties of the atmosphere along the direction of sounding. The sensitivity of lidar is largely determined by its design. The experimental and theoretical data were analyzed characterizing the peak of backscatter enhancement. A scheme of a lidar transceiver with spacing between receiving channels is considered and parameters of the optimal lidar design are determined. Calculations have shown that the optimal diameter of the transmitting-receiving aperture should be 70 mm.
A specialized aerosol lidar has been designed at Institute of Atmospheric Optics SB RAS for remote sounding of atmospheric turbulence. The lidar records aerosol backscattering amplification (BSA) in the turbulent atmosphere. An image jitter sensor (IJS) was manufactured for automated monitoring of turbulence along a path; it allows estimation of the mean intensity of atmospheric temperature turbulence along a horizontal 2-km path. The results of simultaneous operation of BSA-2 lidar and IJS under weak atmospheric turbulence are described. They were used for estimation of the turbulent lidar sensitivity. It was noted that IJS readings of the variance of image jitter differ in horizontal and vertical directions under a weak turbulence. An increase in the vertical component of the jitter variance was caused by some slow atmospheric processes or convection near an IJS target, which did not affect the lidar data.
The quality of adaptive suppression of initial aberrations of the wave front of a main laser beam with the use of the method of aperture sensing by the signal of atmospheric backscattering of the additional (sensing) laser radiation at a different wavelength has been studied experimentally. It is shown that wavefront distortions of the main laser beam were decreased significantly during the setup operation.
Micro pulse lasers have allowed solution of some technical problems and design of a specialized aerosol lidar capable of recording backscattering amplification (BSA) in a turbulent atmosphere (2014) by now. The BSA-lidar has two receiving channels, one of which is affected by a turbulence. The measurement result is the ratio of echo signals, i.e., the coefficient of backscattering amplification. The problem of lidar data inversion and retrieval of “optical” turbulence parameters was recently solved by V.V. Vorob’ev theoretically (2016). A lidar experiment was organized for testing the solution, and the asymptotic solution was applied to echo signals, which allowed estimating the daily behavior of the structural characteristics C_{n}^{ 2} along a horizontal 2-km path. The experiment was accompanied by parallel independent measurements of C_{n}^{2} by an image jitter sensor along the same path. It was shown experimentally that the Vorob’ev solution is applicable to C_{n}^{2} retrieval from BSA-lidar data if β_{0}^{2}≤3; for β_{0}^{2}>3, the saturation of the amplification effect and a decrease in the experimental data with respect to calculation results are observed. The coefficient of correlation between the retrieved structural characteristics C_{n}^{2 }of the lidar and jitter sensor is 0.8–0.9. The C_{n}^{ 2} values retrieved from lidar signals turned out to be 20–40% lower than the C_{n}^{2} values of the image jitter sensor.
In this paper we are talking about the possibility of increasing the accuracy of measurements of turbidity meter. The article analyzes the requirements of the light source being probed volume, as well as in calibration mode. Use as a pulsed light source of high power LEDs, collimator requires optimization which is devoted to this work. The main factor limiting the possibilities for creating the desired collimator are the dimensions of both the collimator, the one and only device. Considerable attention is paid to the uniformity of light-probed volume nephelometer.
An aerosol lidar designed specifically for measurement of the backscatter amplification (BSA) was used in a field experiment, and the procedure for information processing was proposed, which allows the obtained data to be presented in the form convenient for analysis. The BSA-lidar records the value of the relative amplification coefficient, which is proportional to the intensity of atmospheric turbulence. The spatial profile of the backscatter amplification coefficient is the monotonically increasing function, the first derivative of which allows one to localize zones with intense turbulence. Construction of range-time images (RTI) of the derivative of the amplification factor gives a comprehensive picture of the location of such zones and their temporal dynamics. The experiment was conducted in the winter season under urban conditions. An image jitter sensor operated at the same path simultaneously with the lidar for the independent control. It has been found that the night behavior of the amplification factor differed from the daytime one by its volatility, and night values of this factor could exceed twofold the daytime level, increasing up to 3. Maximal gradients of the amplification factor equal to 4 km^{-1} have been obtained.
The paper reports the results of operation of a lidar developed for measurement of the backscatter amplification in the turbulent atmosphere along with the image jitter sensor. The sensing was carried out round-the-clock under urban conditions along a horizontal 2-km path in winter at negative temperatures, when significant temperature gradients took place in the atmosphere. The lidar coefficient of backscatter amplification correlated with the variance of image jitter of a white matte disk of the image jitter sensor, whose path coincided with the lidar path. The correlation coefficient between the backscatter amplification coefficient at the path end and the standard deviation of the disk image jitter was 0.80-0.85. When the turbulence intensity along the sensing path became significantly inhomogeneous, marked differences appeared between readings of the lidar and the jitter sensor.
The paper sums up the principles of construction of all-fiber Doppler coherent wind lidars, which can be divided into two classes by the method of signal conversion: homodyne and heterodyne. Lidars of the first class, usually, with continuous-wave probing radiation employ the direct conversion of the signal, when the carrier frequency coincides with the master oscillator frequency. Systems of the second class are pulsed Doppler lidars with the carrier frequency shifted about the master oscillator frequency, which allows simultaneous measurements of the direction and speed of the radial wind. Now our laboratory has started the works to make a prototype model of the pulsed all-fiber coherent Doppler lidar.
The results of long-term continuous measurements of the atmospheric backscattering amplification on an atmospheric surface path 2 km long with a two-channel micropulse lidar based on waveguide laser are presented. It is shown that the backscatter amplification coefficient has the pronounced diurnal behavior. In the day and night time, the atmospheric backscattering amplification is maximal and the amplification coefficient can exceed two. The amplification is low or absent in the morning and evening hours at the neutral temperature stratification in the atmospheric surface layer. The backscattering amplification coefficient increases with an increase of the structure constant of the refractive index of air, as well as with an increase of random wander of optical image of the probing laser beam spot at the distance 2 km from the lidar.
The design of a two-channel lidar with afocal transceiving telescope receiving the backscattered radiation at the axis of the probing beam and at some distance from the axis is described. The lidar with this design can detect the atmospheric backscatter amplification caused by turbulent fluctuations of the refractive index of air due to the correlation of the probing and scattered radiation. The power ratio of echo signals recorded in two different channels gives the value (coefficient) of backscatter amplification. The developed micropulse two-channel lidar can conduct unattended continuous measurements for a long time.
The experimental layout of aerosol lidar with two receiving channels for registration of the atmospheric backscatter amplification is presented in the paper. One of the receiver is placed at the probing beam axis, the other one is displaced from the beam axis in a perpendicular plane on a distance where the echo signals registered by both receivers are uncorrelated. Ratio of the powers of the registered echo signals is a measure of atmospheric backscatter amplification effect.
Results of application of incoherent spatial filtering for structure analysis of lidar returns from clouds are presented. The
revolver disk with a set of the spatial filter windows provides multiple-field-of-view of the receiver system. There is an
agreement between data recorded and model estimates considering the power contribution of double scattering
component due to diffraction of the sounding radiation in a droplet cloud.
Proc. SPIE. 5027, Ninth Joint International Symposium on Atmospheric and Ocean Optics/Atmospheric Physics. Part II: Laser Sensing and Atmospheric Physics
The results of measurements with an automatic integrating nephelometer intended for recording of molecular and aerosol scattering coeffients are discussed. The diffraction grid was used as a dispersive element. The frequency range of recording of molecular and aerosol scattering coefficients was from 300 to 820 nm. The spectral calibration of the device with the help of gases with known scattering coefficients is discussed, the evaluation of sensitivity of the nephelometer with use of the calibrated source of photons is considered. The measurements of dynamic range of the nephelometer are presented. It is shown that the device has dynamic range about 5 orders. The measurements of scattering coefficients in room conditions and outdoors are considered. The use of the nephelometer for continuous extended in situ observations of the scattering coefficient is discussed. High frequency of data recording permits us to use the nephelometer for estimation of turbulent aerosol flows, if the measurements of components of a wind velocity will be carried out synchronously with use of the nephelometer.
In the paper it is proposed to make a spatial-time analysis of aerosol micro pulse lidar data by a parametric statistical approach based on a mixed model of 'auto-regressive-moving average' (ARMA). The given approach allows obtain the spectral estimations of auto- and cross-spectra with high frequency resolution using short time series of data. The determination of parameters of the ARMA model has been made on a scheme of two-channel spectral estimation by the Nuttall-Strand method. The examples of obtaining an additional information data on dynamics of aerosol inhomogeneities from real micro pulse lidar are presented.
The spatial distribution of laser beam itself and the alignment of the receiving-transmitting system are very important parameters in quantitative analysis of Lidar signal. This is important because the beam shape and the inclination angle between axes of the transmitter and receiver change the physical quality of detecting elements, for example, the aerosol scattering coefficient and the concentration of pollutants, etc. In practical alignment of Lidar system, the inclination angle is allowed to fluctuate within the receiver field of view. At a long distance where geometric overlap is complete, this fluctuation has no influence on the final results, but at a short distance this effect is serious. If we know the distribution of laser beam and inclination angle, this effect can be corrected by using geometrical overlap function. In this study, we have calculated the geometrical overlap function for Gaussian and uniform distributions, respectively. It is found that the ratio-function, which is defined as the ratio of two geometrical functions obtained from different angles between the axes of the transmitter and receiver, behaves in different way for Gaussian and uniform laser beams. The completely different behavior of these two ratio-functions for different beam types can be used more conveniently in identifying the laser beam shape and inclination angle than geometrical functions only. This fact can be used for characterizing the spatial distribution of laser beam and for testing of alignment between receiver and transmitter.
Data of in situ observations obtained in Zarechnyi Polygon of the Institute of Atmospheric Optics near Tomsk in the morning on October 9 1996, are analyzed. Ultrasound acoustic meteorological station was capable of recording with a frequency of 2 Hz three components of the wind velocity and temperature of the air, and a two-angle nephelometer of an open type measured the coefficients of scattering at angles of 45 degree(s) and 165 degree(s). Measurements have been carried out continuously for a long time at a sampling frequency of 2 Hz. Devices were mounted on the mast of 5 m height. When stratification was temperature stable, the quasi-periodic vibrations of meteorological parameters and parameters of aerosol scattering was temperature stable, the quasi- periodic vibrations of meteorological parameters and parameters of aerosol scattering were observed with a period of 25 minutes. The oscillations of the transformation coefficient of scattering phase function as well as of temperature were particularly pronounced. The view of time realizations pointed to the presence in the atmosphere of the known as buoyancy waves or gravity waves. The spectra of fluctuations of parameters were calculated. It is noted that spectra of speed, temperature and aerosol concentration, calculated from 17 minute intervals, in the low-frequency region of the inertial interval had greater slope in frequency region from 0.03 Hz to 0.2...0.3 Hz than the slope of the Kolmogorov spectrum (the -5/3 law). Simultaneously, analogous spectrum for vertical component of the wind velocity had the slope close to the Kolmogorov spectrum in the frequency range from 0.03 Hz to 1 Hz.
Idea of this work is to use the spectral Fourier analysis of time series of lidar return fluctuations between two different heights. Formally, fluctuating signals from two heights can be considered as an input and output signal of a linear system. If the system is linear, the mutual phase spectrum is linear function of temporal frequency. We have used a simple and reliable method to estimate wind velocity, and the algorithm is to fix the frequency at which the phase spectrum reaches the value +/- (pi) or jump through zero of mutual phase spectrum function. The sign of phase angle means the direction of movement, i.e. upwards or downwards. The algorithm is applied to the real data of Micro Pulse Lidar (MPL) system. The MPL data provide an array of aerosol scattering coefficients up to the heights of 20 approximately 30 km with the sampling frequency of approximately 20 cycles per hour. In order to determine the vertical speed, the average time interval about 3 hours is required. The statistical reliability at single height is obtained by averaging several heights with the resolution of 1 km interval. From a spatial-temporal map of vertical motion in cloudy conditions, we find that there are downward movements of the air under the cloudy layers. That is the typical characteristics of cloud because when it moves down, cloudy drops are evaporated and also air temperature is lower. This dynamics promotes further acceleration of this process.
In practical applications of lidar sounding, it is often difficult for the devices which work on various physical measurements to estimate meteorological parameters of the atmosphere. One of the difficulties is to define the slow vertical speed of the aerosol motion. This study is based on statistical processes at a spatial frequency domain after long-term observation of aerosol motion with the help of Micro Pulse lidar or other lidar systems. Basic assumption of the aerosol model is that an inhomogeneous aerosol distribution of certain size has initially uniform spatial arrangement. Simultaneously, we also assume that there are two more different aerosol distributions whose size is two times smaller than the initial one. The resulting aerosol field is a superposition of inhomogeneous aerosol distribution of the different sizes. The randomness of the aerosol distribution is provided with the random-number generator which changes the size and the position of each inhomogeneous aerosol distribution in the given limits. In this work, we have presented a 2D aerosol field whose size is 25.6 km in a vertical direction and 256 km in horizontal direction. The number of the basic aerosol sizes to which random factor is added is equal to seven. The size of inhomogeneous aerosols in a horizontal direction has changed from maximal value and the vertical size of inhomogeneities is setup by the special parameters from which shape of the each inhomogeneity is determined.
Experimental data on statistical properties of an aerosol content in the atmospheric surface layer collected by an in situ method with a nephelometer and ultrasonic meteorological stations are presented. Thus, obtained data are classified on the basis of the Monin-Obukhov similarity theory has been done. The analysis of asymmetry coefficient of the scattering phase function has shown, that the change of microphysical properties of aerosol happens with delay by approximately 6 hours after the establishment of stable thermal stratification. On the contrary the change of sign of stratification results in an immediate reset of microphysical parameters to the initial condition, i.e., without any delay.
In this paper we present experimental data of studies of fluctuation spectra of the backscattering coefficient by the synchronous registration of three components of wind velocity, temperature and backscattering coefficient in the local air volume at height equal to 5 meter. Measurements were carried out by means of the acoustic meteostation and with the aerosol laser locator. Obtained results had verified differences between of power auto-spectra of wind velocity fluctuations and backscattering coefficient fluctuations at stable thermal stratification, presumably explained away as the effects of temperature stratification and of convective turbulence. When stratification of the surface layer is stable there appears a 'sub- area of buoyancy' (the Boljiano-Monin spectrum) in fluctuation spectra of the coefficient of backscattering. Assuming that the field of concentration of particles is formed at upper boundary of the boundary layer, and then it without substantial changes it delivers by downgoing flows to the earth surface.
In this paper experimental and theoretical results on the investigation of the statistical properties of the atmospheric aerosol by the laser sounding method and by means of the acoustic meteostation are presented. Mathematical processing of data allowed to calculate turbulent flows of pulse, heat, and aerosol in the surface air layer. The presence gradient measurements was allowed to define scales for the measurement of velocity, temperature, specific content of aerosol particles and a scale of Monin-Obukhov length, and also to estimate according coefficients of the turbulent exchange for pulse, temperature, and atmospheric aerosol.
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