Boiko K. Kaprielov

Lidar determination of winds by aerosol inhomogeneities: motion velocity in the planetary boundary layer

The paper presents results from lidar measurements of wind velocity in the planetary boundary layer using correlation data processing. Two lidars are used in our experiments: a ruby lidar operating along slant paths and a YAG:Nd lidar operating for near vertical sounding used by us for the first time. On the basis of our experience the optimal sizes of aerosol inhomogeneities (30-300 m), the duration of the experiments (2-10 min), and the repetition rate of laser shots (fractions of hertz to several hertz) are determined. The results are compared to independent data obtained from anemometer measurements, theodolite- and radar-tracked pilot balloons. The range of differences is ~1-2 m/s in speed and 10-15 degrees in direction. Preliminary results from the use of lidar data to remotely sound the wind speed for various atmospheric stratifications and synoptic situations are described as well.

Aerosol Lidar and in situ ozone observations of the planetary boundary layer over Bulgaria during the solar eclipse of 11 August 1999

A complex investigation of the planetary boundary layer (PBL) is presented. Observations were carried out on 11 August 1999 during the solar eclipse over Bulgaria using a Light Detection and Ranging Device (Lidar), ozone meters and ground meteorological stations. The Lidar was used to measure the height of the mixing layer before, during and after the solar eclipse in Sofia city; the ozone meters measured the surface O3 concentrations during the phenomenon, while the ground stations took meteorological parameters of the atmospheric ground layer. Weather conditions in all the regions (Sofia, Shabla, Ahtopol and Rozhen peak) were favourable for the observations. The data of the three types of measurements demonstrate with certainty that the solar eclipse affects the meteorological parameters of the atmosphere near the ground, the ozone concentration and the height of the mixing layer. It was found that a certain time delay exists in the solar eclipses impact on the meteorological parameters, the ozone concentration and the mixing layer height and that this delay was different for each of the different parameters.

Investigation of the aerosol structure over an urban area using a polarization lidar

The paper presents a lidar study of the aerosol structure in the planetary boundary layer in the case of radiation fog and haze. A conceptual model of the dynamics of the depolarization coefficient profile during the mixing layer development, taking into account the presence of a multilayered inversions and radiation fogs, is proposed. Various techniques are employed in the processing of the lidar signal in order to determine the mixing layer height as well as more details of the aerosol structure in the low atmosphere, namely, finding the maximum of the signal returned from the lowest temperature inversion, the crossing point of the S functions first derivative with the x axis, and profiles of the depolarization ratio. After the complete destruction of the stable stratification, a low constant value of the depolarization ratio within the newly formed mixing layer is being observed. The study of stable boundary layer disintegration and convective boundary layer formation in the presence of fogs and/or clouds is of both scientific and practical significance in what concerns the protection of the environment and the aviation meteorology.

Lidar and spectroradiometer measurements of atmospheric aerosol optical characteristics over an urban area in Sofia, Bulgaria

A series of campaigns involving a systematic investigation of the atmosphere over an urban area of Sofia city were carried out. A European Aerosol Research Lidar Network (EARLINET) scanning aerosol lidar, a spectroradiometer, a standard sun photometer and a ground meteorological station were used in the observations. Multiple aerosol layers of variable thickness (200–600 m) were observed systematically in the planetary boundary layer (PBL) over the study area and the experimental data were compared with theoretical data. A study of the optical characteristics of the atmospheric aerosol, including the extinction coefficient, aerosol optical depth (AOD) and Angstrom parameters α and β, was performed and their variations followed during the convective boundary layer (CBL) formation. Values of the AOD obtained using the different instruments during simultaneous measurements were compared. Preliminary results show that the AOD values recorded by the sun photometer and those calculated on the basis of the spectroradiometer data are higher than those retrieved from the lidar data. Determination of the atmospheric optical depth and extinction coefficient using a ground-based spectral instrument is a relatively simple and inexpensive method of monitoring the total aerosol content in the atmosphere as well as the air quality over the region.

Influence of the boundary layer development on the ozone concentration over an urban area

Results obtained during two campaigns (summer 2004 and autumn 2005) of observation of the planetary boundary layer dynamics over the Sofia city urban area are presented. An EARLINET scanning aerosol lidar, an ozone analyser and a ground meteorological station were used during the observations. The stable boundary layer height varied from 200 m to 600 m during the different seasons. The residual layer was found to be at 700–1200 m, being destroyed between 10:30 and 12:30 LST. The mixing layer developed up to heights of about 800–1300 m. The ground level ozone concentration was measured to be from 10 µg/m3 to 90 µg/m3. The convective boundary layer formation in three case studies (a clear sunny day, a partial solar eclipse, and in the presence of internal atmospheric gravity waves) are presented. In particular, the mixing layer development and the residual layer destruction are studied and considered, along with the relevant ground level ozone concentration variation.

Lidar visualization of the aerosol stratification and the internal boundary layer in the coastal area in the case of breeze circulation

This paper presents some results of an elastic- backscattering lidar experiment carried out in the Bulgarian Black Sea coastal area in September 1992. The distribution of aerosol within the planetary boundary layer is studied following the variations of the lidar return signals. A combined interpretation of the lidar data with conventionally measured meteorological parameters is made. The obtained results prove the ability of the lidar the cases of sea-to-land and land-to-sea breezes to be distinguished. Based on the changes of the backscattered return signal the prevailing effect of the radiative or advective processes is determined. An attempt is made the height of the thermal internal boundary layer near the shore line to be determined.

Experimental study of polarization characteristics of lidar signal in case of occlusion front

In this work, experimental data of a light detection and ranging (lidar) polarization study of cloud formations in a case of warm occlusion front in winter are presented. The changes in the low clouds at the different stages of the front advection are followed: before, during and after the cold air mass settles down. The experiment was carried out using a polarization lidar with variable viewing angle of the receiver, which allows the influence of the multiple scattering on the signal depolarization to be estimated. The data are acquired by simultaneously recording two polarized components of the lidar return: parallel and perpendicular with respect to that of the sounding radiation. The depolarization coefficient of the signals from various clouds types (stratus, stratocumulus, nimbus stratus, etc.) is determined by receiving and rejecting the multiply scattered lidar returns. The depolarization of the lidar returns is determined also in the space between the ground and the clouds base during different stages of the front advection including wet snowfall and no precipitation; the typical values obtained are: 3–5% before precipitation, 5–7% during rain, 10–40% during snowfall and 1–2% after precipitation. Conclusions are drawn about the phase composition of the clouds formations and the heights of the ice crystals nucleation during snowfall. So the evolution of the atmospheric formations is followed during the different stages of the warm occlusion front advection.

Intercomparison of the three lidar systems in Sofia

In this work the results of an experimental assessment of the operation accuracy of three lidar systems in Sofia City are presented. The investigation was carried out performing an inter-comparison of profiles of the atmospheric aerosol backscattering coefficient simultaneously measured by the lidars. All three lidar systems are involved in regular measurements within the frame of the EARLINET-ASOS (European Aerosol Research Lidar Network: Advanced Sustainable Observation System) European project. Twenty-four lidar stations, distributed over 16 countries in Europe are associated in this network. The main objective of the project is the establishment of a large statistical database of the aerosol distribution on the European scale. In order for the data to be reliable, preliminary tests of the quality of work of the individual lidar systems should be performed. That was the reason to perform the present study. The experiments on simultaneous operation of the lidar systems were carried out according to the fixed EARLINET-ASOS schedule starting in April 2006. The mean difference in the aerosol backscatter for all profiles obtained simultaneously and the standard deviation of the differences were calculated and compared.

Lidar observation of planetary boundary-layer clouds

In the paper methods and experimental techniques for study of various types of clouds in the PBL (St, Sc, Cu, Stfr, fair weather clouds, etc.) accompanied by the obtained related information are presented and discussed. The abilities of different methods for determination of the cloud base height and its dynamics are also considered. The possibility for determination of the clouds phase composition through analysis of the lidar signal polarization characteristics is shown. The basic physical mechanisms grounding the lidar study of cloud formations and the limits of their applicability are discussed. The algorithms of data processing and desired information extraction are described as well. The studies presented in the paper were carried out using a backscatter aerosol lidar system with a polarization unit and module for the viewing angle variation. The results obtained employing the methods and techniques, subject of the paper, could be summarized as follows: (1) following of the base height of various types of clouds and its dynamics in different atmospheric situations and synoptic conditions, including atmospheric fronts passage; (2) determination of crystal (ice) phase within the volume of different types of clouds; (3) estimation of the density of cloud formations; (4) determination of some microphysical characteristics of the clouds, such as droplets mean size, water content, etc.

Lidar, Radiometer and Ozonemeter Measurements over Urban Area

A scanning aerosol lidar, a NIR‐VIS radiometer, an ozonemeter and a ground meteorological station were used for observation of the atmosphere over the urban area of the Sofia City. The mixing layer height and the aerosol extinction coefficient and its variation during the convective boundary layer formation are determined using the lidar data. The spectral dependence of the extinction coefficient is estimated by means of the radiometric data.