Ivan N. Kolev

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.

Lidar Investigation of the Temporal and Spatial Distribution of Atmospheric Aerosols in Mountain Valleys

Lidar experiments were conducted in the mountainous region of Bulgaria to determine the spatial and temporal distribution of major aerosol sources and the zones of aerosol accumulation. When these lidar data are combined with conventional meteorological observations of temperature and wind profiles they provide a clear picture of the physical processes that lead to the accumulation and subsequent dispersion of aerosols and other pollutants in the valleys. The observations showed that the valley gradually fills with cool air after sunset, producing an inversion that traps aerosols and other pollutants emitted at night. After sunrise a convective boundary layer develops in the valley; its evolution is delayed by the confining valley walls. Insolation causes airflow up the slope, producing divergence near the surface and subsidence of the inversion core. The one winter experiment conducted suggests that weaker winter insolation delays the process until much later than in the summer, sometimes to the extent that the inversion persists throughout the day, or even for several days. The findings described here are in good agreement, qualitatively and quantitatively, with the model described by Whiteman and McKee. The results also demonstrate the power of combining conventional meteorological observations with lidar techniques for determining the nature of boundary layer processes in a valley.

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.

Lidar Observations of Sea-Breeze and Land-Breeze Aerosol Structure on the Black Sea

Abstract This paper describes results of a lidar study of sea-breeze behavior near Akhtopol in the southeastern corner of Bulgaria. The lidar site was a few hundred meters from the western shore of the Black Sea. Analyses are presented of vertical cross sections at different azimuths through the nearshore atmosphere. Three different stages of the sea–land-breeze circulation are discussed: the offshore land-breeze flow, the near-calm period before the onset of a sea breeze, and an onshore sea-breeze flow. The differences in backscatter between the moist, droplet-laden air over the water and the drier air over land with its smaller particles have allowed the characteristic features of the circulations to be identified. Comparisons to simultaneous pilot balloon and surface meteorological observations have supported the interpretations.

Lidar measurement of the aerosol extinction profile in Black Sea coastal zone

The paper presents a lidar study of the volume extinction coefficient in the atmosphere at the Bulgarian Black Sea coast in the presence of a breeze circulation. An attempt is made to evaluate the influence of the meteorological parameters on the optical properties of aerosol. In the case of a land-to-sea breeze and a layered structure of the atmosphere (stable stratification), the height of the thermal internal boundary layer formed over the sea surface is compared with analytical models. In the case of an atmospheric convective boundary layer, an increase in the aerosol extinction with height is observed. The results are compared with an analytical model of the aerosol extinction profile in a well-mixed marine boundary layer. In both cases, the experimental results are in a good agreement with the analytical models.

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.

Optical characteristics of aerosol determined by Cimel, Prede, and Microtops II sun photometers over Belsk, Poland

An experimental campaign was carried out at Central Geophysical Laboratory at Belsk (Poland) in October 2007. Three sun photometers: Cimel CE-318, Prede POM-01L and Microtops II were used to obtain the atmospheric aerosol optical characteristics. Cimel CE-318 and Prede POM-01L are automatic sun-tracking devices common in the world networks for aerosol investigations AERONET and SKYNET. Microtops II is a hand-held manual device often used in field experiments because of its mobility. The results obtained show similar behavior of the aerosol optical depth variations and close values obtained by the three devices at the common wavelengths over rural area. The Microtops II readings are between the readings of the other two sun photometers. The experiment carried out is helpful and shows that the Microtops II sun photometer provides comparable results to these obtained by the Cimel CE-318 and Prede POM-01L sun photometers and potential of using Microtops II for measurement over an urban and rural area.

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.