Valentin Mitev

Long‐range transport of Saharan dust to northern Europe: The 11–16 October 2001 outbreak observed with EARLINET

The spread of mineral particles over southwestern, western, and central Europe resulting from a strong Saharan dust outbreak in October 2001 was observed at 10 stations of the European Aerosol Research Lidar Network (EARLINET). For the first time, an optically dense desert dust plume over Europe was characterized coherently with high vertical resolution on a continental scale. The main layer was located above the boundary layer (above 1-km height above sea level (asl)) up to 3–5-km height, and traces of dust particles reached heights of 7–8 km. The particle optical depth typically ranged from 0.1 to 0.5 above 1-km height asl at the wavelength of 532 nm, and maximum values close to 0.8 were found over northern Germany. The lidar observations are in qualitative agreement with values of optical depth derived from Total Ozone Mapping Spectrometer (TOMS) data. Ten-day backward trajectories clearly indicated the Sahara as the source region of the particles and revealed that the dust layer observed, e.g., over Belsk, Poland, crossed the EARLINET site Aberystwyth, UK, and southern Scandinavia 24–48 hours before. Lidar-derived particle depolarization ratios, backscatter- and extinction-related Angstrom exponents, and extinction-to-backscatter ratios mainly ranged from 15 to 25%, −0.5 to 0.5, and 40–80 sr, respectively, within the lofted dust plumes. A few atmospheric model calculations are presented showing the dust concentration over Europe. The simulations were found to be consistent with the network observations.

Atmospheric temperature measurements made by rotational Raman scattering

Rotational Raman scattering of light from the second harmonic of a Nd:YAG laser is used to measure atmospheric temperature at altitudes of 3 to 20 km. The method relies on taking the ratio of light passed by two filters that differ slightly in their passbands, and is therefore insensitive to variations in atmospheric transmission. The calibration of the experiment relies on only spectroscopic measurements and not on normalizing to nearby radiosondes, as has been the previous practice with this kind of experiment. Comparisons with radiosonde profiles show excellent agreement to within the precision of the measurements and the variability of the atmosphere. The main application of the experiment lies in its ability to measure a time series of temperature profiles with good height resolution throughout a night. Examples of such series that show the passage of a tropopause fold above the lidar are presented.

Comparison between Backscatter Lidar and Radiosonde Measurements of the Diurnal and Nocturnal Stratification in the Lower Troposphere

A collection of boundary layer heights has been derived from measurements performed by a groundbased backscatter lidar in Neuchâtel, Switzerland (47.000°N, 6.967°E, 485 m ASL). A dataset of 98 cases have been collected during 2 yr. From these data, 61 are noon and 37 are midnight cases. The following two different schemes were used to retrieve the mixed layer depth and the height of the residual layer from the measurements: the gradient and variance methods. The obtained values were compared with those derived from the potential temperature profiles as computed from radiosonde data. For nocturnal cases, the height of the first aerosol layer above the residual layer was also compared to the corresponding potential temperature value. Correlation coefficients between lidar and radiosonde in both convective and stable conditions are between 0.88 and 0.97.

Experimental investigation of high-power single-pass Raman shifters in the ultraviolet with Nd:YAG and KrF lasers

Single-pass Raman cells pumped by either a quadrupled Nd:YAG (266-nm) laser or a KrF excimer laser are studied. The Raman-active gases comprise H(2), D(2), or CH(4), as well as a mixture of them, with the addition of He, Ne, or Ar. A parametric study, in which the Stokes conversion efficiency and the beam quality (M(2)) were measured, was made. The first Stokes efficiency increases and all the Stokes thresholds decrease with an increase in the lens focal length or the M(2) parameter of the pump beam. The quality of the Stokes beams deteriorates when the active-gas pressure increases but is improved by the addition of an inert gas. Laser-induced breakdown is shown to be a factor that limits the conversion efficiency and the quality of the Stokes beams. With a mixture of D(2), H(2), and Ar, a 10-15-mJ pulse energy is obtained (depending on the pump M(2) parameter) in the first Stokes beam of D(2) (289 nm) and H(2) (299 nm), with a full-angle divergence of 0.5 mrad (at 86% power).

Relationships between Ice Water Content and Volume Extinction Coefficient from In Situ Observations for Temperatures from 0° to −86°C: Implications for Spaceborne Lidar Retrievals*

AbstractAn examination of 2 yr of Cloud–Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) lidar observations and CloudSat cloud radar observations shows that ice clouds at temperatures below about −45°C frequently fall below the CloudSat radar’s detection threshold yet are readily detectable by the lidar. The CALIPSO ice water content (IWC) detection threshold is about 0.1 versus 5 mg m−3 for CloudSat. This comparison emphasizes the need for developing a lidar-only IWC retrieval method that is reliable for high-altitude ice clouds at these temperatures in this climatically important zone of the upper troposphere. Microphysical measurements from 10 aircraft field programs, spanning latitudes from the Arctic to the tropics and temperatures from −86° to 0°C, are used to develop relationships between the IWC and volume extinction coefficient σ in visible wavelengths. Relationships used to derive a radiatively important ice cloud property, the ice effective diameter De, from σ are also develop...

Raman Frequency Shifting in a CH(4):H(2):Ar mixture pumped by the fourth harmonic of a Nd:YAG Laser.

Mixtures of methane, hydrogen, and argon (CH(4):H(2):Ar) were studied as UV Raman shifters for ozone differential absorption lidar application. They have higher photochemical stability than pure CH(4) and the capability to produce, with high enough efficiency, either first CH(4) Stokes or, simultaneously, CH(4) and H(2) first Stokes with equal energies. These mixtures can be used as an inexpensive replacement for D(2) or a more stable substitute for pure CH(4) in single-pass high-power Raman shifters.

EARLINET correlative measurements for CALIPSO

The European Aerosol Research Lidar Network (EARLINET) was established in 2000 to derive a comprehensive, quantitative, and statistically significant data base for the aerosol distribution on the European scale. At present, EARLINET consists of 25 stations: 16 Raman lidar stations, including 8 multi-wavelength Raman lidar stations which are used to retrieve aerosol microphysical properties. EARLINET performs a rigorous quality assurance program for instruments and evaluation algorithms. All stations measure simultaneously on a predefined schedule at three dates per week to obtain unbiased data for climatological studies. Since June 2006 the first backscatter lidar is operational aboard the CALIPSO satellite. EARLINET represents an excellent tool to validate CALIPSO lidar data on a continental scale. Aerosol extinction and lidar ratio measurements provided by the network will be particularly important for that validation. The measurement strategy of EARLINET is as follows: Measurements are performed at all stations within 80 km from the overpasses and additionally at the lidar station which is closest to the actually overpassed site. If a multi-wavelength Raman lidar station is overpassed then also the next closest 3+2 station performs a measurement. Altogether we performed more than 1000 correlative observations for CALIPSO between June 2006 and June 2007. Direct intercomparisons between CALIPSO profiles and attenuated backscatter profiles obtained by EARLINET lidars look very promising. Two measurement examples are used to discuss the potential of multi-wavelength Raman lidar observations for the validation and optimization of the CALIOP Scene Classification Algorithm. Correlative observations with multi-wavelength Raman lidars provide also the data base for a harmonization of the CALIPSO aerosol data and the data collected in future ESA lidar-in-space missions.

Interpretation of lidar depolarization measurements of the Pinatubo stratospheric aerosol layer during EASOE

During the EASOE campaign in the Arctic, all lidar measurements showed total depolarization values for the Pinatubo stratospheric aerosol layer varying between 2% and 4%. If multiple scattering is negligible, the radiation backscattered by a polydispersion of spheres has the same polarization as the incident light. There is always some depolarization of a lidar return because of the anisotropy of the polarizability of the air molecules, but depolarization in the signal returned from aerosols indicates the presence of non-spherical particles. In this paper, we utilize scattering calculations for non-spherical particles to explore interpretations of lidar signals returned by Pinatubo aerosols. Taking into account several particle shapes and lognormal size distributions, calculations show that only slightly-to-moderate deformed spheres with equivalent-mean-radius less then 0.2 µm yield depolarization values in the range given by the lidar measurements, if such particles form a substantial fraction of the population. The implications of particle non-sphericity on the retrieval of the aerosol size distribution are discussed.

Variation of the aerosol stratification over the Rhine Valley during Foehn development: a backscatter lidar study

The vertical aerosol stratification within and above the Rhine Valley was studied with a backscatter lidar during FORM (Foehn in the Rhine Valley during MAP) IOPs 4-5, October 1-3, 1999. The lidar site was in Trubbach, 9° 28 E, 47° 04 N, 490 m asl. Two scintillometers were used to monitor the horizontal and the vertical wind velocity at 600 m above the Rhine Valley floor. A number of surface stations were operational in the valley, as well as a set of radiosounding stations. This provides a possibility to correlate the measurements of the aerosol stratification with the variation of the meteorological parameters defining the Foehn development. The backscatter lidar measurements during the Foehn development show the variation of the aerosol backscatter at different altitudes of the valley PBL. The combination of lidar signal gradient and lidar signal variance presents the cold-pool as an aerosol-rich layer and suggests a likely mechanism for cold-pool erosion by the Foehn air.

Lidar observation of spherical particles in a −65° cold cirrus observed above Sodankyla (Finland) during S.E.S.A.M.E.

Abstract The absence of LIDAR depolarization in a polar cirrus observed above Sodankyla (Finland) showed the presence of spherical particles at −65°C. The presence of pure liquid water is excluded, since homogeneous freezing should occur at or above −40°C. The cirrus layer was detected at the tropopause, during the horizontal and vertical advection above northern Scandinavia of warm and wet oceanic air. Two alternative explanations are suggested, the first one involving the presence of large, deliquescent tropospheric CN, freezing at very low temperature, and the second one involving the presence of metastable, spherical ice-particles produced in the fast adiabatic cooling of the airmasses. The second hypothesis is less reasonable because the absence of depolarized LIDAR signal implies a strict cylindrical symmetry in the particles, a symmetry that is difficult to maintain during the freezing process.