Ivan Grigorov

EARLINET correlative measurements for CALIPSO: First intercomparison results

A strategy for European Aerosol Research Lidar Network (EARLINET) correlative measurements for Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) has been developed. These EARLINET correlative measurements started in June 2006 and are still in progress. Up to now, more than 4500 correlative files are available in the EARLINET database. Independent extinction and backscatter measurements carried out at high-performance EARLINET stations have been used for a quantitative comparison with CALIPSO level 1 data. Results demonstrate the good performance of CALIPSO and the absence of evident biases in the CALIPSO raw signals. The agreement is also good for the distribution of the differences for the attenuated backscatter at 532 nm ((CALIPSO-EARLINET)/EARLINET (%)), calculated in the 1–10 km altitude range, with a mean relative difference of 4.6%, a standard deviation of 50%, and a median value of 0.6%. A major Saharan dust outbreak lasting from 26 to 31 May 2008 has been used as a case study for showing first results in terms of comparison with CALIPSO level 2 data. A statistical analysis of dust properties, in terms of intensive optical properties (lidar ratios, Angstrom exponents, and color ratios), has been performed for this observational period. We obtained typical lidar ratios of the dust event of 49 ± 10 sr and 56 ± 7 sr at 355 and 532 nm, respectively. The extinction-related and backscatter-related Angstrom exponents were on the order of 0.15–0.17, which corresponds to respective color ratios of 0.91–0.95. This dust event has been used to show the methodology used for the investigation of spatial and temporal representativeness of measurements with polar-orbiting satellites.

Aerosol lidar intercomparison in the framework of the EARLINET project. 2.Aerosol backscatter algorithms

An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET). The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals. The exercise consisted of processing synthetic lidar signals of various degrees of difficulty. One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms. Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar. The intercomparison was performed in three stages with increasing knowledge on the input parameters. First, only the lidar signals were distributed; this is the most realistic stage. Afterward the lidar ratio profiles and the reference values at calibration height were provided. The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance. These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio. The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well.

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.

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.

Lidar measurement of atmospheric aerosol extinction profiles: a comparison between two techniques-Klett inversion and pure rotational Raman scattering methods

Two lidar methods of determining an atmospheric extinction coefficient profile are compared. The methods are the Klett inversion method for elastic lidar return and the log-derivative method for rotational Raman backscattered signal processing. The comparison includes numerical modeling and processing of lidar measurements when both the elastic and the rotational Raman backscattered signals are measured simultaneously. The suggested idea is that such a comparison can be used as a criterion for the reliability of the results of lidar measurements, similar to the comparison between the results of lidar and contact measurements.

Remote monitoring of aerosol layers over Sofia during Sahara dust transport episode (April, 2012)

In this work we present results of lidar remote sensing of aerosol layers in the atmosphere above Sofia during an episode of Sahara dust transport, 02-07 April, 2012. The investigations were made using two lidar systems, one equipped with a CuBr-vapor laser, emitting at wavelength 510.6 nm, and a second one - with Nd:YAG laser, at wavelengths 1064 nm and 532 nm. The results of lidar measurements are presented in terms of vertical atmospheric backscatter coefficient profiles and color maps of the aerosol stratification evolution. The involved into discussions ceilometer data (CHM 15k ceilometer) and satellite data from CALIPSO lidar, enhance the synergy of observations. Conclusion about atmospheric aerosol’s origin was made upon analyses of the information of weather-forecast maps provided by the Forecast system of Barcelona Supercomputing Centre, which are accessible via Internet. Additional information was provided by calculations of the backward air mass trajectories, using online software of NOAA about HYSPLIT model. The comparison between the data from the two lidars and the ceilometer showed similar behavior of aerosol layers development in the atmosphere above Sofia. All information about aerosol layers origin, their altitude above ground, persistence during lidar observations, confirmed the conclusion of observation of a long-distance Sahara dust transport beyond Balkans and Sofia. An interesting completion of CALIPSO lidar and ground based lidars results of measurement is presented in case of thick opaque cloud layer in the atmosphere, which slices the path of lidar sensing in both directions.

LIDAR Atmospheric Sensing by Metal Vapor and Nd:YAG Lasers

LIDAR systems have widely been used for remote investigation of atmospheric parameters (Measures, 1984; Kovalev & Eichinger, 2004; Weitkamp, 2005). They are based on the socalled LIDAR (LIght Detection And Ranging) principle which consists in sending a laser pulse to the atmosphere and subsequent detecting of the radiation backscattered (at angle π) by atmospheric constituents or pollutants. As LIDAR is a time-of-flight technique, the return signal profile detected in the time domain contains range-resolved information about the atmospheric characteristics along the line of laser beam propagation. Advantages of the lidar measurement approaches, as compared to other available active techniques (e.g. radars), are the high spatial and temporal resolution, higher sensitivity and accuracy in sensing atmospheric particles, covering large observation areas, etc. These features make lidar systems powerful instruments for environmental measurements. At present, lidars find a variety of applications in different fields of the human activity. Along with the meteorology, atmospheric physics, and ecological monitoring, lidars are extensively used for volcanic and fire alerting, laser ranging, altimetry and bathymetry, lidar mapping and forestry, coastal morphology and hazards assessment in geology, as well as for many other applications in physics and astronomy, nuclear fusion, military, aviation, robotics, transportation, etc. There exists a variety of ground-based, air-borne and space-borne lidar systems distinguished by their types, schematics, regimes of operation, monitored parameters, constructions, etc. (Kovalev & Eichinger, 2004; Weitkamp, 2005). Among the most widely used systems are the oneor multi-wavelength aerosol lidars exploiting elastic scattering of light.

Lower Troposphere Observation over Urban Area with Lidar at 1064 nm

An episode of relatively thick (till ∼3 km) aerosol formation over the urban area of Sofia city was observed by lidar at a wavelength of 1064 nm. The lidar is part of Sofia lidar station at the Institute of Electronics of Bulgarian Academy of Sciences. Analysis of the weather conditions during the measurement period explains the stable persistence of such formation of human-activity aerosol over the town for the days of observation 20, 21, 23, and 24 June, 2011. The estimated top of the Planetary Boundary Layer for the measurement dated 23 June showed unusually high altitude ∼2200 m above ground. The results are presented in terms of vertical atmospheric backscatter coefficient profiles and color maps of the aerosol stratification evolution.

Analysis of the EARLINET correlative measurements for CALIPSO

Lidar techniques represent the most suitable tool to obtain information on the aerosol vertical distribution and therefore to close this kind of observational gap. Lidar networks are fundamental to study aerosol on large spatial scale and to investigate transport and modification phenomena. These are the motivations why EARLINET, the European Aerosol Research Lidar Network, was established in 2000. At present, EARLINET consists of 25 lidar stations: 7 single backscatter lidar stations, 9 Raman lidar stations with the UV Raman channel for independent measurements of aerosol extinction and backscatter, and 9 multiwavelength Raman lidar stations (elastic channel at 1064 nm, 532 nm, 355 nm, Raman channels at 532 nm and 355 nm, plus depolarization channel at 532 nm) for the retrieval of aerosol microphysical properties. EARLINET data can significantly contribute to the quantification of aerosol concentrations, radiative properties, long-range transport and budget, and prediction of future trends on European and global scale. It can also contribute to improve model treatment on a wide range of scales and to a better exploitation of present and future satellite data. EARLINET is playing an important role in the validation and in the full exploitation of the CALIPSO mission. EARLINET started correlative measurements for CALIPSO since June 2006. A strategy for correlative measurements has been defined on the base of the analysis of the high resolution ground track data provided by NASA. Results in terms of comparisons between EARLINET and available CALIPSO products, both level 1 and level 2 data, are presented.

Raman-elastic-backscatter lidar for observations of tropospheric aerosol

In this work, a combined Raman-elastic backscatter lidar is described. It is based on a Q-switched powerful frequencydoubled Nd:YAG laser (output pulse power: up to 1 J at 1064 nm; up to 100 mJ at 532 nm; pulse duration 15 ns FWHM; repetition rate 2 Hz). A Cassegrain telescope (35 cm diameter, 200 cm focal length) collects backscattered radiation from atmospheric particles and molecules. Lidars spectral receiving module consists of three cannels. The first two channels separate and detect elastic-backscattered lidar signals at laser wavelengths 1064 nm and 532 nm, respectively. In the third lidar channel, a radiation at a wavelength of 607 nm is selected and detected, resulting from Raman backscattering of laser second harmonic (532 nm) by atmospheric nitrogen molecules. Experimental profiles of the aerosol backscattering coefficient demonstrating measurement abilities of the system are shown and discussed. Particularly, attention is paid to profiles containing signals from high-altitude cirrus clouds, low-altitude stratus clouds, hazy atmospheric areas and Saharan dust over the city of Sofia.