In this paper the possible synergy between advanced lidars and ceilometers for the monitoring of atmospheric
aerosols is evaluated. The advanced measurement capabilities of the multi-wavelength Raman lidar are used
to investigate the capability of ceilometers to provide reliable information about the atmospheric aerosol content.
At the CNR-IMAA Atmospheric Observatory (CIAO), a ceilometer is operational since September 2009
providing vertical profiles of atmospheric backscatter at 1064nm up to 15km; at the same location, the Potenza
EArlinet Raman Lidar (PEARL), a quality-assured, multi-wavelength Raman lidar operates in the framework
of EARLINET and performs regular measurements plus measurements of special events (Sahara dust outbreaks,
volcanic eruptions etc.). Using the PEARL data products as a reference, the capability of ceilometers to detect
aerosol layers and provide quantitative information about the atmospheric aerosol load is investigated. The
variation of ceilometers' performance in different atmospheric conditions is analysed. A procedure for obtaining
backscatter coefficient profiles from ceilometer signals is proposed and its limitations are discussed.
The development of the Global Position System (GPS) satellite network provides new opportunities to
characterize atmospheric parameters using innovative techniques. The GPS Radio Occultation Technique
(GPS RO) is one of the most recent and promising atmospheric remote sensing technique applied to GPS
measurements. The GPS RO technique allows obtaining profiles of refractivity, temperature, pressure and
water vapor in the neutral atmosphere and electron density in the ionosphere. In the last years, other
missions confirmed the RO efficiency, like GPS/MET, COSMIC (Constellation Observing System for
Meteorology, Ionosphere, and Climate), Formosa Satellite Mission 3 and the last Radio Occultation
Sounder Antenna for the Atmosphere.
In this work, water vapor mixing ratio profiles retrieved from COSMIC observations are presented and
validated using ground based water vapor Raman lidar profiles. As far as we know, this is the first time
water vapor mixing ratio profiles provided by COSMIC are compared with a ground based Raman lidar.
COSMIC profiles used in this study are retrieved applying a one-dimensional variational method that make
use of ECMWF low resolution analysis data as a guess of atmospheric water vapor. Raman lidar
measurements of the water vapor mixing ratio profiles are provided by PEARL (Potenza EArlinet Raman
Lidar) system running at CIAO, located in Potenza, South Italy.
Performance of COSMIC retrieval are studied over a period of one year (2008) of systematic water vapor
Raman lidar measurements. A possible strategy for reducing the impact of the co-location mismatch
between satellite footprint and the lidar station is presented and the problem of the vertical resolution of
COSMIC profiles respect to the Raman lidar is also discussed.
The statistical analysis for the selected cases shows good performance of COSMIC in the identification of
the vertical gradients of the water vapor field, even though the average difference between the Raman lidar
and the COSMIC profiles suggests that caution should be taken in using COSMIC data as an absolute or
reference measurement of water vapor, in particular in the low and middle troposphere.
At the Istituto di Metodologie per l'Analisi Ambientale of the Italian National Research Council (CNR-IMAA) an
advanced observatory for the ground-based remote sensing of the atmosphere is operative. This facility is equipped with
several instruments including two multi-wavelength Raman lidars, one of which mobile, a microwave profiler, a 36 GHz
Doppler polarimetric radar, two laser ceilometers, a sun photometer, a surface radiation station and three radiosounding
CNR-IMAA atmospheric observatory (CIAO) is located in Southern Italy on the Apennine mountains (40.60N, 15.72E,
760 m a.s.l.), less than 150 km from the West, South and East coasts. The site is in a valley surrounded by low mountains
(<1100 m a.s.l.) and this location offers an optimal opportunity to study different kinds of weather and climate regimes.
CIAO represents an optimal site where testing possible synergies between active and passive techniques for improving
the profiling capabilities of several atmospheric key variables, such as aerosol, water vapour and clouds, and for the
development of an integration strategy for their long-term monitoring.
CIAO strategy aims at the combination of observations provided by active and passive sensors for providing advanced
retrievals of atmospheric parameters exploiting both the high vertical resolution of active techniques and the typical
operational capabilities of passive sensors. This combination offers a high potential for profiling atmospheric parameters
in an enlarged vertical range nearly independently on the atmospheric conditions. In this work, we describe two different
integration approaches for the improvement of water vapour profiling during cloudy condition through the combination
of Raman lidar and microwave profiler measurements. These approaches are based on the use of Kalman filtering and
Tikhonov regularization methods for the solution of the radiative transfer equation in the microwave region. The
accuracy of the retrieved water vapour profiles during cloudy conditions is improved by the use of the water vapour
Raman lidar profiles, retrieved up to a maximum height level located around the cloud base region (depending on their
optical thickness), as a constraint to the obtained solution set. The presented integration approaches allow us to provide
physically consistent solution to the inverse problem in the microwave region retrieving water vapour vertical profiles
also in presence of thick clouds. The integration of Raman lidar and microwave measurements also provides a
continuous high-resolution estimation of the water vapour content in the full troposphere and, therefore, a useful tool for
the evaluation of model capability to capture mean aspects of the water vapour field in nearly all weather conditions as
well as for the identification of possible discrepancies between observations and models.
EARLINET, the European Aerosol Research Lidar NETwork, established in 2000, is the first coordinated lidar network
for tropospheric aerosol study on the continental scale. The network activity is based on scheduled measurements, a rigorous quality assurance program addressing both instruments and evaluation algorithms, and a standardised data
exchange format. At present, the network includes 27 lidar stations distributed over Europe.
EARLINET performed almost continuous measurements since 15 April 2010 in order to follow the evolution of the
volcanic plume generated from the eruption of the Eyjafjallajökull volcano, providing the 4-dimensional distribution of
the volcanic ash plume over Europe. During the 15-30 April period, volcanic particles were detected over Central Europe
over a wide range of altitudes, from 10 km down to the local planetary boundary layer (PBL). Until 19 April, the
volcanic plume transport toward South Europe was nearly completely blocked by the Alps. After 19 April volcanic
particles were transported to the south and the southeast of Europe. Descending aerosol layers were typically observed
all over Europe and intrusion of particles into the PBL was observed at almost each lidar site that was affected by the
volcanic plume. A second event was observed over Portugal and Spain (6 May) and then over Italy on 9 May 2010. The
volcanic plume was then observed again over Southern Germany on 11 May 2010.
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
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.
The synergistic use of the measurements carried out using active and passive techniques represent a powerful
solution to fully exploit the capabilities of each remote sensing techniques and to contemporarily overrun its main
limitations. The ground-based facility operational at the CNR-IMAA for the study of the atmosphere is an optimal site
where testing possible synergies between active and passive techniques for improving the profiling capabilities of
atmospheric key variables, such as aerosol, water vapor and clouds.
The combination of the measurements provided by a lidar and a passive sensor is a particularly promising
approach because it puts together the high-resolution measurements obtained using a lidar and the operational
capabilities typical of passive sensors. In particular, the combination of the Raman lidar and sunphotometry
measurements allows to describe the aerosol optical and microphysical properties, supporting the lidar retrievals during
daytime and in presence of thick clouds. Moreover, the use of Raman lidar and microwave measurements, integrated
using an approach based on the Kalman filter, is an optimal way to provide high-resolution measurements of the
tropospheric water vapor in nearly all weather conditions. A strong improvement in the supercooled liquid water
retrieval, obtained through the inversion of the microwave brightness temperature, is also achievable using the cloud
base height information retrieved using the lidar backscattering ratio profile to constrain the microwave retrieval.
The international experiment EAQUATE (European AQUA Thermodynamic Experiment) was held in September 2004 in Italy and in the United Kingdom. The Italian phase, performed in the period 6-10 September 2004, was mainly devoted to assessment and validation of performances of new IR hyperspectral sensors and benefits from data and results of measurements of AQUA and in particular of AIRS. It is also connected with the preparatory actions of MetOp mission with particular attention to calibration and validation of IASI products (as water vapour and temperature profiles), characterization of semitransparent clouds and study of radiative balance, demonstrating the role of ground-based and airborne systems in validation operations.
The Italian phase of the campaign was carried out within a cooperation between NASA Langley Research Center, University of Wisconsin, the Istituto di Metodologie per l'Analisi Ambientale (CNR-IMAA), the Mediterranean Agency for Remote Sensing (MARS) and the Universities of Basilicata, Bologna and Napoli. It involved the participation of the Scaled Composites Proteus aircraft (with NAST thermal infrared interferometer and microwave radiometer, the Scanning HIS infrared interferometer, the FIRSC far-IR interferometer), an Earth Observing System-Direct Readout Station and several ground based instruments: four lidar systems, a microwave radiometer, two infrared spectrometers, and a ceilometer. Radiosonde launches for measurements of PTU and wind velocity and direction were also performed as ancillary observations. Four flights were successfully completed with two different AQUA overpasses. The aircraft flew over the Napoli, Potenza and Tito Scalo ground stations several times allowing the collection of coincident aircraft and in- situ observations.
The European AQUA Thermodynamic Experiment was devoted to study atmosphere, ocean and land with high resolution measurements. It consisted of two phases: the first one took place in Italy in the 6-10 September period and the second one in England on 13-22 September. In the framework of the EAQUATE Italian phase, an intensive lidar measurement campaign was performed at CNR-IMAA, sited in Tito Scalo (40°36'N 15°44'E, 760 m a.s.l.). Independent measurements of aerosol extinction and backscatter coefficient at 355nm, and aerosol backscatter coefficient at 532 nm were obtained by means of an elastic\Raman lidar. Another Raman lidar allowed the vertical profiling of the water vapour mixing ratio. Both the lidar systems have high vertical and temporal resolution (15 m - 1 minute), allowing a characterization of the Planetary Boundary Layer as well as of the Free Troposphere also in terms of dynamical behaviour. Ancillary instruments were utilized contemporaneously with lidar measurements. In particular 17 Vaisala radiosondes for PTU measurements were launched during the campaign, 10 of these equipped with RS90 sensors, while 7 utilized RS92 sondes equipped with GSP sensors for wind velocity and direction measurement. Furthermore a 12 channels microwave radiometer providing all around the clock measurements of temperature, relative humidity and water vapour content, was used during the campaign together with a ceilometer for continuous indication of the cloud cover.
At CNR-IMAA located in Tito Scalo (40°36'N, 15°44'E, 760 m a.s.l.), two lidar systems are systematically operational: the first is devoted to tropospheric aerosol characterization, in the framework of EARLINET, and the second performs water vapour measurements. The aerosol lidar system provides independent measurements of aerosol extinction and backscatter coefficient at 355 nm and at 532 nm, aerosol backscatter profiles at 1064 nm and particles depolarization ratio at 532 nm. The Raman lidar for the water vapor allows the vertical profiling of the water vapour mixing ratio with high spatial and temporal resolution up to the tropopause. The system has been calibrated by means of intensive measurement campaign of simultaneous and co-located radiosonde launches. CNR-IMAA is also provided with a DIAL mobile system for pollutants 3-dimensional spatial distribution. Besides these lidar systems, the CNR-IMAA ground based facility for Earth Observation includes ancillary instruments: a radiosounding system for PTU, ozone and wind measurements; a Sun photometer operative since December 2004 in the framework of AERONET; a 12 channels microwave radiometer for continuous measurements of temperature, relative humidity and water vapor, operative since February 2004; a ceilometer for continuous cloud cover monitoring. Lidar systems together with these ancillary instruments make the CNR-IMAA a heavily instrumented experimental site for integrated observations of aerosols, clouds and water vapor to be used for climatological studies and for the validation of satellite data.