The radiative effects caused by a massive desert dust outbreak that took place in the Western Sahara Desert, in the proximity of the Atlantic Ocean, in June 2020 are studied. This outbreak featured two significant dust plumes, the second of which is the focus of the present study. For the identification of the dust plume, we adopted a multi-platform set of remote sensing data, including satellite retrievals from the sensors VIIRS, MODIS and SEVIRI onboard the NOAA-20, Aqua and MSG spacecrafts, respectively. The analysis of aerosol-radiation effects is based on a regional simulation with the WRF-Chem model, implementing the coupling between the aerosols of the GOCART speciation and the radiative modules defined in WRF-Chem model by the "New Goddard Shortwave and Longwave Schemes". In this context, two sets of simulations are proposed: the first one (CTL) without any feedback, and the second (CPL) adopting the fullycoupling strategy. From the comparison between the simulated and the observed (SEVIRI) incoming SW/LW radiation it follows that: (i) the presence of dust in the domain causes a reduction of the incoming SW radiation in the CPL runs; (ii) this reduction is fully in agreement with experimental data; (iii) conversely, the LW component appears to be insensitive to model coupling. Hence, the Goddard radiative coupling is effective in reducing the incoming SW radiation: the difference in terms of average daily values between model and SEVIRI is 123.7 vs 37.0 W m- 2 for CTL and CPL runs, respectively. The incoming longwave radiation seems to be less correlated with the coupling strategy, being the difference of 31.15 vs 15.8 W m- 2 for CTL and CPL runs, respectively. The dust radiative forcing (DRF), that characterises the aerosol-radiation interactions, results in an average surface cooling of -16.9 W m- 2 between the CPL and the CTL runs that could be attributed to a reduction of SW radiation absorbed and scattered by dust particles in the coupled run. At the top of the atmosphere (TOA), a warming, caused by the decrease in atmospheric transparency to the terrestrial thermal radiation, results in an average DRF of +1.5 W m- 2. Finally, the space- and time-averaged surface energy balance results in a difference (E(CPL) - E(CTL)) of about -7.5 W m- 2. Similarly, the dynamic response of the dust forcing within the Planetary Boundary Layer is characterised by a mean temperature difference (CPL-CTL) of about -0.7 K near the surface. Our findings strongly encourage the use of fully-coupled modelling strategy, between aerosols and meteorology, for regional-scale studies and in climate risk assessments.

In this work, we analyzed the radiative effects caused by a giant Saharan dust intrusion that occurred in June 2020, transporting desert dust from the western Sahara to the Caribbean. Our analysis employs (i) remote sensing data from the VIIRS and SEVIRI sensors aboard the NOAA-20 and Meteosat spacecrafts, respectively, and (ii) a regional simulation using the WRF-Chem model, which integrates the aerosol speciation of GOCART with the New Goddard shortwave and longwave radiation schemes. Our results highlight a significant surface cooling effect, with a maximum reduction of shortwave radiation of about − 50 W m− 2, consistent with previous studies, while the longwave radiation component showed limited sensitivity to aerosol interactions. The analysis confirms the critical role of dust in reducing surface solar radiation through scattering and absorption processes. This study serves as an initial exploration of a more comprehensive work presented in a subsequent full paper, where additional simulations and detailed sensitivity analyses are provided. The results encourage the use of fully coupled aerosol-meteorology models in regional studies of dust-induced climate variability. However, limitations of the current study include the underestimation of aerosol optical depth in the early simulation period due to insufficient spin-up time, which may affect the accuracy of initial results.

Irradiance and LiDAR measurements at the surface combined with satellite products from SEVIRI (Spinning Enhanced Visible and InfraRed Imager) and MODIS (MODerate resolution Imaging Spectroradiometer) were used to detect and characterize the Etna volcano (Italy) plume that crossed southeastern Italy on 3 December 2015, from about 10:00 up to 11:30 UTC, and estimate its radiative impact. The volcanic plume was delivered by a violent and short paroxysmal eruption that occurred from 02:30 to 03:10 UTC of 3 December 2015, about 400 km away from the monitoring site. Measurements from the LiDAR combined with model results showed that the aerosol optical depth of the volcanic plume, located from about 11 to 13 km above sea level (asl), was equal to 0.80 +/- 0.07 at 532 nm. A low tropospheric aerosol load, located up to about 7 km asl, with optical depth equal to 0.19 +/- 0.01 at 532 nm was also revealed by the LiDAR measurements. Short-Wave (SW) downward and upward irradiance measurements revealed that the instantaneous SW direct radiative forcing at the surface (DRFsurf) decreased to -146 +/- 16 Wm(-2) at 10:50 UTC because of the volcanic plume passage. A Two-Stream radiative transfer model integrated with experimental measurements, which took into account the volcanic plume and the low tropospheric aerosol properties, was used to reproduce the SW radiative flux measurements at the surface and estimate the aerosol DRF both at the top of the atmosphere (TOA) and at the surface, in addition to the aerosol heating rate vertical profile. We found that the clear-sky, instantaneous, SW DRF at the TOA and the atmospheric forcing were equal to -112 and 33 W m(-2), respectively, at 10:50 UTC that represented the time at which the volcanic plume radiative impact was the highest. The SW aerosol heating rate reached the peak value of 1.24 K day(-1) at 12 km asl and decreased to - 0.06 K day(-1) at 11 km asl, at 10:50 UTC. The role of the aerosol load located up to about 7 km asl and the corresponding radiative impact has also been evaluated.

In this study, the Weather Research and Forecasting model with online coupled chemistry (WRF-Chem) is applied to simulate an intense Saharan dust outbreak event that took place over the Southern Italy in March 2016. The WRF model is found to reproduce well the synoptic meteorological conditions driving the dust outbreak: an omegalike pressure configuration associated with a weak cyclogenesis in the Iberian Peninsula. At the end of the simulated period the merging of two minima produce a large depression in the Peninsular Italy. The model performances in reproducing the atmospheric desert dust load is evaluated using a multi-platform observational dataset of aerosol and desert dust properties, including optical properties from satellite and ground-based sun-photometers, plus in-situ particulate matter mass concentration (PM) data. This comparison allows us to investigate the model ability in reproducing both the horizontal and the vertical displacement of the dust plume, and its evolution in time. The preliminary comparison with satellite (MODIS-AQUA) and sunphotometers (AERONET) showed that the model is able to reproduce well the horizontal field of the aerosol optical depth (AOD) and its evolution in time. The routinely measurements of ARPA-Puglia revealed the intense dust outbreak with peak PM10 value larger than 300 μg/m during march 23. On the other side, the model-measurements comparison for PM10 shows a good temporal matching. The model-to-measurements comparisons allows the evaluation and the tuning of physics-based emission scheme that is part of the WRF-Chem package release.

This paper introduces the recent European Aerosol Research Lidar Network (EARLINET) quality-assurance efforts at instrument level. Within two dedicated campaigns and five single-site intercomparison activities, 21 EARLINET systems from 18 EARLINET stations were intercompared between 2009 and 2013. A comprehensive strategy for campaign setup and data evaluation has been established. Eleven systems from nine EARLINET stations participated in the EARLINET Lidar Intercomparison 2009 (EARLI09). In this campaign, three reference systems were qualified which served as traveling standards thereafter. EARLINET systems from nine other stations have been compared against these reference systems since 2009. We present and discuss comparisons at signal and at product level from all campaigns for more than 100 individual measurement channels at the wavelengths of 355, 387, 532, and 607 nm. It is shown that in most cases, a very good agreement of the compared systems with the respective reference is obtained. Mean signal deviations in predefined height ranges are typically below ±2 %. Particle backscatter and extinction coefficients agree within ±2 × 10−4 km−1 sr−1 and ± 0.01 km−1, respectively, in most cases. For systems or channels that showed larger discrepancies, an in-depth analysis of deficiencies was performed and technical solutions and upgrades were proposed and realized. The intercomparisons have reinforced confidence in the EARLINET data quality and allowed us to draw conclusions on necessary system improvements for some instruments and to identify major challenges that need to be tackled in the future.

This paper presents a detailed description of LIRIC (LIdar-Radiometer Inversion Code) algorithm for simultaneous processing of coincident lidar and radiometric (sun photometric) observations for the retrieval of the aerosol concentration vertical profiles. As the lidar/radiometric input data we use measurements from European Aerosol Research Lidar Network (EARLINET) lidars and collocated sun-photometers of Aerosol Robotic Network (AERONET). The LIRIC data processing provides sequential inversion of the combined lidar and radiometric data. The algorithm starts with the estimations of column-integrated aerosol parameters from radiometric measurements followed by the retrieval of height dependent concentrations of fine and coarse aerosols from lidar signals using integrated column characteristics of aerosol layer as a priori constraints. The use of polarized lidar observations allows us to discriminate between spherical and non-spherical particles of the coarse aerosol mode.The LIRIC software package was implemented and tested at a number of EARLINET stations. Intercomparison of the LIRIC-based aerosol retrievals was performed for the observations by seven EARLINET lidars in Leipzig, Germany on 25 May 2009. We found close agreement between the aerosol parameters derived from different lidars that supports high robustness of the LIRIC algorithm. The sensitivity of the retrieval results to the possible reduction of the available observation data is also discussed.

Lidar-Radiometer Inversion Code (LIRIC) for the retrieval of vertical aerosol properties from combined lidar/radiometer data: development and distribution in EARLINET A. Chaikovsky, O. Dubovik, B. Holben, A. Bril, P. Goloub, D. Tanré, G. Pappalardo, U. Wandinger, L. Chaikovskaya, S. Denisov, Y. Grudo, A. Lopatin, Y. Karol, T. Lapyonok, V. Amiridis, A. Ansmann, A. Apituley, L. Allados-Arboledas, I. Binietoglou, A. Boselli, G. D’Amico, V. Freudenthaler, D. Giles, M. J. Granados-Muñoz, P. Kokkalis, D. Nicolae, S. Oshchepkov, A. Papayannis, M. R. Perrone, A. Pietruczuk, F. Rocadenbosch, M. Sicard, I. Slutsker, C. Talianu, F. De Tomasi, A. Tsekeri, J. Wagner, and X. Wang Institute of Physics, NAS of Belarus, Minsk, Belarus LOA, Universite de Lille, Lille, France NASA Goddard Spaceflight Center, Greenbelt, Maryland, USA

Abstract. The paper investigates numerical procedures that allow determining the dependence on altitude of aerosol properties from multi wavelength elastic lidar signals. In particular, the potential of the LIdar/Radiometer Inversion Code (LIRIC) to retrieve the vertical profiles of fine and coarse-mode particles by combining 3-wavelength lidar measurements and collocated AERONET (AErosol RObotic NETwork) sun/sky photometer measurements is investigated. The used lidar signals are at 355, 532 and 1064 nm. Aerosol extinction coefficient (αL), lidar ratio (LRL), and Angstrom exponent (AL) profiles from LIRIC are compared with the corresponding profiles (α, LR, and A) retrieved from a Constrained Iterative Inversion (CII) procedure to investigate the LIRIC retrieval ability. Then, an aerosol classification framework which relies on the use of a graphical framework and on the combined analysis of the Angstrom exponent (at the 355 and 1064 nm wavelength pair, A(355, 1064)) and its spectral curvature (ΔA = A(355, 532)–A(532, 1064)) is used to investigate the ability of LIRIC to retrieve vertical profiles of fine and coarse-mode particles. The A-ΔA aerosol classification framework allows estimating the dependence on altitude of the aerosol fine modal radius and of the fine mode contribution to the whole aerosol optical thickness, as discussed in Perrone et al. (2014). The application of LIRIC to three different aerosol scenarios dealing with aerosol properties dependent on altitude has revealed that the differences between αL and α vary with the altitude and on average increase with the decrease of the lidar signal wavelength. It has also been found that the differences between AL and corresponding A values vary with the altitude and the wavelength pair. The sensitivity of Angstrom exponents to the aerosol size distribution which vary with the wavelength pair was responsible for these last results. The aerosol classification framework has revealed that the deviations between LIRIC and the corresponding CII-procedure retrieval products are due to the fact that LIRIC does not allow to the modal radius of fine mode particles to vary with the altitude. It is shown that this represents the main source of uncertainties in LIRIC results. The plot on the graphical framework of the A-ΔA data points retrieved from the CII-procedure has indicated that the fine-mode-particle modal radius can vary with altitude when particles from different sources and/or from different advection routes contribute to the aerosol load. Analytical back trajectories combined with linear particle depolarization ratio profiles from lidar measurements at 355 nm and dust concentrations from the Barcelona Supercomputing Center-Dust REgional Atmospheric Model (BSC-DREAM) have been used to demonstrate the dependence on altitude of the aerosol properties.

Introduction Conclusions References

The peculiarity of lidar systems is to provide profiles of optical properties of the atmosphere. The use of specific wavelengths and the selection of different kinds of backscattering (elastic, Raman, polarisation selective) permit to obtain information about suspended particles (aerosols). The authors show here a case study in which particle signals are detected from the boundary layer up to the stratosphere. Information on the size distribution of the different layers can be obtained, using a graphical method relying on the spectral dependence of aerosol extinction. The authors apply this method, for the first time to their knowledge, to stratospheric aerosol.

An approach based on the graphical method of Gobbi and co-authors (2007) is introduced to estimate the dependence on altitude of the aerosol fine mode radius (Rf) and of the fine mode contribution (η) to the aerosol optical thickness (AOT) from three-wavelength lidar measurements. The graphical method of Gobbi and co-authors (2007) was applied to AERONET (AErosol RObotic NETwork) spectral extinction observations and relies on the combined analysis of the Ångstrom exponent (å) and its spectral curvature Δå. Lidar measurements at 355, 532 and 1064 nm were used in this study to retrieve the vertical profiles of å and Δå and to estimate the dependence on altitude of Rf and η(532 nm) from the å–Δå combined analysis. Lidar measurements were performed at the Department of Mathematics and Physics of the Universita' del Salento, in south-eastern Italy. Aerosol from continental Europe, the Atlantic, northern Africa, and the Mediterranean Sea are often advected over south-eastern Italy and as a consequence, mixed advection patterns leading to aerosol properties varying with altitude are dominant. The proposed approach was applied to ten measurement days to demonstrate its feasibility in different aerosol load conditions. The selected days were characterized by AOTs spanning the 0.26–0.67, 0.15–0.39, and 0.04–0.27 range at 355, 532, and 1064 nm, respectively. Mean lidar ratios varied within the 31–83, 32–84, and 11–47 sr range at 355, 532, and 1064 nm, respectively, for the high variability of the aerosol optical and microphysical properties. å values calculated from lidar extinction profiles at 355 and 1064 nm ranged between 0.1 and 2.5 with a mean value ± 1 standard deviation equal to 1.3 ± 0.7. Δå varied within the −0.1–1 range with mean value equal to 0.25 ± 0.43. Rf and η(532 nm) values spanning the 0.05–0.3 μm and the 0.3–0.99 range, respectively, were associated with the å–Δå data points. Rf and η values showed no dependence on the altitude. 60% of the data points were in the Δå–å space delimited by the η and Rf curves varying within 0.80–0.99 and 0.05–0.15 μm, respectively, for the dominance of fine-mode particles in driving the AOT over south-eastern Italy. Vertical profiles of the linear particle depolarization ratio retrieved from lidar measurements, aerosol products from AERONET sun photometer measurements collocated in space and time, analytical back trajectories, satellite true colour images, and dust concentrations from the BSC–DREAM (Barcelona Super Computing Center-Dust REgional Atmospheric Model) model were used to demonstrate the robustness of the proposed method.

The eruption of the Icelandic volcano Eyjafjallajökull in April–May 2010 represents a "natural experiment" to study the impact of volcanic emissions on a continental scale. For the first time, quantitative data about the presence, altitude, and layering of the volcanic cloud, in conjunction with optical information, are available for most parts of Europe derived from the observations by the European Aerosol Research Lidar NETwork (EARLINET). Based on multi-wavelength Raman lidar systems, EARLINET is the only instrument worldwide that is able to provide dense time series of high-quality optical data to be used for aerosol typing and for the retrieval of particle microphysical properties as a function of altitude. In this work we show the four-dimensional (4-D) distribution of the Eyjafjallajökull volcanic cloud in the troposphere over Europe as observed by EARLINET during the entire volcanic event (15 April–26 May 2010). All optical properties directly measured (backscatter, extinction, and particle linear depolarization ratio) are stored in the EARLINET database available at http://www.earlinet.org. A specific relational database providing the volcanic mask over Europe, realized ad hoc for this specific event, has been developed and is available on request at http://www.earlinet.org. During the first days after the eruption, volcanic particles were detected over Central Europe within a wide range of altitudes, from the upper troposphere down to the local planetary boundary layer (PBL). After 19 April 2010, volcanic particles were detected over southern and south-eastern Europe. During the first half of May (5–15 May), material emitted by the Eyjafjallajökull volcano was detected over Spain and Portugal and then over the Mediterranean and the Balkans. The last observations of the event were recorded until 25 May in Central Europe and in the Eastern Mediterranean area. The 4-D distribution of volcanic aerosol layering and optical properties on European scale reported here provides an unprecedented data set for evaluating satellite data and aerosol dispersion models for this kind of volcanic events.

Abstract. A new approach is introduced to characterize the dependence on altitude of the aerosol fine mode radius (Rf) and of the fine mode contribution (η) to the aerosol optical thickness (AOT) by three-wavelength lidar measurements. The introduced approach is based on the graphical method of Gobbi et al. (2007), which was applied to AERONET spectral extinction observations and relies on the combined analysis of the Ångstrom exponent (å) and its spectral curvature Δå. Lidar measurements at 355, 532 and 1064 nm were used in this study to retrieve the vertical profiles of å and Δå and to determine the dependence on altitude of Rf and η (532 nm) from the å–Δå combined analysis. Lidar measurements were performed at the Mathematics and Physics Department of Universita' del Salento, in south eastern Italy. Aerosol from continental Europe, the Atlantic, northern Africa, and the Mediterranean Sea are often advected over south eastern Italy and as a consequence, mixed advection patterns leading to aerosol properties varying with altitude are dominant. The proposed approach was applied to eleven measurement days to demonstrate its feasibility in different aerosol load conditions. The selected-days were characterized by AOTs spanning the 0.23–0.67, 0.15–0.41, and 0.04–0.25 range at 355, 532, and 1064 nm, respectively. Lidar ratios varied within the 28–80, 30–70, and 30–55 sr range at 355, 532, and 1064 nm, respectively, for the high variability of the aerosol optical and microphysical properties. å(355 nm, 1064 nm) values retrieved from lidar measurements ranged between 0.12 and 2.5 with mean value ±1 standard deviation equal to 1.4 ± 0.5. Δå varied within the −0.10–0.87 range with mean value equal to 0.1 ± 0.4. Rf and η (532 nm) values spanning the 0.02–0.30 μm and the 0.30–0.99 range, respectively were associated to the å–Δå data points. Rf and η values showed no dependence on the altitude. 72% of the data points were in the Δå–å space delimited by the η and Rf curves varying within 0.70–0.95 and 0.15–0.05 μm, respectively for the dominance of fine mode particles in driving the AOT over south eastern Italy. Volume depolarization vertical profiles retrieved from lidar measurements, aerosol products from AERONET sunphotometer measurements collocated in space and time, the BSC-DREAM model, analytical back trajectories, and satellite images were used to demonstrate the robustness of the proposed method.

Volcanic aerosols resulting from the Eyjafjallajökull eruption were detected in south-eastern Italy from 20 to 22 April 2010, at a distance of approximately 4000 km from the volcano, and have been characterized by lidar, sun/sky photometer, and surface in-situ measurements. Volcanic particles added to the pre-existing aerosol load and measurement data allow quantifying the impact of volcanic particles on the aerosol vertical distribution, lidar ratios, the aerosol size distribution, and the ground-level particulate-matter concentrations. Lidar measurements reveal that backscatter coefficients by volcanic particles were about one order of magnitude smaller over south-eastern Italy than over Central Europe. Mean lidar ratios at 355 nm were equal to 64 ± 5 sr inside the volcanic aerosol layer and were characterized by smaller values (47 ± 2 sr) in the underlying layer on 20 April, 19:30 UTC. Lidar ratios and their dependence with the height reduced in the following days, mainly because of the variability of the volcanic particle contributions. Size distributions from sun/sky photometer measurements reveal the presence of volcanic particles with radii r > 0.5 μm on 21 April and that the contribution of coarse volcanic particles increased from 20 to 22 April. The aerosol fine mode fraction from sun/sky photometer measurements varied between values of 0.85 and 0.94 on 20 April and decreased to values between 0.25 and 0.82 on 22 April. Surface measurements of particle size distributions were in good accordance with column averaged particle size distributions from sun/sky photometer measurements. PM1/PM2.5 mass concentration ratios of 0.69, 0.66, and 0.60 on 20, 21, and 22 April, respectively, support the increase of super-micron particles at ground. Measurements from the Regional Air Quality Agency show that PM10 mass concentrations on 20, 21, and 22 April 2010 were enhanced in the entire Apulia Region. More specifically, PM10 mass concentrations have on average increased over Apulia Region 22%, 50%, and 28% on 20, 21, and 22 April, respectively, compared to values on 19 April. Finally, the comparison of measurement data with numerical simulations by the FLEXPART dispersion model demonstrates the ability of FLEXPART to model the advection of the volcanic ash over the 4000 km from the Eyjafjallajökull volcano to Southern Italy.

The almost six-years long database of aerosol and cloud vertical profiles provided by CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) is at the present the longest database of aerosol optical properties at global scale. This database is a unique tool for the characterization of aerosol 4D distribution at global scale. However, CALIPSO has a small footprint and a revisiting time of 16 days, and therefore how well these measurements represent the atmospheric conditions of a surrounding area over a longer time is a big issue to be investigated. Because of its geographic coverage and the large number of advanced Raman aerosol lidars, EARLINET, the European Aerosol Research Lidar Network, offers a unique opportunity for the validation and full exploitation of the CALIPSO mission. CALIPSO Level 2 and Level 3 data products are investigated in terms of their representativeness by comparison with EARLINET measurements. Comparisons for the Level 2 profiles available in both version 2 (40 km as horizontal resolution) and version 3 (5 km) releases highlight the issue of finding a best compromise between the high resolution and the high signal-to-noise ratio. Representativeness of monthly averages provided in Level 3 data is investigated using EARLINET-CALIPSO correlative measurements. The study will furthermore benefit from the availability of the 12-years long-term database of EARLINET climatological data available from May 2000.

The aim of this paper is the characterization of volcanic aerosol over a site of Southern Italy, located 4000 km far from the volcanic eruption. As stated by the authors, the optical and physical properties of aged volcanic aerosol is not well known and they are interesting to be studied. For this reason I consider the paper suitable for a publication. However measurements refer only to one site and it is not possible to perform a deep study of how the aerosol characteristics changed during the transport across Europe. Moreover the first feeling reading the paper in that it is mainly focused to certify that volcanic aerosol was really over the site. On my opinion there are different points, already treated in the paper, that must be highlighted to make this work scientifically interesting and really original. For this reason I believe the present work needs a

A methodology has been implemented to compare ground-based Raman lidar measurements at 355 nm to measurements at 532 nm with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the Cloud-Aerosol-Lidar and Infrared-Pathfinder-Satellite-Observation (CALIPSO) and contribute to the validation of level 2 (version 2) aerosol products. In particular, lidar measurements performed at Lecce (40 degrees 20'N,18 degrees 6'E) in 2006, 2008, and 2009 within the European Aerosol Research Lidar Network have been compared to CALIOP measurements co-located in time and performed at radial distances from Lecce varying from 6 up to 19 km. Extinction- and backscatter-related Angstrom exponents from sun/sky photometer measurements have synergistically been used to calculate aerosol extinction (alpha(z)) and backscatter (beta(z)) coefficients at 532 nm from the ones at 355 nm retrieved from ground-based lidar measurements. Selected clear-sky study-cases have been analyzed to illustrate the implemented methodology and address the difficulties that may occur to fill the missing information of ground based measurements at 532 nm. Direct profile-to-profile comparisons have demonstrated either the reliability of the implemented methodology and the CALIOP capability to detect aerosol at least up to similar to 0.3 km from ground in cloud-free conditions. The agreement on the aerosol layer top height is within +/- 0.1 km for the selected study cases. It has been shown that the spatial variability of aerosols can greatly complicate the validation of CALIOP backscatter and extinction coefficient profiles by direct comparison with corresponding profiles from ground-based instruments co-located in time, but not in space. (C) 2011 Elsevier By. All rights reserved.

A lidar system is used to determine the diurnal evolution of the planetary boundary layer (PBL) height on a summer day characterized by anticyclonic conditions. The site is located some 15 km distant from the sea, on a peninsula in south-east Italy. Contrary to expectations, the PBL height, after an initial growth consequent to sunrise, ceases to increase about 2 h before noon and then decreases and stabilizes in the afternoon. An interpretation of such anomalous behaviour is provided in terms of trajectories of air parcels towards the lidar site, which are influenced by the sea breeze, leading to a transition from a continental boundary layer to a coastal internal boundary layer. The results are analyzed using mesoscale numerical model simulations and a simple model that allows for a more direct interpretation of experimental results.

The regional climate model RegCM3 coupled with a radiatively active aerosol model with online feedback is used to investigate direct and semi-direct radiative aerosol effects over the Sahara and Europe in a test case of July 2003. The aerosol model includes dust particles in addition to sulfates, hydrophobic and hydrophilic black carbon and organic carbon. The role of the aerosol online feedback on the radiation budget and the direct radiative forcing (short-wave and long-wave) by dust particles are investigated by intercomparing results from three experiments: REF, including all interactive aerosol components, Exp1, not accounting for the aerosol radiative feedback, and Exp2 not accounting for desert dust particles. The comparison of results in the REF experiment with satellite observations, sun/sky radiometer measurements, and lidar profiles at selected Central Mediterranean sites reveals that the spatio-temporal evolution of the aerosol optical depth is reasonably well reproduced by the model during the entire month of July. Results for the dust outbreaks of 17 and 24 July, averaged over the simulation domain, show that the daily-mean SW direct radiative forcing by all particles is −24 Wm−2 and −3.4 Wm−2 on 17 July and −25 Wm−2 and −3.5 Wm−2 on 24 July at the surface and top of the atmosphere, respectively. This is partially offset by the LW direct radiative forcing, which is 7.6 Wm−2 and 1.9 Wm−2 on 17 July and 8.4 Wm−2 and 1.9 Wm−2 on 24 July at the surface and top of the atmosphere, respectively. Hence, the daily-mean SW forcing is offset by the LW forcing of ~30% at the surface and of ~50% at the ToA. It is also shown that atmospheric dynamics and hence dust production and advection processes are dependent on the simulation assumptions and may significantly change within few tens of kilometers. The comparison of REF and Exp1 shows that the aerosol online feedback on the radiation budget decreases the domain-average daily-mean value of the 2 m-temperature, aerosol column burden (CB), and short-wave (SW) atmospheric forcing by −0.52 °C, 14%, and 0.9%, respectively on 17 July and by −0.39 °C, 12% and 12%, respectively on 24 July. The comparison of REF and Exp2 reveals that on 17 July, radiatively-active dust particles decrease the daily-mean 2 m-temperature averaged over the whole simulation domain by 0.4% even if are responsible for 99.8% and 97% of the daily-mean aerosol column burden and SW atmospheric forcing, respectively.