Maria Gavrouzou

Greece and the eastern Mediterranean are among the regions that are most exposed to climate-driven natural hazards, with wildfires, floods, and extreme rainfall events consistently producing significant socioeconomic and environmental impacts. Although previous literature has addressed each hazard type individually, a systematic, comparative analysis of the atmospheric circulation regimes associated with all three hazard categories within a unified Lagrangian framework has not yet been conducted for Greece. In this study, a 96 h HYSPLIT back-trajectory analysis driven by ERA5 reanalysis data, combined with k-means clustering, is employed to characterise the air mass origins associated with extreme events in Greece from 2000 to 2020 at two atmospheric levels: 750 m and 3000 m above sea level. Wildfire events are predominantly linked to short-distance northeast airflow at 750 m, and are directly associated with the Etesian wind system and to a coherent northwest-west Mediterranean signal at 3000 m, reflecting the influence of the summer blocking anticyclone over Europe. Conversely, flood events are dominated by northerly flow at 750 m, driven by the eastern flank of Mediterranean depressions. These results indicate that flooding in Greece is primarily conditioned by surface cyclogenesis, regardless of the upper-level flow geometry. Extreme rainfall events exhibit the most complex structure, with a dominant upper-level cluster that describes a recurving trajectory consistent with cut-off low dynamics. Cross-hazard comparisons demonstrate that similar near-surface trajectory patterns may arise from different atmospheric drivers, underscoring the necessity of integrating Lagrangian trajectory classification with additional context, such as thermodynamic and seasonal, to enable robust multi-hazard attribution and enhance early warning capabilities in the eastern Mediterranean.

In the present study, the all-sky Dust Direct Radiative Effect (DDRE) is estimated during intense Dust Aerosol (DA) episodes that took place from 2005 to 2018 over the broader Mediterranean Basin (MB). DA episodes are identified using a satellite algorithm, which is based on aerosol optical properties. The DDREs are estimated using a vertically resolved version of the FORTH spectral radiative transfer model (RTM). The RTM ran on a 3-hourly temporal and 0.5° × 0.625° horizontal spatial resolution for each of the 162 identified DA episode days (DAEDs), initialized with 3D dust and total (all) aerosol optical properties, namely optical depth, single scattering albedo, and asymmetry parameter from the MERRA-2 reanalysis. Cloud data (cloud amount, optical depth, and top pressure) are taken from the ISCCP-H. The RTM computes the upwelling and downwelling solar fluxes at TOA, at the surface, and at 50 levels in the atmosphere. The runs were made with all aerosols, and all but DA. The DDREs were calculated by the difference between the two sets of output fluxes. According to the model results averaged over the 162 studied episodes, DA reduces the net surface solar radiation over the MB, causing a cooling effect as large as −77 W/m² (climatological mean value at 12:00 UTC) over areas with high dust optical depth. On the contrary, they increase the atmospheric absorbed solar radiation, resulting in considerable (up to 79 W/m²) atmospheric radiative heating. At TOA, either heating (up to 30 W/m² over desert areas) or cooling (as strong as −21 W/m² over oceanic areas) are found, depending on the significantly different underlying surface albedos. DDREs are even higher during single episodes, with a surface cooling effect as large as −200 W/m² and a corresponding atmospheric heating effect of up to +187 W/m².

In the present study, a satellite algorithm is used to identify Dust Aerosol Episodes (DAEs) over the wide region of Asia and the Caspian Sea, aiming to determine their spatial and temporal distribution by applying threshold values on key aerosol optical properties, emphasizing their frequency of occurrence and intensity, during 2005–2021. Based on the algorithm outputs, a climatology of spatiotemporally extended Asian DAEs is built, emphasizing their interannual variability and tendencies over the 17-year study period.

In this study, we try to characterize local aerosols over the city of Ioannina for the first time using continuous AERONET CIMEL Sun–Sky spectral photometer measurements. The instrument, which belongs to the Laboratory of Atmospheric Physics of the Aristotle University of Thessaloniki, was installed in 2022 at the University of Ioannina and operated for a 5-month period from 23 February to 30 June 2022. Based on its measurements and retrievals, we investigate aerosol optical properties on a monthly, daily and hourly mean basis and reveal relationships between Aerosol Optical Depth (AOD) and local particulate matter (PM). It is found that the 5-month mean AOD is 0.17, from which 0.08 is ascribed to fine-mode and 0.09 to coarse-mode aerosols, while the corresponding mean Angstrom Exponent is 0.95. The total PM, PM₁₀ and PM_2.5 concentrations for the same period are equal to 32.51, 22.39 and 11.40 μg/m³, respectively. The correlation coefficient between PM₁₀ and AOD_500nm is equal to 0.79, and the one between the PM_2.5/PM₁₀ ratio and the Fine-Mode Fraction of AOD is equal to 0.76. Events of episodic fine and coarse aerosol conditions, which took place during the study period, are also analyzed using AERONET Volume Size Distribution (VSD) inversion products, along with back trajectories obtained with the NOAA’s HYSPLIT model, to assure the origin of the particles.

A global climatology of sea-salt aerosols for the 17-year period 2005–2021 is produced with a satellite algorithm that uses input data of spectral aerosol optical depth (AOD) and single scattering albedo (SSA) at 388 nm, taken from MODIS-Aqua Collection 6.1 and OMI-Aura Satellite databases, respectively. The results show that the maximum frequency of occurrence of sea-salt aerosols is observed over the Southern Tropical Pacific and Indian oceans, with values up to 60 and 70 days per year, respectively, while on a global mean basis, the sea-salt aerosols are observed with a decreasing frequency from 2005 to 2021.

In the present study, we used the FORTH deterministic spectral Radiation Transfer Model (RTM) to estimate detailed three-dimensional distributions of the Direct Radiative Effects (DREs) and their consequent modification of the thermal structure of the regional atmosphere during an intense dust episode that took place from 16 to 18 June 2016 over the Mediterranean Basin (MB). The RTM operated on a 3-hourly temporal and 0.5 × 0.625° spatial resolution, using 3-D aerosol optical properties (i.e., aerosol optical depth, single scattering albedo, and asymmetry parameter) and other surface and atmospheric properties from the MERRA-2 reanalysis and cloud properties (i.e., cloud amount, cloud optical depth, and cloud top height) from the ISCCP-H dataset. The model ran with and without dust aerosols, yielding the upwelling and downwelling solar fluxes at the top of the atmosphere, in the atmosphere, and at the Earth’s surface as well as at 50 levels in the atmosphere. The dust direct radiative effect (DDRE) was estimated as the difference between the two (one taking into account all aerosol types and one taking into account all except for dust aerosols) flux outputs. The atmospheric heating rates and subsequent convection induced by dust radiative absorption were calculated at 50 levels to determine how the DDRE affects the thermal structure and dynamics of the atmosphere. The results showed that such a great and intense dust transport event significantly reduces the net surface solar radiation over the MB (by up to 62 W/m² on a daily mean basis, and up to 200 W/m² on an hourly basis, at 12:00 UTC) while increasing the atmospheric solar absorption (by up to 72 W/m² daily and 187 W/m² hourly, at 12:00 UTC). At the top of the atmosphere, both heating (over desert areas) and cooling (over oceanic and other continental areas) are observed due to the significantly different surface albedos. Transported dust causes considerable heating of the region’s atmosphere, which becomes maximum at altitudes where the dust loadings are highest (0.14 K/3 h on 17 June 2016, 12:00 UTC, at 3–5 km above sea level). The dust solar absorption and heating induce a buoyancy as strong as 0.014 m/s², resulting in considerable changes in vertical air motions and possibly contributing to the formation of middle- and high-level clouds over the Mediterranean Basin.

Perturbation of the Earth’s radiation budget is a key factor for climate change. Such perturbations are caused either from changes in the incoming solar radiation at the top of atmosphere (TOA), i.e. astronomical changes, or from modifications in the absorbed and scattered solar radiation within the Earth-atmosphere system. It is known that the current climate change is mainly attributed to greenhouse gases and aerosols. However, opposite to the achieved significant improvement of our knowledge of the role of greenhouse gases, there is still high uncertainty in the estimations of the aerosol radiative effect, due to their high spatial and temporal variability and complex and changing physical, chemical and optical properties.Dust Aerosols (DA) is a major contributor of the global aerosol burden, while they modify the Earth’s radiation budget through the absorption and scattering of solar radiation and the absorption and re-emission of terrestrial radiation. Such dust-radiation interactions are known as Direct Radiation Effect (DRE) and generally result in a shortwave cooling effect and a smaller longwave heating effect both at the Top of Atmosphere (TOA) and the Earth’s surface. However, these radiative effects vary significantly in space and time, depending on the DA physical and optical properties, as well as on the underlying surface reflectivity or their vertical position relative to clouds, resulting in changes of the magnitude or even the sign of DREs. These dust-radiation interactions are expected to be maximized when the DA loads and the available solar radiation amounts are high. Therefore, the study of DREs under episodic dust conditions over areas such as the climatically sensitive and threatened Mediterranean Basin (MB), especially on a three-dimensional basis, is of primary importance. This becomes even more challenging when the study involves spectral detailed radiative transfer models (RTMs) and three-dimensionally resolved aerosol optical and atmospheric properties.Here, all-sky DRE of DA is estimated during a spatially and temporally extended Dust Aerosol Episode Case (DAEC) took place from 16 to 18 June 2016 over the MB. The studied DAEC is identified using a satellite algorithm, which uses aerosol optical properties. The dust DREs are computed using 3-D dust optical properties, namely dust optical depth, single scattering albedo and asymmetry parameter from the MERRA-2 reanalysis, and cloud (i.e., cloud amount, optical depth and top pressure) and other atmospheric properties from the International Satellite Cloud Climatology Project (ISCCP) as input data to the FORTH (Foundation for Research and Technology-Hellas) spectral radiative transfer model. The model runs, with and without DA, on a 3-hourly temporal and 0.5˚×0.625˚ horizontal spatial resolution for the 4-day period from 15 to 18 June 2016. The RTM output includes upwelling and downwelling solar fluxes, as well as DREs, at TOA, at the surface, and at 50 levels in the atmosphere. The vertical and horizontal variation of DA DREs are computed by producing and examining the respective DRE cross-sections, and finally the heating rates caused by the evolving dust episode are estimated in order to yield the radiative effect of dust on the dynamics of the Mediterranean atmosphere.

The wide region of Asia is one of the most densely populated places of the Earth, hosting a large percentage of the Εarth's population. Thus, changes in climate and weather conditions affect the lives of many people. In Asia, there are many desert areas, from which large amounts of Dust Aerosols (DA) are emitted into the atmosphere, where they remain suspended from a few hours up to several days. DA are able to travel thousands of miles away from their source areas, among which the largest ones are the Taklamakan and Gobi Deserts in Central & East Asia and the Tar Desert in the Indian subcontinent. Apart from them, there are also other smaller deserts in Asia, i.e. Badain Jaran, Tengger, which also contribute significant amounts of DA. Furthermore, the Aralkum, Kyzylkum and Karakum areas East of the Caspian Sea contribute high dust loadings, too.DA is a major contributor of aerosol burden in the Earth’s atmosphere, significantly affecting weather and climate conditions, through various interactions with radiation and clouds, while also deteriorating air quality and causing a series of health problems. DA alter the energy balance of the Earth-Atmosphere system, as they absorb and scatter primarily the solar, but also the thermal infrared radiation, thus influencing climate from the local to regional and global scales. Besides, DA act as effective Cloud Condensation Nuclei (CCN) or Ice Nuclei (IN), modifying cloud albedo and coverage, as well as the produced precipitation. All these dust effects are intensified under Dust Aerosol Episodes (DAEs), i.e. conditions of unusually high dust loadings, which occur every year with varying frequency and intensity, but with distinct seasonal and spatial characteristics. DAEs are originally determined on, and refer to, a pixel level, whilst days with an extended spatial coverage of DAEs are named Dust Aerosol Episode Days (DAEDs). Finally, series of consequent DAEDs constitute Dust Aerosol Episode Cases (DAECs), which are spatiotemporally extended and intense dust episodes that deserve to be identified and studied in areas like Asia.In the present study, a satellite algorithm is used to identify DAEDs over Asia and the Caspian Sea, aiming to determine their spatial and temporal distribution emphasizing their frequency of occurrence and the associated dust loadings. The algorithm uses as input daily spectral Aerosol Optical Depth (AOD) and Aerosol Index (AI) data from MODIS C6.1 and OMI OMAERUV databases, respectively, spanning the 16-year period from 2005 to 2020. It operates on a daily basis and 1deg x 1deg pixel level and detects the presence of DA by applying appropriate thresholds on Ångström Exponent (AE) (calculated using spectral AOD from MODIS) and AI. Subsequently, the algorithm determines the occurrence of DAEDs and DAECs, yielding their frequency of occurrence, as well as the associated dust optical depth (DOD) on monthly and annual timescales. Thus, the algorithm outputs enable to build a climatology of spatiotemporally extended Asian dust episodes, as well as to derive their year to year variability and tendencies over the 16-year study period.

 Sea salt (SS) are natural strongly scattering coarse aerosols, which yield the largest fraction of aerosol burden over many places on the Earth. They are important to the physics and chemistry of the marine atmosphere, affecting visibility, remote sensing, atmospheric chemistry, and air quality. The production, entrainment, transport and removal of SS aerosol are affected by several meteorological and environmental factors, such as wind speed, surface ocean and air temperature, relative humidity, atmospheric stability, precipitation and sea bottom depth and topography. The key meteorological factor that governs the SS production and life cycle is wind, which causes waves to break, forming whitecaps, thus influencing the injection of SS to upper atmospheric levels and their horizontal transport. Although, most of SS aerosols can be transported with atmospheric circulation only to short distances from their sources, the relatively smaller bubbles can live for a longer time in the atmosphere and thus can be transported not only over oceanic, but also over adjacent continental areas. Sea salt aerosols are highly hygroscopic, adsorbing water, and thus behave as Cloud Condensation Nuclei (CCN), affecting the formation, physical and optical properties of clouds. Therefore, their quantification and spatiotemporal variability is essential for the accurate determination of their climatic ole.In the present study, SS aerosols are detected on a global scale and for the 16-year period from 2005 to 2020, using a satellite algorithm, which is based on aerosol optical properties. This algorithm uses as input daily spectral Aerosol Optical Depth (AOD) and Aerosol Index (AI) or single scattering albedo (SSA) data from MODIS C6.1 and OMI OMAERUV databases, respectively.  It operates on a daily basis and 1°×1° pixel level and detects the presence of SS aerosols by applying suitable thresholds on Ångström Exponent (AE) (calculated using spectral AOD from MODIS) and AI or SSA. The algorithm outputs the absolute and relative frequency of occurrence of SS aerosols, as well as the associated AOD, on a monthly and annual basis. The results are given on a pixel as well as on regional and global scales. By running the algorithm for each year of the study period, the climatological mean values and the interannual variability and trends of the frequency of occurrence and AOD of SS aerosols are estimated.

A spatiotemporally extended dust aerosol episode that occurred over the Mediterranean Basin (MB) from 16 to 18 June 2016 is investigated using observational satellite and reanalysis data, focusing on the effects of high dust loads on cloud formation and temperature fields, including the creation of temperature inversions. The atmospheric conditions before and during the 3-day dust aerosol episode case (DAEC) are also analyzed. The dust episode, which is identified using a contemporary satellite algorithm, consists of long-range transport of African dust to the western and central MB. The day to day, before and during the DAEC, atmospheric circulation, dust-cloud interactions, and dust effect on temperature are examined using a variety of Moderate Resolution Imaging Spectroradiometer (MODIS) Level-3 Collection 6.1 satellite and Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis data. According to the obtained results, the dust export from N. Africa, which occurs under the prevalence of a trough over the western MB, and a ridge over the central MB, extends from southwest to northeast along two axes, one in the western and another in the central Mediterranean, covering remote areas up to the coasts of southern Europe, including the Balearic and Tyrrhenian Seas, the Italian peninsula, the Ionian and Adriatic Seas, and the Balkan peninsula. The analysis provides evidence of the formation of mixed-phase clouds, with high cloud-top heights (CTH higher than 10 km) and low cloud-top temperatures (CTT as low as 230 K), which spatiotemporally coincide with the high dust loadings that provide the necessary CCN and IN. Dust aerosols are transported either in the boundary layer (within the first 1–2 km) of areas close to the North African dust source areas or in the free troposphere over the Mediterranean Sea and the Italian and Balkan peninsulas (between 2 and 8 km). Distinct and extended layers of remarkable temperature inversions (up to 20 K/km) are created below the exported dust layers in the boundary layer of Mediterranean Sea areas, while weak/reduced lapse rates are formed over continental areas of MB undergoing the dust transport. Such modifications of temperature fields are important for the dynamics of the atmosphere of MB.