Range Corrected Signal of an elastic backscatter lidar (at 355 nm, 532 nm and 1064 nm) and photometer's Aerosol Optical Depth and radiances (at 440 nm, 675 nm, 870 nm and 1020 nm) serve as input in GRASP model to retrieve the aerosol optical and microphysical properties. Several sets of simultaneously measurements in 2022 are analyzed. Collocated Doppler wind lidar is examined when available in terms of wind speed and direction. Hysplit trajectories are used to reveal the potential sources.

This work proposes a comprehensive approach for improving air quality monitoring and forecasting in Romania through the integration of Copernicus Atmosphere Monitoring Service products (CAMS) into the national environmental framework. The main objectives of the paper focus on: i) understanding the mechanisms of extreme pollution events and ii) investigating their early predictability. We use high resolution modelling of atmospheric motion and chemistry (Chimere model), coupled with CAMS boundary conditions. The added value of this dynamical downscaling — allowing harmonization of CAMS data with observational in-situ measurements — is analyzed through the implementation and test of dynamic downscaling procedures. Refining CAMS data from relatively low resolution ($10 \times 10 \text{km}$ resolution) to highresolution ($2 \times 2 \text{km}$) formats, provides a thorough context for local and regional use, especially in urban areas. This initiative is highlighted by the study of pollution events, such as an atypical mineral dust event that occurred over South-Eastern Romania on $22{ }^{\text{nd }}$ of March 2022, when the rapid increases of $\text{PM}_{10}$ concentrations were recorded at the Bucharest's air quality monitoring stations, reaching up to 580 micrograms per cubic meter. The data harmonization will enable more efficient air quality monitoring, support informed policy decisions, and lead to improved public health and environmental outcomes. Furthermore, the enhanced capacity for air quality monitoring and forecasting will contribute to Romania's climate targets and foster innovation in environmental research and technology.

Long-term changes (1940–2023) in thermal stress and its drivers in Europe were investigated based on the Universal Thermal Climate Index (UTCI) extracted from the ERA5-HEAT reanalysis. Furthermore, a vulnerability index was introduced to assess the impact of thermal stress. The results reveal a rapid continental shift in thermal stress toward milder conditions. Although cold stress is declining fastest in northern regions (more than 4 hours · decade ^-1 ), heat stress is intensifying across southern Europe (more than 3 hours · decade ^-1 ). Such changes are driven by an increase in 2 m air temperature (between 0.2 up to 0.6 ^∘ C · decade ^-1 ) and mean radiant temperature, particularly over northern and central Europe, and by changes also in relative humidity and wind speed. One of the key findings of this study is the decline in wind speed in specific areas of central and eastern Europe, leading to an increase in UTCI. Monthly analyzes show that winters warm the fastest (e.g., January cold stress decreases by 1.93 · decade ^-1 ), while summer heat stress peaks in July (0.27 · decade ^-1 ). In particular, the late spring months (e.g., May) also exhibit a positive trend, contributing to an extended warm season in Europe. Although European UTCI trends generally show a migration of thermal stress categories toward no to moderate-stress, city-level findings highlight more pronounced effects in continental cities such as Milan, Italy, and less pronounced effects in coastal areas like Madrid, Spain compared to cities of similar latitude. Furthermore, Heat Vulnerability Index reveals that highest vulnerability in southern and eastern Europe. These findings support policymakers in developing science-based measures to mitigate thermal risks, addressing both climatic and socioeconomic vulnerabilities in an era of rapid urbanization and climate change.

Convective weather, through its heavy showers, strong winds and hail, significantly impacts human activities, having the potential to inflict serious damage on social and environmental sectors. Limited research has been conducted on this phenomenon within Romanian territory, and currently there is no referenced climatological study primarily aimed at air traffic management users in this context. This study aims to assess the climatological aspects related to convective events based on sub-hourly observation data recorded at 17 airport weather stations throughout Romania during an 11-year period (2012–2022). The spatiotemporal distribution of convective events was analyzed based on occurrences of Cumulus Congestus (TCU) clouds, Cumulonimbus (CB) clouds, thunderstorms (TSs), heavy showers (+SHs), and hail (GR). With the data being extracted from meteorological aerodrome reports (METARs) and special meteorological aerodrome reports (SPECIs). Short-term trends were determined using Sen’s slope estimator, and statistical significance was assessed through the Mann–Kendall test. The main findings indicated that the highest occurrence of convective events is located over central and western Romania, with June emerging as the extreme month in terms of convective events, while the hourly distribution emphasizes that the highest frequency of convective events occurred in the afternoon. Trend analysis in TCU, CB, and TS show tendencies toward higher frequency of convective events while the results related to +SH and GR indicate a high variability across Romanian territory. Trend analysis disclosed more substantial changes in the TS variable. The results of this study bear potential significance for a broad spectrum of human activities and the management of natural environments.

The changes in the characteristics of heatwaves over Romania have been analyzed using the excess heat factor calculated for two climate change scenarios (RCP4.5 and RCP8.5) from the EURO-CORDEX project. The changes were evaluated for the near future (2021–2050) using the historical period (1971–2000) as reference. The frequency of occurrence and the duration of heatwaves is projected to increase for both climate scenarios in particular over southern Romania. In this region, the percentage of change in the near future for the number of heatwaves is between 50 and 60% for the RCP4.5 scenario and 60–80% for the RCP8.5 scenario. Also for the same region, the duration of heatwaves will increase by 30–50% for the RCP4.5 scenario and 60–80% for the RCP8.5 scenario. These results indicate that the human exposure to heatwaves will increase in Romania in the near future. To increase awareness on heatwaves and their impact, we propose a series of immediate actions that include (1) improving the communication of the impact of heatwaves, (2) identification of the regions where the population is more vulnerable to heatwaves, and (3) better understanding of the mortality and morbidity associate with heatwaves in Romania.

Doppler wind lidar measurements were used for the first time in Romania to analyse the wind and turbulence statistics for a peri-urban site located at Măgurele, southwest of Bucharest. Vertical and scanning measurements between December 2019 and November 2021 were processed using an existing toolbox. The statistics over the two-year period were performed on seasonal and diurnal cycle bases. The analyses showed a diurnal cycle for the horizontal wind speed, with lower values during daytime. In the upper part of the planetary boundary layer (PBL), the wind speed is lowest during the day and highest at night (near surface, the behaviour is reversed). The diurnal cycle has variations during the year (from approximately 500 m during midnight winter to approximately 1250 m during summer noon). The wind direction during autumn shows similarities with the summer season, with prevailing directions from east and northeast. The winter season is characterised by westerly winds. The most variable diurnal wind direction is observed during summer, with nighttime westerly winds and changing directions (from northeast to west) during daytime. The ERA5 reanalysis shows similar patterns for wind speed with Doppler wind lidar (slightly underestimated) and direction. The planetary boundary layer classes over the altitude region analysed shows the predominant convection during daytime and non-turbulent behaviour during nighttime. To a lesser extent, the intermittent turbulent class is observed during the growth and the decay of the mixing layer.

Understanding the macrophysical and microphysical processes within a convective environment is highly important to strengthen the resilience and adaptation to severe weather. The ACCuReSy project targets a topic a topic under debate within the international scientific community but less tackled in Romania: the aerosol-cloud interactions. By using state-of-the-art remote sensing instruments from new exploratory platforms placed within relevant two locations chosen by their relevance for the environmental factors that meet the conditions for observing convective development either thermal and dynamical, the project aims to perform advanced research on the atmospheric environment before, during and shortly after the convective events and to investigate the aerosol-cloud interactions with special attention on the factors that contribute to the hail formation. Here we present briefly the work performed during the first phase of the project and the future steps in achieving our goals. Key words: aerosol-clouds interactions, cloud macro- and microphysics, convective clouds, severe weather Acknowledgment: This work is funded by a grant of the Ministry of Research, Innovation and Digitization, CCCDI - UEFISCDI, project number PN-III-P2-2.1-PED-2021-1938, ctr. No. 713PED, and partially supported by the Romanian National Core Program Contract No.11N/03.01 2023 and by the Romanian Ministry of Research, Innovation and Digitalization, through Program 1- Development of the national research-development system, Subprogram 1.2 - Institutional performance - Projects to finance the excellent RDI, Contract no. 18PFE/30.12.2021

The general interest in tornado studies has been stimulated by scientific challenges, such as a better understanding of their genesis and development, and by applications and practical issues, such as forecast, building engineering, and risk analysis. This context justifies the interest in reconstructing tornado characteristics based on different approaches. The recently released ERA5 reanalysis data set at 31 km-horizontal resolution, and hourly temporal resolution holds significant potential for many climate applications. In this paper, the tornado environments in Romania between 1981-2020 are reconstructed using a series of thermodynamic and kinematic parameters (e.g., 0-1 km bulk wind shear, 0-6 km bulk wind shear, 0-500m mixed-layer CAPE, ML WMAX*SHEAR) calculated based on ERA5 data. A previous paper on the climatology of tornadoes in Romania showed that in the 1980s there was a minimum in the tornado activity (1 tornado report) followed by a period with a large number of reports (138 reports, approximately 4 tornadoes per year). The aim of the paper is twofold. First, we want to investigate the changes in the occurrence of tornado environments in Romania and thus understand if the minimum in tornado reporting in 1980 is due to tornadoes being ignored by the researchers and meteorologists (and reported as swerve wind events) or natural variability. Second, to study the spatial and temporal variability of the tornadoes in Romania in relation to large-scale patterns. The results show that the frequency of occurrence of tornado environments has increased in Romania since 1980, especially over southeastern Romania. This work has been financed within project „Dezvoltarea Centrului de Competență pentru Adaptarea Comunităților Locale la Schimbările Climatice prin parteneriate de tip public-privat în domeniul CDI” cod 3/16.11.2022 finanțat prin Planul Național de Redresare și Reziliență Apel nr. PNRR-III-C9-2022 – I5"

The Remote Sensing department from National Institute of R&D for Optoelectronics INOE, Romania, operates a complex research infrastructure and has multidisciplinary expertise spanning from laboratory/in-situ chemical analysis to Earth Observation (EO) techniques and applications. Infrastructure and personnel are mostly supported from projects financed by European Commission through FP7/Horizon 2020 (ended programs) and Horizon Europe (current programme) and other international calls (e.g., European Space Agency). In order to facilitate support of R&D projects submission/execution/reporting to Horizon Europe framework program competitions and other international calls, INOE is building a dedicated center until mid-2023, through PREPARE project - -Support Center for European Project Management and European Promotion-, funded under the Competitiveness Operational Program (2014- 2020), the action 1.1.3 "Creating synergies with the RDI actions of the European Union's HORIZON 2020 framework program and other international RDI programs". This work presents the results of our journey of drawing the main components of the management and promotion center that will mainly support large projects related to ACTRIS - Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS). The center is intended to guide and support scientists- approach from idea to grant negotiation stages and also to project implementation and reporting.

Lidar measurements of 11 smoke layers recorded at Măgurele, Romania, in 2014, 2016, and 2017 are analyzed in conjunction with the vegetation type of the burned biomass area. For the identified aerosol pollution layers, the mean optical properties and the intensive parameters in the layers are computed. The origination of the smoke is estimated by the means of the HYSPLIT dispersion model, taking into account the location of the fires and the injection height for each fire. Consequently, for each fire location, the associated land cover type is acquired by satellite-derived land cover products. We explore the relationship between the measured intensive parameters of the smoke layers and the respective land cover of the burned area. The vegetation type for the cases we analyzed was either broadleaf crops or grasses/cereals. Overall, the intensive parameters are similar for the two types, which can be associated with the fact that both types belong to the broader group of agricultural crops. For the cases analyzed, the smoke travel time corresponding to the effective predominant vegetation type is up to 2.4 days.

The bioclimatology of thermal stress over Europe between 1979 and 2019 was analysed using the Universal Thermal Climate Index (UTCI) derived from ERA5‐HEAT reanalysis. The bioclimatology of different European regions was assessed using Köppen–Geiger climate classification. The annual number of hours with heat stress (UTCI > 32°C) increased significantly during the study period for all the analysed Köppen–Geiger climate subclasses, showing also a clear increase towards southern Europe. The highest percentage of hours (20% of all hours) with cold stress (UTCI < −13°C) occur over northern Europe. A significant increasing trend (>0.05 hr·year −1 ) in the number of hours with heat stress was observed for 23 out of 32 analysed European cities representative for the Köppen–Geiger climate subclasses. For these cities not only the number of hours with heat stress has increased but also the heat stress is more persistent, while the number of cases and the persistence of the periods with cold stress have decreased over the last four decades. The UTCI values showed a statistically significant increase between 0.6 and 3.2°C for all the analysed cities over the study period reflecting the rising of global mean temperatures.

Abstract The goal of this article is to investigate how the Romanian air space would be affected by a potential large eruption of Vesuvius volcano. In order to characterize the potential impact of such an event on the Romanian air space, we simulated the ash plume transport from a hypothetical explosive eruption with a significant magnitude, that is, Volcanic Explosivity Index (VEI 4). The volcanic ash cloud transport towards Romania was described by using the mass distribution from the HYbrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) for Volcanic Ash numerical model. To assess the dominant air circulation types, an adaptation of the WetterLagenKlassification (WLK) classification scheme was used. The results emphasized that a long‐wave trough and southwesterly flow in upper levels would favour the direct transport of volcanic ash cloud towards the Romanian air space. Non‐negative matrix factorization method has been used to identify the atmospheric layer with the maximum values of mass. The simulation results reveal that the maximum values of averaged mass were obtained at an altitude around 7 km.

Significant progress in tornado research and management can be claimed over the last few decades worldwide. However, tornado forecasting and warning continue to be permanent challenges for most European national meteorological services because they require particular skills and experience. Moreover, tornado warnings may generate panic. Therefore, one can remark that the main difficulties are related to (1) forecasting the tornado genesis, and (2) finding the most efficient way to communicate to the general public the possibility of tornado occurrence. This article presents the main characteristics of two convective events that occurred in Romania in order to emphasize the similarities and disparities between the tornado event (30 April 2019) and the non-tornado event (6 May 2019), from the warning perspective. Further, we investigate, for the first time in Romania, the general public’s comprehension, risk perception and reactions regarding the tornado events. The survey performed in 2020 emphasized that the Romanian public is able to recognize tornadoes (60%), understand the risks (over 80%), can manage the panic (over 70%), and is rather desirous to receive clear (over 90%) and real-time (95%) tornado warnings. The lessons learned may support the further development of tornado forecasting and warning procedures, and foster the public’s awareness related to tornado events.

A high-impact freezing rain event affected parts of southeastern Romania on 24–26 January 2019. The freezing rain caused extensive damages in Bucharest, the capital city of Romania. The meteorological analysis highlighted the presence of a particular synoptic pattern involving a high-pressure system advecting cold air mass at low levels, while at mid-levels a warm and humid intrusion was associated with a low-pressure system of Mediterranean origin. At Bucharest, the vertical profiles from ERA5 and radiosondes emphasized the presence of a thick warm layer between 1000–1400 m above the re-freezing layer close to the surface. A climatology of freezing rain events in Bucharest was built to understand the frequency and intensity of this phenomenon. On average, there were approximately 5 observations of freezing rain in Bucharest per year between 1980–2018. The number of consecutive freezing rain days was used as a proxy for the event severity. Moderate-duration events (2 consecutive days) represented 16 periods of all 59 non-overlapping freezing rain periods in Bucharest and long-duration events (3 consecutive days) represented 3 periods. The monthly distribution showed that freezing rain occurs more frequently between December–February with a maximum in December. The moderate and long-duration freezing rain events were associated with two main sub-synoptic patterns related to the Carpathians lee cyclogenesis.

On the morning of 23 March 2018, an unusual phenomenon was observed over Romania where the southeastern part of the country was covered in a fresh-layer of orange snow. The event was extensively reported in mass-media and social-media and raised questions about the origin and the possible impact of the orange snow. Even if this type of events, intrusions of Saharan dust, have been reported before in Romania, and in Europe in general, their occurrence during negative temperature conditions is very rare. Saharan dust intrusion occurs over Europe mainly during spring and, in general, is not accompanied by snow at low altitudes. In this article, for the first time, the synoptic-scale conditions leading to the Saharan dust intrusion over Romania and the chemical and physical properties of the deposited dust particles in a snow layer were analyzed. The Saharan dust event affected a permanent atmospheric measurement research infrastructure located southwest of Bucharest, the capital city of Romania. In-situ and remote sensing measurements conducted at this research infrastructure allowed the identification of the dust source as the north Sahara. The source was confirmed by the elemental ratios of the main components (e.g., Al, Ca, Mg, Fe, K). For example, the (Ca+Mg)/Fe ratio of 1.39 was characteristic for the north Sahara. The dust morphology and the minerals were analyzed by scanning electron microscopy with energy disperse X-ray spectrometry (SEM/EDX). The size distribution of the particle geometric diameter showed that they are centred on 1 μ m, but larger particles up to 40 μ m are also present. To visualize the minerals, an approach was developed which emphasized the presence of the calcite, quartz or clay minerals. The optical parameters of dust were measured by re-suspending the particles. Values of the optical parameters (i.e., asymmetry parameter at 550 nm was 0.604, single scattering albedo was 0.84–0.89) were similar to those measured for Saharan dust intrusions over the Iberian Peninsula. Also, the non-refractory particles found in the dust-contaminated snow layer were analyzed, indicating the presence of HULIS-like compounds, most probably advected from the Mediterranean sea.

In this study, AERONET (Aerosol Robotic Network) and EARLINET (European Aerosol Research Lidar Network) data from 17 collocated lidar and sun photometer stations were used to characterize the optical properties of aerosol and their types for the 2008–2018 period in various regions of Europe. The analysis was done on six cluster domains defined using circulation types around each station and their common circulation features. As concluded from the lidar photometer measurements, the typical aerosol particles observed during 2008–2018 over Europe were medium-sized, medium absorbing particles with low spectral dependence. The highest mean values for the lidar ratio at 532 nm were recorded over Northeastern Europe and were associated with Smoke particles, while the lowest mean values for the Angstrom exponent were identified over the Southwest cluster and were associated with Dust and Marine particles. Smoke (37%) and Continental (25%) aerosol types were the predominant aerosol types in Europe, followed by Continental Polluted (17%), Dust (10%), and Marine/Cloud (10%) types. The seasonal variability was insignificant at the continental scale, showing a small increase in the percentage of Smoke during spring and a small increase of Dust during autumn. The aerosol optical depth (AOD) slightly decreased with time, while the Angstrom exponent oscillated between “hot and smoky” years (2011–2015) on the one hand and “dusty” years (2008–2010) and “wet” years (2017–2018) on the other hand. The high variability from year to year showed that aerosol transport in the troposphere became more and more important in the overall balance of the columnar aerosol load.

This study documents a complex fire event that occurred on October 2016, in Middle East belligerent area. Two fire outbreaks were detected by different spacecraft monitoring instruments on board of TERRA, CALIPSO and AURA Earth Observation missions. Link with local weather conditions was examined using ERA Interim Reanalysis and CAMS datasets. The detection of the event by multiple sensors enabled a detailed characterization of fires and the comparison with different observational data.

Atmospheric aerosols play a crucial role in the Earth's system, but their role is not completely understood, partly because of the large variability in their properties resulting from a large number of possible aerosol sources. Recently developed lidar-based techniques were able to retrieve the height distributions of optical and microphysical properties of fine-mode and coarse-mode particles, providing the types of the aerosols. One such technique is based on artificial neural networks (ANNs). In this article, a Neural Network Aerosol Typing Algorithm Based on Lidar Data (NATALI) was developed to estimate the most probable aerosol type from a set of multispectral lidar data. The algorithm was adjusted to run on the EARLINET 3β + 2α( + 1δ) profiles. The NATALI algorithm is based on the ability of specialized ANNs to resolve the overlapping values of the intensive optical parameters, calculated for each identified layer in the multiwavelength Raman lidar profiles. The ANNs were trained using synthetic data, for which a new aerosol model was developed. Two parallel typing schemes were implemented in order to accommodate data sets containing (or not) the measured linear particle depolarization ratios (LPDRs): (a) identification of 14 aerosol mixtures (high-resolution typing) if the LPDR is available in the input data files, and (b) identification of five predominant aerosol types (low-resolution typing) if the LPDR is not provided. For each scheme, three ANNs were run simultaneously, and a voting procedure selects the most probable aerosol type. The whole algorithm has been integrated into a Python application. The limitation of NATALI is that the results are strongly dependent on the input data, and thus the outputs should be understood accordingly. Additional applications of NATALI are feasible, e.g. testing the quality of the optical data and identifying incorrect calibration or insufficient cloud screening. Blind tests on EARLINET data samples showed the capability of NATALI to retrieve the aerosol type from a large variety of data, with different levels of quality and physical content.

The present study is focused on microphysics analysis of different cloud types observed by a multi-wavelength Raman lidar. Particle’s content from a wide variety of cloud types, produced in various atmospheric environments, were investigated using data provided by lidar and validated using ECMWF’s ERA Interim reanalysis. The results emphasized the capability of lidar instruments to detect multiple cloud layers, and to discriminate between ice particles and water droplets found within the studied clouds.