Abstract. The winter of 2025/26 in Romania generated intense media coverage characterizing cold and snowfall episodes in Bucharest and south-eastern Romania as historically exceptional. Here, we evaluate this characterization against a multi-source climatological analysis spanning up to 148 years of instrumental records. Using daily minimum temperature data from Bucuresti Filaret station (1879–2026) and monthly mean temperatures from 65 stations across Romania, we show that both January and February 2026 were anomalously warm relative to the 1971–2000 baseline (country-mean anomalies of +2.3 °C and +2.2 °C, respectively), with positive temperature anomalies recorded at nearly all 65 stations. Cold spell detection, using the 10th percentile threshold of the daily minimum temperature (TN10p), confirmed zero cold spell days throughout the winter 2025/26. The principal high-impact event, the 18th of February 2026 blizzard, was driven by a classical Mediterranean cyclone characterized by an upper-level potential vorticity streamer and an anomalously high integrated water vapor transport directed towards south-eastern Romania. Content analysis of 89 Romanian and international media items (~112,400 words) reveals 692 alarm-vocabulary occurrences (6.2 per 1,000 words). Outlet alarm-vocabulary density correlates strongly and negatively with the provision of historical context (Pearson r = −0.88; p < 0.001). We interpret this amplification as consistent with both the Social Amplification of Risk Framework and a shifting experiential baseline mechanism. Our study demonstrates that heavy snowfall in a warming climate can occur in the complete absence of any thermal cold extreme, and underscores the practical consequences of conflating precipitation and temperature hazards in emergency communication, and proposes a standardized context protocol for national meteorological services.

The European Severe Storms Laboratory (ESSL) was founded in 2006 as a non-profit research organization building on a pre-existing informal network of European scientists. ESSL was created to foster pan-European collaboration in the study and forecasting of severe convective storms, tornadoes, and extreme weather events. Since then, it has grown into a leading centre for severe weather research, education, and operational support with various activities aimed at reducing the impact of extreme meteorological events on society.ESSL’s core activities include the development and maintenance of the European Severe Weather Database (ESWD) together with its network of volunteers. The ESWD has grown to contain approximately 500,000 events and forms the basis for a wide range of research and development work as shown by its citations in over 400 peer-reviewed publications. The Additive Regression Convective Hazard Models (AR-CHaMo), and the Weather Data Displayer and Radar Displayer, a sophisticated platform for visualizing and analysing meteorological data, are among the most important severe weather analysis tools that ESSL has developed. The AR-CHaMo models were originally developed for climate analysis and are an important tool for risk assessments by both public authorities and the reinsurance sector. They have also been adapted for use in the prediction of severe storm hazards, in collaboration with the European Centre for Medium-Range Weather Forecasts (ECMWF). Since 2025, the Displayer and tailored AR-CHaMo-based products can be licensed by ESSL Services, a private sector spin-off of ESSL.Over the years, ESSL has published a wide range of peer-reviewed publications on topics ranging from the relation between severe storms and climate change to analyses of severe storms, their impacts and their detection. Currently, ESSL is leading the initiative for a large-scale multi year field campaign named TIM for the collection of data on severe storms to improve the understanding of their evolution near mountain ranges.ESSL's annual Testbeds and its training programs, including specialized courses for forecasters, often held at its Research and Training Centre in Wiener Neustadt, Austria. These initiatives are conducted in close cooperation with EUMETSAT, ECMWF and weather services, for example focusing on preparing meteorologists for new satellite capabilities, such as those offered by the Meteosat Third Generation. Joint workshops and expert-led sessions ensure that forecasters are equipped with the latest tools and knowledge to enhance severe weather prediction and response.Through its partnerships with national or regional meteorological services and international organizations, many of whom are one of ESSL's 32 Full Institutional Members, the laboratory continues to advance the science of severe storms, and strengthen the resilience of European communities to high-impact weather events.

Strong storms in the North Atlantic are a significant natural source of low-frequency seismic and acoustic signals (microseisms and microbaroms), commonly detected by monitoring stations in Romania. Storm Éowyn was an intense extratropical cyclone that impacted Ireland and the United Kingdom on 24 January 2025, driven by an exceptionally strong jet stream. The storm produced maximum wind gusts of 183 km/h and sustained winds of 135 km/h in western Ireland, breaking national records dating back to 1945.This study presents a joint seismo-acoustic analysis of Storm Éowyn as an intense source of oceanic ambient noise, using simultaneous seismic and infrasonic observations from the Romanian arrays BURAR, BURARI, and IPLOR. Infrasonic and seismic data were processed using the PMCC correlation-based method to characterize the temporal variability of microbarom and microseism signals between 21 and 27 January 2025. Seismo-acoustic detections in the 0.1–0.6 Hz frequency range were analyzed with DTK-PMCC and DTK-DIVA software packaged into CTBTO NDC-in-a-Box.The storm trajectory was computed using CyTRACK, an open-source Python toolbox for cyclone detection and tracking. ERA5 hourly reanalysis data from the Copernicus Climate Data Store provided mean sea level pressure, 10-m wind speed, and relative vorticity fields. Seismo-acoustic detections were compared with ARROW products from IFREMER describing microseism and microbarom source models. To assess detection performance and backazimuth discrepancies, we calculated the effective sound speed ratio (Ceff) at 50 km altitude using temperature and wind profiles from ECMWF operational analyses obtained via CAMS.During the storm's peak impact on 24 January, power spectral density analysis revealed microbarometric peaks at 0.23 Hz (BURARI) and 0.22 Hz (IPLOR), while the microseismic peak at BURAR reached 0.29 Hz. Results demonstrate good agreement between observed signals and modeled source locations.This study confirms the capability of Romanian infrasound and seismic arrays to monitor microbaroms and microseisms generated by intense North Atlantic storms. These findings provide a foundation for investigating other seismo-acoustic low-frequency signals from North Atlantic cyclones, which dominate winter detections at Romanian stations.

This study presents an updated, quality-controlled tornado climatology for Romania, increasing the catalog to 245 cases by incorporating newly recovered archival records and entries from the European Severe Weather Database. Spatial analysis shows that tornadoes are primarily concentrated in the plains of northeastern and southeastern Romania. Seventy percent of events occur between May and July, with a diurnal maximum in the early afternoon (1400-1500 UTC; 1600-1700 local time). Most tornadoes are rated using the International Fujita scale as IF1 and IF1.5 (39 cases), with 29 rated IF2 or higher. Notably, previously undocumented deadly tornadoes were uncovered in the archival research, including the 13 May 1912 Transylvania tornado, the deadliest recorded event. Analyses of convective environments from ERA5 reanalysis (1990-2024) indicate that Romanian tornadoes typically form in rapidly evolving, high-shear/low-CAPE settings. Mixed-layer CAPE increases to 700-1000 J kg21 in the 6-2 h before tornado onset, while 0-1-km bulk shear exceeds 7 m s21, and 0-500-m storm-relative helicity surpasses 50 m2 s22. Although the U.S.-derived significant tornado parameter rarely exceeds 0.2 in Romania, its marked increase prior to events suggests operational value when used with regional thresholds. Decadal trend analysis (1990-2024) reveals generally weak and spatially variable changes: Early summer mixed-layer CAPE shows modest increases in northwestern areas, while low-level shear has declined in central and southern regions during June-July. This revised climatology provides a more comprehensive foundation for understanding severe convective storms in Romania.

Fog is a type of cloud that forms in direct contact with the Earth’s surface. It is composed of extremely small water droplets or ice particles suspended in the air, similar to those found in clouds. For atmospheric conditions to be classified as fog, horizontal visibility must be reduced to less than 1 kilometer due to the presence of these fine particles, which scatter and absorb light and significantly limit what can be seen near the ground. Fog also plays an important role in the Earth system because it influences the surface radiation budget, in daytime causing cooling and in nighttime causing warming. Fog is a significant phenomenon that impacts the safety of terrestrial, maritime, and especially aviation transportation.This study investigates variations in fog microphysics and the correlations with horizontal visibility. The analysis is performed on datasets gathered during in situ continuous measurements conducted in wintertime 2025-2026 in Bucharest using the Fog Monitor FM-120 from Droplet Envea Group. A weather station from Luft (WS600-UMB) monitored meteorological parameters: temperature, pressure, humidity, wind direction and wind speed.The measurements were taken at the National Institute for Aerospace Research (INCAS) in Bucharest (coordinates: 44.4672° N, 26.0814° E), that is located in a Bucharest area with high traffic likely providing plenty of condensation nuclei. We present very recent observational evidence on the fog droplet signature in real time, linking temporal droplet size distribution changes and visibility evolution. We focused on assessing the microphysical parameters of fog, including number concentration (N), effective diameter (ED), liquid water content (LWC), and mean volume diameter (MVD), across a dimensional spectrum from 2 to 50 µm. The observational datasets were then used to test some visibility parameterizations, with the goal of determining a specific parameterization, linking visibility to the fog microphysics, best suited for the Bucharest area.The results add to the past studies aiming to contribute to a better understanding of fog characteristics and visibility parameterization using regional characteristics, ultimately aiding in improving safety measures in various transport sectors.

Air pollution remains a major environmental and public health challenge in Romania, where exceedances of European Union air quality standards persist. However, a national-scale assessment identifying regions of simultaneous multi-pollutant burden and coherent worsening trends has been lacking. Here we analyse hourly concentrations of PM2.5, PM10, NO2, and O3 from the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis over 2013-2024, validated against 317 in situ stations-pollutant validation series. The Mann-Kendall test, Sen's slope estimator, and Local Indicators of Spatial Association (LISA) are used to quantify trends and detect spatially coherent clusters. CAMS demonstrates a solid correlation with the measured data for various pollutants: PM2.5 shows a strong correlation (coefficient of 0.83), followed closely by O3 with 0.82, and PM10 at 0.78. However, the performance for NO2 is noticeably weaker, with a correlation coefficient of only 0.48. Atmospheric pressure serves as the primary factor influencing pollutant levels, surpassing other variables. Following this, the proximity to urban centres and the height of the boundary layer also emerge as significant determinants in understanding air quality dynamics. Trend analysis reveals seasonally structured patterns: winter PM2.5 and PM10 increases of up to ＋10 μgm-3decade-1 in northern Transylvania with maxima recorded in the Cluj area of +10.01 μgm-3decade-1 (95% CI: 3.91-15.62 μgm-3decade-1) for PM2.5 and +10.22 μgm-3decade-1 (95% CI: 5.19-17.38 μgm-3decade-1) for PM10, and widespread summer O3 increases exceeding ＋15 μgm-3decade-1 in southeastern and western Romania. LISA clustering identifies High-High trend regions in northwestern Transylvania, the Bucharest-Giurgiu corridor, and northeastern Romania. Conversely, Low-Low clusters in southwestern Romania and the Galaţi area reflect lignite-fired power plant decommissioning and industrial restructuring, respectively. Combining a composite pollution-burden score with LISA clustering delineates priority areas where multi-pollutant exposure is persistently elevated and continues to increase, providing spatially explicit evidence for geographically targeted air-quality management.

During thunderstorms in northern Romania, numerous infrasonic signals are emitted due to the process of lightning and thunder. Association between infrasound detections into 0.5 to 7 Hz frequency band and lightning flashes detected by the Arrival Time Difference lightning network (ATDnet) managed by the Met Office within 50 km from the BURARI infrasound station is systematically investigated. Statistical results are presented based on infrasound and lightning observations during summer months (June to August) from 2020 to 2022. Assuming direct wave propagation path, infrasound detections can be successfully correlated with ATDnet lightning detections up to distances of 50 km from the infrasound array. Long-duration trains of frequent sharp spikes in the amplitude observed into infrasound recordings during thunderstorms are associated with lightning discharges. Acoustic signatures of lightning activity show short-lived disturbances with dominant frequency of approx. 3 Hz and amplitudes ranging from 0.01 up to about 0.5 Pa. In order to associate BURARI measurements with ATDnet detections, a relationship between infrasound time-of-arrival and time of discharge signals is applied. A maximum deviation of 10o between observed infrasound back-azimuth and back-azimuth of ATDnet detections is allowed. For several cases (days with the largest number of lightnings), detection conditions of infrasound from lightning are detailed, and some characteristics are analyzed (e.g., amplitude, frequency, trace velocity and spectrograms in the frequency range from 0.5 to 10 Hz). Correlations with synoptic charts, regional lightning activity maps and electric field measurements could be performed.

The Central and Eastern European Infrasound Network (CEEIN) has been operating since 2019 in a collaboration of Hungarian, Czech, Romanian, Austrian, and Ukrainian research institutes. The study aims to extend the process of categorisation of infrasound signals that has been previously applied to the Hungarian infrasound array (PSZI) to the other CEEIN stations. The method of associating infrasound signals with thunderstorms relies on correlating the detections both spatially and temporally to lighting data from the Worldwide Lightning Location Network (WWLLN), which is considered ground truth. As a result, over 30,000 infrasound detections were categorized as thunderstorm-originated in the period between 2019 and 2023. Based on the results, we analyse the capabilities of the CEEIN to detect thunderstorms.

Severe thunderstorms are among the most damaging and impactful weather phenomena. In Europe, notable clusters occur in the vicinity of complex terrain. These areas not only experience frequent thunderstorms but also show a strong climate change signal with an increasing storm frequency. Despite the relevance of the subject, our understanding of severe convection in complex terrain, particularly in a changing climate, remains incomplete. This White Paper presents the current state of the research on thunderstorms in complex orography, covering storm severity, modification of pre-storm environments, convection initiation, storm-scale interactions with complex terrain, impactful hazards, numerical modeling and forecasting, climatologies and climate change signals, and innovative storm observations. Highlighting the gaps in our understanding, this review underscores the need for a coordinated European field campaign on thunderstorm intensification from mountains to plains (TIM). Initial plans for the TIM campaign, developed by the participating authors and institutions of this article, are briefly outlined. Obtaining coordinated and dense data on orographically driven storms is a key step toward improving warnings, forecasts, future climate projections, and adaptation measures.

The increasing air traffic at Romanian airports in recent years has necessitated a heightened focus on aviation safety standards. Simultaneously, climate change has altered the frequency of meteorological phenomena that impact aviation activities. This study aims to establish a baseline climatology and identify evolving trends of hazardous weather phenomena for the main airports in Romania between 1941 and 2022. The study uses ERA5 data to derive the most important parameters to study the occurrence of thunderstorms, low-level wind shear, reduced visibility, and snowfall, the weather phenomena that are relevant for the air traffic safety. The results show an increase in the number of hours with thunderstorms in the eastern part of the country and a rising trend in events with low-level wind shear in the western and central regions. Conversely, events with significant snowfall are decreasing in the eastern Carpathians and Muntenia region, while those with limited visibility are decreasing in the coastal regions and northeastern Romania. These findings can inform aviation safety regulations and help mitigate the impact of hazardous weather conditions on aeronautical activities.

Lightning strike fatalities across Europe from 2001 to 2020 based on reports from the European Severe Weather Database (ESWD) were analyzed. The ESWD records are based on publicly available sources such as local authorities, police and fire departments, newspapers, and news agencies, collected by the European Severe Storms Laboratory (ESSL) and its partners. In total, 1282 individual fatalities during this period were recorded in the ESWD. We report on the spatial distribution of fatalities, the distribution of fatalities per age group and gender, whether the victims were at work or undertaking leisurely activities, and in which kind of location they were struck. Furthermore, we considered the circumstances that led to lightning fatalities that occur indoors. We found that an average of 64 people are killed by lightning strikes every year and that the majority (77.8%) are male. The countries with the highest lightning fatality rate are Bulgaria (0.37 fatalities per million inhabitants per year), Moldova (0.36), and Romania (0.27). The lowest rates are found in Belgium, France, Norway, Portugal, and the United Kingdom (0.01 each). The proportion of fatalities in work activities increases from western to southeastern Europe. When working, people are most typically killed when farming, while during leisure, hiking/walking outdoors is their most common activity. Young people between 10 and 19 years of age have the biggest share of all age decenniums with a peak at the age of 15 (33 fatalities). Unlike male fatalities, that see a maximum fatality rate in the age decade of 10-19, female fatalities reach a maximum between 50 and 59 years of age.

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.

The COVID-19 pandemic has provided an opportunity to examine the impact of reduced human activity on air quality. This study assesses the levels of particulate matter (PM10) in three cities—Bucharest, Brașov, and Iași—during the pandemic restrictions in 2020, comparing them with data from the pre-pandemic period (2017–2019) and post-lockdown period (2021–2022). The results show a significant decrease in PM10 levels during the lockdown, which is closely associated with reduced traffic and mobility. Notably, while PM10 concentrations initially spiked at the beginning of 2020, they markedly declined following the enforcement of lockdown measures, during which mobility to workplaces in these cities decreased by about 60% in Bucharest, 50% in Brașov, and 45% in Iași. Health risks related to PM10 exposure were evaluated using the hazard quotient method, following EU and WHO guidelines. Despite the reduction in pollution levels in 2020, the findings suggest long-term human health risks for residents of these cities. This research highlights the critical need for sustainable strategies to address air quality issues in urban areas and protect public health.

Thunderstorms and their associated hazards pose a significant threat to society and the economy. In Europe, between 1980 and 2022, thunderstorms caused an estimated EUR 190 billion in economic losses. In Romania, the rate of cloud-to- ground (CG) lightning fatality is one of the highest in Europe. This study aims to reevaluate the risk of lightning in Romania by updating the CG lightning climatology for the period 2010-2022 and analysing long-term changes (1941-2022) in the frequency of thunderstorm environments. Data were obtained from the Arrival Time Difference Lightning Network (ATDnet) and the ERA5 reanalysis. On average, approximately 760,000 flashes occurred each year in Romania during the study period, with a maximum mean annual flash density of approximately 6.5 flashes km(-2) yr(-1) over the Southern Carpathians. Approximately 75% of all flashes were detected in June-August, with a peak in the afternoon hours. There is a general decrease in flash density in Romania, but this trend is statistically significant only in western Romania. The highest number of hours with thunderstorm environments (270-390 hours yr(-1)) was predominantly observed in the mountainous regions of central and western Romania. There is a significant increase in the number of hours with thunderstorms per decade, especially in eastern and southern Romania (>4 hours per decade). The results of this study allow for a better understanding of the risk posed by CG lightning in Romania, improved forecasting of thunderstorms, and serve as a basis for developing mitigation strategies to reduce the impact of CG lightning.

As national air quality networks and international research infrastructures, such as ACTRIS (EU) and ASCENT (US), continue to expand, the deployment of online aerosol chemistry measurements increases worldwide. These research infrastructures are focused on the ability to compare atmospheric properties from one region to another, making it crucial to understand instrument operation in various settings. This paper is part of a series of publications dedicated to better understanding the operation of these instruments using a series of laboratory tests. A particular focus was made on evaluating the organic aerosol (OA) measurement performance of six Aerosol Chemical Speciation Monitors (ACSMs) when sampling known mixtures of organic and inorganic aerosols and in ambient air. The study focuses on assessing the impact of instrument-to-instrument variability on ACSM data processing as well as identifying and quantifying the previously identified m/z44/NO3 artifacts that can affect the accuracy of the measurements. A high degree of variability was observed in instrument measurements of the m/z44/NO3 artifact when compared to results obtained two years earlier (e.g., an increase from similar to 0 in 2016 to 0.16 in 2018 or a decrease from 0.12 in 2016 to 0.05 in 2018), confirming the need for frequent evaluation and quantification during calibration. This study underlines that the product between organic aerosol relative ionization efficiency and the instrument collection efficiency value is instrument dependent and that the variability in these values (1.78 +/- 0.35) should be considered when estimating the measurement uncertainties. Using a range of specific compounds, an average RIEOA for levoglucosan (1.29 +/- 0.23) close to the default value commonly used in ACSM was determined, obtaining a value more specific to each instrument. This study provides valuable information for the calibration and operation of ACSM instruments, ensuring that future studies can build on this work to evaluate and improve instrument performance.

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.

Until now, studies on lightning fatalities have been available from a few individual countries but not for Europe as a whole. We here report on lightning strike fatalities in the period from 2001 to 2020 that happened across Europe, all of Turkey, the Caucasus countries (Armenia, Azerbaijan, and Georgia), based on reports from the European Severe Storms Database (ESWD). The ESWD records are based on publicly available data derived from local authorities, police and fire departments, newspapers, and news agencies, which are collected by ESSL and its partners. In total, 1280 individual fatalities were recorded in the ESWD during this period. We report on the spatial distribution of fatalities, and the distribution of fatalities per age group and sex, whether the victims were at work or undertaking leisurely activities, and in which kind of landscape they were hit. Furthermore, we considered the circumstances that led to lightning fatalities that occur indoors. We found that an average of 64 people are killed by lightning strikes every year, and that the majority (78%) are male. The countries with the highest rate of lightning fatalities per capita are Bulgaria (8.6 per million per year), Moldova (7.7), and Romania (5.8). The lowest rates are found in Belgium (0.1), Malta (0.1), and Portugal (0.1). The proportion of fatalities in work activities increases from Western to Southeastern Europe. When working, people are most typically killed when working on the fields while during leisure, people are most typically killed when hiking. Of the rare indoor fatalities (1.6%), a majority (11 of 15 known cases, 72%) were killed by fire and smoke that broke out after the lightning struck. Referring to the age of fatalities, people at the age of 10-19 have the biggest share of all age decennium with a peak at the age of 15 (33 fatalities). Divided by sex, male fatalities also show the biggest share in the age decade of 10-19, while female fatalities peak in the age decade of 50-59.

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.

The aerosol particles that become condensation nuclei affect the formation and evolution of clouds, their cycle lifetime, and their optical properties, having thus a major impact on the atmosphere, the radiative balance and leading therefore to climate changes. The aim of the study is to investigate the second indirect aerosol effect and the relationship between cloud droplet effective radius and cloud albedo for cleaner and polluted clouds over two sites in Romania (Bucharest and Cluj-Napoca), using satellite data collected from March 2000 to March 2022. Present study is the first one over sites in Romania. A series of physical parameters (albedo, cloud cover fraction, cloud optical depth, liquid water path and cloud water radius) were extracted from the Clouds and the Earth's Radiant Energy System (CERES) database for Bucharest, as a high polluted city, and for Cluj - Napoca, as a cleaner city in Romania. The time series for albedo and low cloud characteristics contained 193.584 hourly profiles. In addition, based on the optical depth from CERES, we calculated low cloud albedo using a parameterization currently used in the climate models. The study was also focused on the life cycle of the low clouds and on the cloud cover fraction over the two sites. The annual and seasonal variations of the cloud physical parameters were investigated and compared for both sites. We highlight how the cloud droplet effective radius modifies differently the cloud albedo for polluted clouds over Bucharest (presence of more and smaller cloud droplets and thus, a higher cloud albedo and less drizzle size drops) than for the cleaner clouds over Cluj-Napoca (presence of fewer and larger cloud droplets, and therefore a lower cloud albedo). The results also shows comparatively the frecqueny of occurence of this type of clouds over both sites and the temporal trend of analyzed physical characteristics. Modifications and variations of cloud characteristics at a city scale help us to better understand the second indirect aerosol effect, life cycle and the climatology of low clouds.AcknowledgmentGLSG, SS and GI acknowledge the support from NO Grants 2014-2021, under contract no 31/01.09.2020, Project EEA-RO-NO-2019-0423. GLSG work was also supported by the University of Bucharest, PhD research grant and by the Romanian Nucleu Programme.