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The assessment and interpretation of an extreme hydrometeorological event and the consequent impacts such as flash floods, especially within urban environment, is of high interest, especially in the Anthropocene era. Global warming intensifies Urban Heat Island resulting in local intense convective precipitation over a densely urbanized area. Urbanization on one hand, increases the sensible heat flux and transports more water vapor, mixing it into the upper atmosphere and on the other hand, increases the surface roughness and as a consequence mechanical turbulence drives low-level convergence resulting in enhanced intensity and frequency of convectional extreme precipitation (Nastos and Zerefos, 2007; Li et al., 2023).In this study, we focus on an extreme summer convective event on August 24, 2022, in Athens, Greece, where the precipitation intensity reached instantly 116 mm/h and the visibility was limited to 600m. More specifically, we analyzed and compared the H03B product of EUMETSAT Hydrology Satellite Applications Facility (HSAF) with the ground-based OTT PARSIVEL Laser-based optical Disdrometer measurements.Product H03B is based on the infrared (IR) electromagnetic spectrum from the SEVIRI instrument on board of Meteosat Second Generation satellites. The product is generated at the 15-min imaging rate of SEVIRI, and the spatial resolution is consistent with the SEVIRI pixel. Precipitation is obtained by combining IR equivalent blackbody temperatures at 10.8 μm with rain rates from polar microwave measurements.OTT PARSIVEL Disdrometer is ideal for simultaneous measurement of PARticle SIze and VELocity of all liquid and solid precipitation. It detects and identifies 8 different precipitation types as drizzle, mixed drizzle/rain, rain, mixed rain/snow, snow, snow grains, freezing rain and hail. Besides, it provides drop size distributions on the ground and a function to derive a local Z/R relation – ready to be used to adjust the radar data.The findings of the combined analysis showed that, towards early warning of convective precipitation in urban areas, combined high temporal resolution measurements carried out by satellites and ground based disdrometer measurements could provide a high-performance regional early warning system.Keywords: extreme convective precipitation, EUMETSAT HSAF, PARSIVEL, rain rate, AthensAcknowledgement: This work is co-financed by Iceland, Liechtenstein, Norway European Economic Area (EEA) Grants 2014 – 2021 and Greek Public Investments Program.References:Panagiotis T. Nastos, Christos S., Zerefos (2007), On extreme daily precipitation totals at Athens, Greece. Advances in Geosciences10, DOI: 5194/adgeo-10-59-2007 Chenxi Li, Xihui Gu, Louise J. Slater, Jianyu Liu, Jianfeng Li, Xiang Zhang, Dongdong Kong (2023), Urbanization-Induced Increases in Heavy Precipitation are Magnified by Moist Heatwaves in an Urban Agglomeration of East China. Journal of Climate, 36 (2), 693–709, DOI: https://doi.org/10.1175/JCLI-D-22-0223.1

This study presents the results of high-resolution dynamical downscaling of 5 km on maximum (TX) and minimum (TN) air temperature and precipitation, for Greece, with the Weather Research and Forecasting (WRF) model. The ERA-Interim (ERA-I) reanalysis dataset is used for initial and boundary conditions. The model results (WRF_5) are evaluated against available ground observations for the period 1980–2004 through the calculation of mean climatology, statistical metrics, and distributions of extreme events on daily, monthly and seasonal scales. WRF_5 model captures very well the geographical distribution of TX and TN of the study area, and illustrates finely the seasonal differences. Statistical results for TX (TN) indicate a cold (warm) bias of − 0.6 °C (1 °C) regarding WRF_5 and − 3 °C (0.5 °C) for ERA-I. The efficiency metrics for temperatures showed a highly improved performance of the model compared to reanalysis for all temporal scales investigated. The observed mean annual cycle and inter-annual variability of precipitation are also well represented by model simulation. Although WRF_5 overestimates rainfall during most of the year, the seasonal pattern of WRF_5 presented similar correlation coefficients for all stations with a range of 0.6–0.85, showing a good model ability to simulate the precipitation in Greece. The results reveal the capability of the configured WRF high resolution model to reproduce the main climatological variables of the study area, outperforming the coarse resolution ERA-Interim in a region that is dominated by highly variable topographic characteristics. This is deemed necessary for undertaking any further studies concerning future climate change impacts in various sectors.

The Mediterranean region is an area of increasing interest due to its unique climate. Nowadays, climate change has already evident consequences, such as the rise of extreme weather events, which significantly affect peoples’ life in the highly populated urban areas of the Mediterranean. Thus, in this study, ten metropolitan cities from the wider Mediterranean region with different climatic characteristics have been selected to study the frequency and the multidecadal trends of extreme events, as well as their possible connection with the large scale and synoptic scale atmospheric variability.Four combined extreme indices have been evaluated on annual and seasonal basis for the period 1950-2018 using the high-resolution E-OBS gridded daily mean temperature and precipitation datasets (0.1° x 0.1°; v.19e) from the European Climate Assessment & Dataset (ECA&D, Klein Tank et al. 2002, www.ecad.eu). These combined extreme indices refer to the joint modes of temperature and precipitation extremes, concerning the co-occurrence of Cold/Dry days (CD), Cold/Wet days (CW), Warm/Dry days (WD), Warm/Wet days (WW), which can reflect extreme conditions better than temperature or precipitation statistics considered separately (Beniston, 2009; 2011). The links of the extreme events with the atmospheric variability are investigated based on large-scale teleconnection indices and spatiotemporal distribution of cyclonic activity. Toward this, the comprehensive climatology of Mediterranean cyclones assembled was used by applying a cyclone tracking algorithm (Murray and Simmonds, 1991; Flocas et al., 2011) with respect to the ECMWF ERA5 Interim mean sea level pressure fields since 1950.The findings of the analysis showed distinct temporal and spatial variations of the combined extremes occurrences in the cities across the Mediterranean, which can be attributed to the effects of its complex topography, as well as to the non-uniform influence of the atmospheric variability. Specifically, the CD and WD indices have higher annual occurrences than the CW and WW, which indicates that the wider Mediterranean region experiences more dry days, either cold or warm, than wet days. The urban areas most affected by cold/dry events are located on the western Africa, while almost all urban areas around the Mediterranean coast are impacted by higher number of warm/dry events, with increasing trends.References: Beniston M., 2009, Geophys. Res. Lett., 36, L07707; Beniston M., and Coauthors, 2011, Int. J. Climatol., 31, 1257-1263; Murray and Simmonds, 1991 Aust Met Mag 39 155 166; Flocas et al., 2010, J Climate, 23(19), 5243-5257

Climate and weather are significant factors in planning to visit a touristic destination, especially taking into consideration the global warming. In this respect, touristic areas will be significantly affected by the potential adverse effects of climate change. This is the case of the Mediterranean which is considered one of the most vulnerable regions in global climate change, regarding the observed but also the expected future thermal risk. The goal of this study is to highlight the bioclimatic conditions anticipated to prevail in the eastern Mediterranean, studying five touristic destinations; namely, Heraklion, Crete Island (Greece), Antalya (Turkey), Limassol (Cyprus), Valletta (Malta) and Cairo (Egypt).One of the most popular and widely applied human thermal index, the Physiologically Equivalent Temperature (PET), based on human energy balance, is utilized to quantify the regime of present and simulated future bioclimatic conditions. Air temperature, humidity, wind speed and global solar radiation (estimated at 1.1m where is the gravity center of the human body and builds the reference level for human biometeorological studies) are the parameters needed for the assessment of PET, using the RayMan model. The meteorological datasets concern 3-hour data from the SMHI RCA4 regional climate model (Rossby Center, Swedish Meteorological and Hydrological Institute, Norrkoping Sweden, spatial analysis 11 km), for the historical period 1971-2000 (reference period), as well as for the periods 2021-2050 and 2071-2100 under the two Representative Concentration Pathways, RCP4.5 (intermediate) and RCP8.5 (extreme).As far as the historical period is concerned, the findings of the analysis showed that Heraklion, Crete Island, appears the most favorable bioclimatic conditions at midday (UTC 12:00), followed by Limassol and Malta, while Cairo and Antalya show high frequency and duration of strong/extreme heat stress within the year. Regarding evening hours (UTC 21:00) during summertime, there is a slight cold stress/thermal comfort in Heraklion and Malta, followed by Limassol, Antalya and Cairo with worse bioclimatic conditions.Based on climate model simulations for 2021-2050 (near future) and 2071-2100 (far future) under RCP4.5 and RCP8.5, for midday (UTC 12:00), Heraklion, Crete Island depicts the best bioclimatic conditions, which can be attributed to the increased frequency and intensity of the Etesians winds, blowing during the summer period of the year contributing to the mitigation of the strong/extreme heat stress. The frequency and the duration of thermal comfort in the evening (UTC 21:00) is expected to increase for both RCPs. Malta shows similar bioclimatic behavior to Heraklion during evening hours, followed by Limassol, Antalya and Cairo, where the light/moderate heat stress occurs with increasing frequency and seasonality.The results of the bioclimatic analysis will help in the management/dissemination of climate information for tourism purposes. In addition, the methodology used, based on bioclimatic (PET) diagrams, can provide detailed information in order to assess and quantify the climate change impacts on the tourism potential in the eastern Mediterranean.

Attica, which is the region with the largest population in Greece, suffers from numerous high rainfall intensity events inducing flash floods and, consequently, significant damages in the urbanized areas. High rainfall intensities are frequently connected with convection, a quiet often phenomenon during either the transitional seasons or the late summer thunderstorms. The objective of this paper is to implement and evaluate a stratiform-convective classification technique for the rainfall events occurred in Attica during the period 2005–2016. As this methodology requires only the precipitation records, the satisfactory evaluation of this classification approach allows further applications, such as, a climatological analysis regarding convective and stratiform trends in the study area and generally in Greece. Particularly, the rain gauge stations' records used correspond to 10-minute time series from 32 rainfall stations for the same period, and these data are analyzed and processed in order to fit a suitable exponential curve describing the stratiform component. A critical annual threshold of rainfall intensity in mm/6h is then calculated, separating the prevailing convective and stratiform regime above and below this value, respectively. The analysis is performed in three individual subareas of Attica region demarcated initially according to their morphological characteristics and subsequently by checking the cross-correlation for each pair of stations. This critical intensity threshold is determined for each year separately, in order to illustrate the uncertainty and the difficulty in identifying a standard threshold, as it is related to the annual behavior of precipitation. Using this threshold, a total of about 180 six-hour case events is detected as purely convective cases, however this number ranges for each subarea. For these cases, a further analysis is performed with the aim of verifying their convective character. Verification includes time-accurate lightning data, surface synoptic observations from the Hellenic National Meteorological Service and six-hour surface analysis charts from UK MetOffice. Findings, which are presented via indicative verification indices, show in general encouraging performance, considering the highly variable nature of convective precipitation which poses considerable problems in its identification with quite simple techniques.

To date, due to climate change, heatwaves are more frequent, with greater intensity and duration resulting in deleterious impacts on human health. To be able to manage heatwaves and quantify the impacts on human health, it is crucial to define them and implement policy preventive measures. However, heatwaves are relative to the climate of a location: The same meteorological conditions can constitute a heatwave in one place but not in another. Due to different climatic conditions, social characteristics, and adaptation, heatwaves should be defined on a local scale, which poses difficulties when it comes to comparison of different definitions. The aim of the present study is to define heatwaves, implementing robust statistical analysis for three different indicators (temperature, physiological equivalent temperature (PET), and universal thermal climate index (UTCI)) for three causes of mortality (i.e., cardiological and respiratory mortality and cardiorespiratory mortality) using Attica (Greece) as a case study. Our results define a heatwave for Attica as a period of at least 3 days when the mean temperature is higher than the 97.5th percentile. Afterwards, we encapsulate the harvesting effect by implementing robust statistical analysis, using the Superposed Epoch analysis. Consequently, quantifying heatwaves is crucial so as to create early warning systems and prevent avoidable mortality.

Risk assessment constitutes the first part within the risk management framework and involves evaluating the importance of a risk, either quantitatively or qualitatively. Risk assessment consists of three steps, namely risk identification, risk estimation and risk evaluation. Nevertheless, the risk management framework also includes a fourth step, i.e., the need for feedback on all the risk assessment undertakings. However, there is a lack of such feedback, which constitutes a serious deficiency in the reduction of environmental hazards at the present time. Risk identification of local or regional hazards involves hazard quantification, event monitoring including early warning systems and statistical inference. Risk identification also involves the development of a database where historical hazard information and hazard effects are included. Similarly, risk estimation involves magnitude–frequency relationships and hazard economic costs. Furthermore, risk evaluation consists of the social consequences of the derived risk and involves cost-benefit analysis and community policy. The objective of this review paper is twofold. On the one hand, it is to address meteorological hazards and extremes within the risk management framework. Analysis results and case studies over Mediterranean ecosystems with emphasis on the wider area of Greece, in the eastern Mediterranean, are presented for each of the three steps of risk assessment for several environmental hazards. The results indicate that the risk management framework constitutes an integrated approach for environmental planning and decision-making. On the other hand, it sheds light on advances and current trends in the considered meteorological and environmental hazards and extreme events, such as tornadoes, waterspouts, hailstorms, heat waves, droughts, floods, heavy convective precipitation, landslides and wildfires, using recorded datasets, model simulations and innovative methodologies.

Most people living in Europe's cities are still exposed to levels of air pollution deemed harmful by the World Health Organization. In the modern world, air pollution is the foremost concern because of its impact in human health and economy. This strong connection appears gaining a lot of concern, driven by new installed low-cost electrochemical sensors monitoring systems. Highly accuracy, real-time monitoring, daily and yearly statistics, data access from experts or simple users, low-cost equipment and forecasting needs, enforce the market to develop new air quality monitoring systems using advanced technologies and protocols. In this study, a comparison via low-cost electrochemical sensors and of static, fixed site measurement monitoring station, is taking place in Athens, Greece, along with the data quality and Air Quality Index (AQI) including data accuracy and quality of data concerning adverse health effects due to air pollution. The findings presented in this work, relate to different flexible and affordable alternatives adopted during the evaluation and calibration of low-cost gas sensors for the monitoring. The significance of the positive results is particularly useful, especially considering the founding for interference, environmental conditions affections and air quality information including indexes and health recommendations for a specific location.

Olive and vine cultivations are two of the most important crops in Europe, yielding high quality and value food products. The climate change over the Balkans may elevate the agroecological pressure for the established crops and shift their cultivations areas. One of the widely-used agroclimatic indices is the growing degree days (GDD) which accumulates the necessary thermal units for the selected crops. Despite the advances on the agroclimatic research, there are few available methods for spatiotemporal estimation of this useful index. So, this research is focused on the construction of simple and reliable equations for the calculation and projection of olive and vine cultivations&rsquo, GDD over the Balkans. The models&rsquo, input parameters are the time, the altitude, the distance from the seashore, and the latitude. Its assembly is made by the extracted spatial data, combined with the Agri4Cast dataset for the period of 1980 to 2018 incorporating the regional climate change trend. The results indicate that the most influential parameter is the time, followed by the latitude, for both cultivations. According to the projections, as quantified by GDD, a vast sprawl of olive and vine cultivation areas will have been formed to the northern parts of the studied area. To be more precise, the viticulture could expand spatially by 28.8% (of the Balkans area) by 2040, and by 15.1% to 2060, when the olive cultivations&rsquo, area could sprawl 23.9% by 2040 and 20.3% by 2060.

Risk assessment constitutes the first part within the risk management framework and involves evaluating the importance of a risk, either quantitatively, or qualitatively. Risk assessment consists of three steps, namely risk identification, risk estimation and risk evaluation. Nevertheless, the risk management framework also includes a fourth step, i.e. the need for a feedback of all the risk assessment undertakings. However, there is a lack of such feedback, which constitutes a serious deficiency in the reduction of environmental hazards at the present time. Risk identification of local or regional hazards involves hazard quantification, event monitoring including early warning systems and statistical inference. Risk identification also involves the development of a database, where historical hazard information and their effects is included. Similarly, risk estimation involves magnitude/frequency relationships and hazard economic costs. Furthermore, risk evaluation consists of the social consequences of the derived risk and involves cost-benefit analysis and community policy. The objective of this review paper is twofold: On one hand, to address meteorological hazards and extremes within the risk management framework. Analyses results and case studies over Mediterranean ecosystems with emphasis on the wider area of Greece, in the eastern Mediterranean, are presented for each of the three steps of risk assessment for several environmental hazards. The results indicate that the risk management framework constitutes an integrated approach for environmental planning and decision making. On the other hand, it shed light to advances and current trends in the considered meteorological and environmental hazards and extreme events, such as tornadoes, waterspouts, hailstorms, heat waves, droughts, floods, heavy convective precipitation, landslides and wildfires, using recorded datasets, model simulations and innovative methodologies.

The Predicted Mean Vote (PMV) and the Predicted Percentage of Dissatisfied (PPD) indices are used to assess the indoor environment in terms of human thermal comfort-discomfort. In this study, an experimental combined objective and subjective investigation of thermal comfort perception has been performed in students between 16-18 years old, in a non-air-conditioned school building in Gerakas municipality within the urban greater Athens area. The objective approach included instrumentation measurements and data processing according to ISO 7730, whereas, the subjective one was based on answers collection following ISO 10551. The study is mainly devoted to the verification of Fanger’s approach in a building, in free running conditions, under a mild (moderate) climate. The comparison between instrumentation data and questionnaire results presented an underestimation of the mean vote, predicting a cooler sensation than the actual one. Finally, the knowledge of human thermal perception indoors is a first approach in order to control the energy demands/savings as well as the emissions of a building complex having impact in urban microclimate and urban air quality.

The goal of this study is to create a high-resolution grid of precipitation indices for the wider Greek region using real data from meteorological stations for the 1980-2010 period. Under the risk of increased extreme events caused by climate change, it is important to be able to have a high-resolution gridded extreme precipitation indices in order to overcome the lack of density of observations in both time and space. The development of such a grid can be used to validate model outputs and inform decision makers to better mitigate the damage from extreme precipitation.The first step of the analysis is to calculate the extreme precipitation indices based on daily observations derived from more than 100 meteorological stations covering a wide range of altitudes and spatial climate patterns existing in Greece. Thereafter, the extreme indices will be multilinearly downscaled to a 12-meter resolution grid. The geophysical parameters used in the downscaling procedure consists of altitude, latitude, longitude, slope, aspect, solar irradiance and Euclidian distance from the water bodies. The altitude information came from the highly accurate 12-meter resolution TanDEM-X Elevation Model, which is a product generated from the TerraSAR-X satellite mission data. The resulting high-resolution patterns will give insight of the spatial and temporal variability of extreme precipitation, over the complex terrain of the wider Greek region.

Mt. Ainos in Kefalonia Island, Greece, hosts a large variety of plant species, some of them endemic to the region. Because of its rich biodiversity, a large portion of the mountain area is designated as National Park and is protected from human activities such as hunting or logging. Therefore, the area presents a lot of opportunities for ecotourist activities, such as trekking, birdwatching, and mountain climbing. In order to estimate its touristic activities potential, it is essential to assess the mountain’s biometeorological conditions. To achieve that, the human thermal index PET (physiologically equivalent temperature) was used, which is based on a human energy balance model. However, it is difficult to get the specific meteorological data over mountainous areas (air temperature, humidity, wind speed, and global solar radiation), appropriate as input variables for PET modeling. In order to overcome this limitation, artificial neural networks (ANNs) were developed for the estimation of PET index in ten sites within the Ainos National Park. In the process, the spatiotemporal distributions of the PET thermal index were illustrated, taking into consideration the ANN modeling. The findings of the performed analysis shed light that Mt. Ainos offers the greatest touristic opportunities from May to September, when thermal comfort conditions appear. The study also proves that the highest frequency of thermal comfort appears within the aforementioned time period over the highest altitudes, while on the contrary, slightly warm class appears as the altitude decreases on both sides of the mountain.

In the present study, the ability of the Advanced Weather Research and Forecasting numerical model (WRF-ARW) to perform climate regionalization studies in the topographically complex region of Greece, was examined in order to explore the possibility of a more reliable selection of physical schemes for the simulation of historical and future high resolution (5 km) climate model experiments to investigate the impact of climate change. This work is directly linked to a previous study investigating the performance of seven different model setups for one year, from which the need was derived for further examination of four different simulations to investigate the model sensitivity on the representation of surface variables statistics during a 5-year period. The results have been compared with observational data for maximum and minimum air temperature and daily precipitation through statistical analysis. Clear similarities were found in precipitation patterns among simulations and observations, yielding smoothly its inter-annual variability, especially during the wettest months and summer periods, with the lowest positive percentage BIAS calculated at about 19% for the selected combination of physics parameterizations (PP3). Regarding the maximum and minimum temperature, statistical analysis showed a high correlation above 0.9, and negative bias around 1&minus, 1.5 &deg, C, and positive bias near 2 &deg, C, respectively.

Clouds play a major role in the radiative balance of our planet and largely control dynamic and thermal atmospheric processes that affect the climatic system. In addition to cloud cover (CC), different types of clouds are of special interest, as they contribute in different ways to the energy budget between the earth and the atmosphere. The study analyzes historical sub-daily reports from cloudiness observations conducted at the National Observatory of Athens, dating since the late nineteenth century. Long-term trends and diurnal, intra-annual, and inter-annual variability of both CC and frequency of prevailing cloud types were estimated. The analysis revealed statistically significant positive trends (p

The aim of the present study is the evaluation of the thermodynamic characteristics of a high impact tornadic event in Skala Lakonia, southern Peloponnese, on September 21, 2015. The study was carried out using the Weather Research and Forecasting (AR-WRF) mesoscale model allowing high spatial resolution for the analysis. For the particular model setup used in this study, different planetary boundary layer (PBL) parameterization schemes were tested. The current study downscales ECMWF reanalysis initial and lateral boundary conditions to 1 km spatial resolution to assess the mixing layer convective available potential energy (MLCAPE – MCAPE hereafter), the reflectivity perceived by radar (dBZ), the storm relative helicity (SRH) and the energy helicity index (EHI) for the different PBL schemes. The validation was conducted using in situ and remote sensing observations of the surrounding area. The results illustrate deep convective activity with MCAPE values indicating a strong instability in the region of interest using both local and non-local closure PBL schemes. The study evaluates the affectability of the PBL physical processes with regard to the thermodynamic variables on the model performance. Finally, Mellor Yamada Janjic local (MYJ) closure PBL scheme is the best PBL scheme for studying this tornado since its simulation results display deep convection activity along with the highest validation scores.

In the Mediterranean region, a rapidly rotating storm system characterized by gale winds, severe precipitation, and low pressure center, accompanied with a spiral pattern of thunderstorms, is occasionally observed. These tropical-like cyclones (TLC), known as Medicanes or Mediterranean ‘hurricanes’, have similar characteristics with hurricanes and a significant amount of research has been done in recent years to investigate their atmospheric characteristics and impacts. The Laboratory of Climatology and Atmospheric Environment (LACAE), National and Kapodistrian University of Athens, has established a systematic effort since 2009, to document the impact of severe atmospheric phenomena (e.g. medicanes, cyclones, tornadoes, waterspouts), especially over the eastern Mediterranean region. The objective of this study is to discuss on one hand, the intra annual and inter annual distribution of medicanes along with the composite daily means and anomalies of synoptic conditions and instability indices at the middle and lower troposphere, sea level pressure (SLP) and sea surface temperature (SST) as well, during medicane days, based on the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data sets. On the other hand, the geographical distribution of medicane impacts over Mediterranean region with respect to period from 1969 to 2014, is illustrated by utilizing the Geographic Information Systems (GIS). The findings shed light on the high activity of medicanes during autumn and winter with maxima in September, decreasing thereafter with a long tail extending in spring and minimizing in summer. The average number of the selected medicanes is 1.4 ± 1.3 events per year while their inter-annual variability is remarkable, although there is not any statistically significant trend (95% confidence level) within the examined period. We found from the analysis of the composite daily anomalies of the synoptic conditions that medicanes initiate and further develop in Mediterranean regions where cold air intrusions appear in the upper atmosphere associated with warm sea waters. Last but not least, the impacts of medicanes over coastal areas, including infrastructure damage and casualties due to associated strong winds, heavy rainfall, and in rare cases, tornadoes, are well presented, indicating the high concern of such extreme atmospheric phenomena should be given by stake holders towards resilience of the society.

This study is a first attempt to obtain high resolution simulations of present climate in Greece, by applying dynamical downscaling using the Advanced Weather Research and Forecasting numerical model (WRF-ARW). We performed two simulations and thus, the model was set up with two different horizontal resolutions for the European domain (D01), of 20 and 25 km, driven by ERA-INTERIM reanalysis data for a total period of one year. For each simulation, the European domain (D01) was dynamically downscaled to the domain of Greece (D02), with grid spacing of 5 km. The first objective concerned the selection of the appropriate horizontal resolution of the first domain (at European scale) and the second was to validate the performance of various physics parameterizations of the WRF-ARW at high resolution on temperature and precipitation predictions, in the domain of Greece. In total, seven different sets of physics parameterizations were applied to investigate the model predictions against available observational data, by utilizing appropriate statistical metrics. It was deduced that there were no significant differences in the results imposed by the grid resolution of 25 km or 20 km of the parent (European) domain. The biases remained systematically negative (underestimation) for maximum temperature and positive (overestimation) for the minimum temperature for all configurations of about 2–2.4 °C and 1–1.5 °C, respectively. Simulation results in precipitation were not particularly satisfactory for any of the physical schemes, indicating overall a strong overestimation in absolute values, but with similarities found in the precipitation patterns.

The COnsortium for Small-scale MOdeling (COSMO) was formed in October 1998, and its general goal is to develop, improve and maintain a non-hydrostatic limited-area atmospheric model. The COSMO model has been designed both for operational numerical weather prediction (NWP) as well as various scientific applications on the meso-β and meso-γ scale. Two tornado case studies were selected to investigate the ability of COSMO model to depict the characteristics of severe convective weather, which favoured the development of the associated storms. The first tornado (TR01) occurred, close to Ag. Ilias village, 8 Km north-western of Aitoliko city over western Greece on February 7, 2013, while the second tornado (TR02) was developed close to Palio Katramio village, 8 Km southern from Xanthi city over northern Greece on November 25, 2015. Although both tornadoes had a short lifetime, they caused significant damages. The COSMO.GR atmospheric model was initialized with analysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The resulting numerical products with spatial resolution of 0.02° (∼ 2 km) over the geographical domain of Greece depicted very well the severe convective conditions close to tornadoes formation. The Energy Helicity Index (EHI) diagnostic variable in both numerical simulations showed a gradual increase of values closing to the location and time of the tornadogenesis. Similar to EHI, the storm relative helicity (SRH) spatio-temporal analysis followed a gradual increase prior to the tornadogenesis events and was reduced after them.

Towards the improvement of the mesoscale modeling for offshore wind application, the real time observational nudging capability of the Weather Research and Forecasting (WRF) model has been implemented aiming for enhanced model performance. Utilizing three different horizontal levels of the offshore meteorological mast, FINO3, in the North Sea, wind speed observations were integrated into the model core. The performance of this modified model was then assessed for three different atmospheric stability conditions. Results from this study, illustrate that for all three stratification cases, there is a significant improvement in model performance when using observational nudging showing a reduction in Root Mean Square Error of up to 27% when compared to the observations from FINO1 platform. This study suggests that observational nudging takes a step towards more accurate simulations in wind resource assessment (WRA).

Extreme weather phenomena affect human societies, and especially thermal stress is a considerable threat. Therefore, the goal of this study is twofold: On one hand, to assess the frequencies of classes of the Universal Thermal Climate Index (UTCI) in Iran. The findings showed that a total of 83.7% of the stations have experienced UTCI classes of very strong (VSHS) or even extreme (EHS) heat stress indicating that Iran faces more VSHS and EHS events against cold stress events. On the other hand, in the process, first order Markov chain model was applied to estimate the probability of extreme heat stress conditions. One of the most important findings of this section was that 63.8% of the stations experienced VSHS conditions in the long term that maximum frequency of the probability of this condition appeared in the zone along the coastal strip of the Persian Gulf and Sea of Oman against to the rest of Iran, it is suggested that early warning system for heat stress should be designed, providing alerts on time to eliminate casualties and strengthen the resilience of the local population.

Potential evapotranspiration (PET) is a key parameter for climate classification and aridity assessment. The widely used UNEP (1992) classification system is based on the estimation of the aridity index AI, which requires annual average values of precipitation P and PET. For the calculation of PET the Thornthwaite's (1948) formula is, in principle, suggested. Recent studies use more advanced and accurate methods for PET estimation but apply to AI the same thresholds proposed by UNEP for aridity classification. This work deals with the uncertainties introduced by the use of different PET methods in the estimation of the aridity index AI. Specifically, the Hargreaves-Samani (HS) method and four of its modifications, three modifications of Thornthwaite's formula and the equation of Hamon, are evaluated against the widely used Thornthwaite's original method, by assessing their impact on the AI. Climatic data as monthly average values of at least 30 years of measurements from 122 stations in the Greek peninsula are used. Results show that AI is highly affected by the PET method adopted, resulting thus to changes in climatic classification of a region. Further, results imply the need for an adjustment of the threshold values that determine aridity classes according to the method each time adopted. Therefore, new threshold values for the aridity classes are developed and presented in this work. The proposed threshold values cover a range of sites that belong to semi-arid SA, sub-humid SH and humid H aridity classes of the Greek peninsula.

The thermal comfort conditions in a complex urban area is influenced by the surrounding structures and obstacles which modify the incoming radiation fluxes. A measure of this modification is the sk...

This Special Issue of the Natural Hazards and Earth System Sciences (NHESS) entitled “Advances in Meteorological Hazards and Extreme Events” hosts fifteen (15) selected papers presented at the 11th International Conference on Meteorology, Climatology and Atmospheric Physics – COMECAP 2012 – Athens, 29 May–1 June 2012. COMECAP 2012 covered various topics related to the science of the Atmospheric Environment, which is in fact an interdisciplinary field, giving the opportunity to understand the physical systems and environmental processes in an integrated manner. The featured papers shed light to advances and current trends in the considered meteorological hazards and extreme events, such as tornadoes, heat waves and extreme temperature indices, droughts, floods, convective precipitation, landslides, medicanes (Mediterranean tropical like cyclones) and wildfires, using recorded datasets, model simulations and innovative methodologies. Hazard (or cause) may be defined as a potential threat to humans and their welfare and risk (or consequence) as the probability of a hazard occurring and creating loss. Unlike hazard and risk, a disaster is an actual happening, rather than a potential threat, thus, a disaster may be defined as the realization of hazard. The term environmental hazard has the advantage of including a wide variety of hazard types ranging from natural (geophysical) events, through technological (man-made) events to social (human behaviour) events. Meteorological hazards and extremes constitute natural environmental hazards caused by atmospheric disturbances. Disaster risk arises when hazards interact with physical, social, economic and environmental vulnerabilities. The impact of disaster can be transferred from one region to another. This, compounded by increasing vulnerability related to several factors, such as population growth, land pressure, urbanization, social inequality, climate change, political change, economic growth, technological innovation, social expectations, global interdependence, environmental degradation, competition for scarce resources and the impact of epidemics, points to a future where disasters could increasingly threaten, among others, the sustainable development of agricultural regions (Smith, 2013). Sustainable development, socio-economic improvement, good governance, and disaster risk reduction are mutually supportive objectives. Recent research findings suggest that variability of climate, if encompassing more intense and frequent extremes, such as major large-scale environmental hazards like droughts, heatwaves or floods, results in the occurrence of natural disasters that are beyond our socio-economic planning levels. This is expected to stretch regional response capabilities beyond their capacity and will require new adaptation and preparedness strategies (Salinger et al., 2005). Disaster prevention and preparedness should become a priority and rapid response capacities to climate change need to be accompanied by a strategy for disaster prevention. Nevertheless, each type of extreme events has its own particular climate, cultural and environmental setting, and mitigation activities must use these settings as a foundation of proactive management. There is an urgent need to assess the forecasting skills for natural disasters affecting mainly agriculture and other sectors of the economy in order to determine those where greater research is necessary. It is well known that lack of good forecast skill is a constraint to improve adaptation, management and mitigation. Seasonal to interannual climate forecasting is a new branch of climate science, which promises reducing vulnerability. Improved seasonal forecasts are now being linked to decision making for cropping. The application of climate knowledge to the improvement of risk management is expected to increase the resilience of farming systems. A more integrated approach to environmental hazards has been gradually attempted using common methodologies

The goal of this study is the assessment of the reliability of the V7 TRMM (Tropical Rainfall Measuring Mission) 3B43 gridded precipitation estimates over Greece. The validation was carried out using gridded monthly precipitation data derived from 96 meteorological stations over the wider region of Greece. The TRMM 3B43 gridded estimates are on a calendar month temporal resolution and 0.25° × 0.25° spatial resolution concerning the 11-year period 1998–2008. In order to compare directly the TRMM 3B43 products with the rain gauge data, the latter were interpolated at the same spatial resolution grid with that of TRMM 3B43 (0.25° × 0.25°) and a new Greek gridded precipitation database (GGPDB) was constructed, using Kriging spatial interpolation method, taking into account the altitude of the meteorological stations. Further, to ensure reliable ground truth, we performed the comparison using data only over the 0.25° grid box when/where at least one reporting gauge is available. The annual and seasonal spatial distribution of the differences between the two datasets along with the spatial distribution of the correlation coefficients are presented and analyzed. The findings of the analysis revealed that the correlations between the two examined gridded databases (satellite and ground based) in winter were very high (r > 0.90) for the entire Greek domain with small exceptions over sparse data continental mountainous regions and restricted areas over north and south Aegean Sea. Regarding the other seasons, small correlations occur in the northern high altitude areas (Olympus Mt.) and the central/south mountainous Peloponnese. Besides, it is worth noting the remarkable small correlations over the eastern Aegean Sea and the Cyclades complex, in summer and the Cyclades complex in autumn. The TRMM 3B43 gridded data overestimate the precipitation over the Aegean and Ionian Sea, the coastal areas of Asia Minor and western Greece, in winter (> 50 mm), spring (> 25 mm) and autumn (> 60 mm). On the contrary, underestimation is demonstrated on central continental highlands (Pindus mountain range) and central mountainous Peloponnese in winter (> 150 mm), spring (25–100 mm) and autumn (60–90 mm). Summer is generally characterized by small irregularities. Generally, the findings of the performed validation indicate an underestimation at higher precipitation appeared over high altitude areas against some overestimation at lower precipitation over plains and coastal regions. The TRMM 3B43 products performance was assessed by using three contingency statistics for probability of detection (POD), false alarm ratio (FAR) and critical success index (CSI). TRMM 3B43 has POD > 80% and CSI > 60% when the monthly precipitation is lower than 84.0 mm. Moreover, the smallest POD of 44% and CSI of 34% appear when the monthly precipitation is greater than 191.5 mm. This result implies that satellite products have poor performance in estimating high monthly precipitation totals, over the Greek region.

Recent research revealed that western Greece and NW Peloponnese are regions that favor prefrontal tornadic incidence. On March 25, 2009 a tornado developed approximately at 10:30 UTC near Varda village (NW Peloponnese). Tornado intensity was T4–T5 (TORRO scale) and consequently caused an economic impact of 350,000 € over the local society. The goals of this study are: (i) to analyze synoptic and remote sensing features regarding the tornado event over NW Peloponnese and (ii) to investigate the role of topography in tornadogenesis triggered under strong synoptic scale forcing over that area. Synoptic analysis was based on the European Centre for Medium-Range Weather Forecasts (ECMWF) data sets. The analysis of daily anomaly of synoptic conditions with respect to 30 years' climatology (1981–2010), was based on the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data sets. In addition, numerous remote sensing data sets were derived by the Hellenic National Meteorological Service (HNMS) weather station network in order to better interpret the examined tornado event. Finally, numerical modeling was performed using the non-hydrostatic Weather Research and Forecasting model (WRF), initialized by ECMWF gridded analyses, with telescoping nested grids that allow the representation of atmospheric circulations ranging from the synoptic scale down to the meso-scale. The two numerical experiments were performed on the basis of: (a) the presence and (b) the absence of topography (landscape), so as to determine whether the occurrence of a tornado – identified by diagnostic instability indices – could be indicated by modifying topography. The energy helicity index (EHI), the bulk Richardson number (BRN) shear, the storm-relative environmental helicity (SRH), and the maximum convective available potential energy (MCAPE, for parcels with maximum θ e ) were considered as principal diagnostic instability variables and employed in both numerical experiments. Furthermore, model verification was conducted, accompanied by analysis of the absolute vorticity budget. Synoptic analysis revealed that the synoptic weather conditions on March 25, 2009 are in agreement with the composite synoptic climatology for tornado days over western Greece. In addition, maximum daily anomalies at the barometric levels of 500, 700, 850 and 925 hPa were found, compared to the climatology of composite mean anomalies for tornado days over western Greece. Numerical simulations revealed that the topography of NW Peloponnese did not constitute an important factor during the tornado event on March 25, 2009, based on EHI, SRH, BRN, and MCAPE analyses.

Cloud-to-ground (CG) lightning activity recorded by the Hellenic National Meteorological Service (HNMS) Precision Lightning Network (PLN) is analysed over the wider area of Greece. In addition, the spatial and temporal relationships between TRMM 3B42 (Tropical Rainfall Measuring Mission) datasets and lightning are presented. The analyses concern the period from January 14, 2008 to December 31, 2012. The Laboratory of Climatology and Atmospheric Environment, University of Athens, has established a detailed dataset of lightning impacts over Greece from 1895 to 2013, based on digitized archive editions of newspapers. The mean seasonal variability of CG lightning activity revealed autumn as the most dominant season with 303 LD, while the mean monthly variability of CG indicated October as the most lightning active month and May as the month with a mean of 27 LD. The mean annual spatial distribution of CG lightning per km 2 , depicted the maximum frequency over Pindus mountain range (> 7 CG/km 2 ). During the autumn season, the northern Ionian Sea experienced a mean frequency of more than 5 CG/km 2 , compared to the southern Ionian Sea and NW Peloponnesus, where values of more than 7 CG/km 2 are depicted. During the summer season, the maximum frequency appeared along Pindus mountain range, around Attica, Thessaly and central Macedonia highlands. The spatial distribution of seasonal correlations between the number of CG flashes/day and gridded (TRMM 3B42) daily rainfall totals for the period 2008–2012 over Greece, indicated that correlations were mainly positive all over the under study area, within all seasons, and especially during summer and autumn. Regarding the lightning impacts in Greece, based on the 1895–2013 study period, more than 343 fatalities and at least 224 injured people have been recorded. The spatial analysis of lightning impacts, showed that the majority of events has been recorded over Greek mainland and only few scattered events have been reported over Ionian and Aegean Seas. The results of the performed research for Greece, during 1895–2013 (2000–2013), indicated that fatalities/injuries caused by lightning, were estimated at 2.9 (2) deaths/1.9 (1.6) injuries per year, respectively.