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On 1st and 2nd February 2015, the central and eastern Mediterranean, as well as parts of the Black Sea, were affected by an extreme desert dust outbreak. This exceptional dust event took place under the prevalence of a strong southwesterly airflow over the region, with maximum wind speeds up to 70 (36 ms−1) and 35 knots (18 ms−1) at 700 hPa and at mean sea-level pressure, respectively. Based on MODIS-Terra/Aqua aerosol optical depth (AOD) retrievals, the intensity of the transported dust loads maximized over the central Mediterranean (4.2) while very high AODs (up to 2.5) were recorded over Greece and the Black Sea. In addition, according to in-situ measurements obtained at Finokalia (Crete, southern Greece), the maximum PM10 concentrations reached up to 758 μgr m−3 (maximum levels for 2015). Through the implementation of the WRF-Chem model, the clear-sky direct radiative effects (DREs) have been computed for the SW, LW and NET (SW + LW) radiation, at the top of the atmosphere (TOA), within the atmosphere (ATM) as well as for the downwelling (SURF) and the absorbed (NETSURF) radiation at the ground. According to our simulations, during daytime, the instantaneous NET DREs reach down to −278.1 Wm−2 (Solar Zenith Angle (SZA) = 53°) and − 252.2 Wm−2 for SURF and NETSURF, respectively, indicating a strong surface cooling, while the corresponding values for ATM can be as large as 147.3 Wm−2 (SZA = 50°) revealing a strong atmospheric warming. At TOA, the computed DREs vary from −120.1 Wm−2 (planetary cooling; SZA = 51°) to 59.4 Wm−2 (planetary warming; SZA = 64°) being negative and positive over dark and bright surfaces, respectively. During nighttime, reverse effects of lower magnitude are found at all levels of the Earth-Atmosphere system. At a regional scale, throughout the forecast period, the clear-sky NET DREs range from −16.60 Wm−2 to −0.76 Wm−2, from −46.18 Wm−2 to 16.11 Wm−2, from −32.31 Wm−2 to 13.70 Wm−2 and from −14.63 Wm−2 to 20.40 Wm−2 for TOA, SURF, NETSURF and ATM, respectively. As a response to the perturbation of the surface radiation budget, the 2 m air temperature decreases/increases by up to 1.5 °C (5.3 °C) during daytime/nighttime. Moreover, at noon, the low-level dust layer (up to 3 km above sea level), cools (heats) the lowest troposphere by up to 0.5 °C (1.4 °C) over land (sea). At night, the temperature of the atmospheric layers where the dust aerosols are confined decreases (by up to −1.8 °C), whereas a warming effect (by up to 2.8 °C) is evident in the air masses beneath the dust layer. Through the inclusion of dust-radiation interactions in the numerical simulations, the model's predictive skill in terms of reproducing the downwelling SW radiation at the ground is improved. This is justified via the comparison of the RADON (dust-radiation interactions are activated) and RADOFF (dust-radiation interactions are deactivated) outputs against cloud-free observations derived by 31 NOAAN stations. More specifically, from RADOFF to RADON, the bias reduces from 157.63 to 26.02 Wm−2 and the slope decreases from 1.25 to 0.98. On the contrary, the evaluation analysis of the temperature at 2 m does not reveal a remarkably better performance of the RADON run, particularly during sunlight hours, reflecting thus the predominance of first order model errors over the expected improvements attributed to dust-radiation interactions.

The Price-Rind 92 parameterization (Price and Rind, 1992) that utilizes cloud top height as predictor for the estimation of lightning density is widely used by modelers in an attempt to forecast electrical activity in thunderstorms. In the present paper new parameterizations for the estimation of lightning density of convective clouds are formulated. LINET lightning data, NWC SAF (Satellite Application Facility on support to Nowcasting and Very Short-Range Forecasting) products and ERA-Interim (ECMWF-Re Analysis) data, covering the summer of 2016 over continental Europe, are used. The proposed models estimate the lightning density of convective clouds, using cloud top height, cloud top pressure and cold cloud depth as predictors. Model efficiency statistics calculated over an independent dataset, suggest that the proposed models can be considered successful over the specific area (continental Europe) and period (summer). The new cloud top height model differs from the PR92 and other parameterizations, which is not an unexpected result, since every model has its own characteristics, strengths and weaknesses. The new parameterizations could be utilized in numerical model simulations to produce quantitative estimations of the amount of strokes over convective areas.

The Mediterranean region is a unique area characterized by a large spectrum of atmospheric phenomena, some of which have a high impact on many aspects of human activities, safety and wellbeing. The area is long considered as a hot spot of such atmospheric phenomena deserving multidisciplinary scientific attention. The scientific research that has been carried out on these high impact atmospheric processes that occur in the Mediterranean area is indeed widespread and the available international literature is very extensive. The paper touches initially the temperature and precipitation regimes, followed by a discussion of floods and droughts. The exciting cyclogenetic patterns of explosive cyclones and medicanes are presented in separate sections. The lightning activity and the presence of dust and other pollutants are also presented herein. The atmospheric chemistry of the region which is increasingly becoming of utmost importance for the area under study is distinctly discussed. Attempts to modify the weather (the precipitation, in particular) are outlined too. The effects of climatic change on various atmospheric processes are considered throughout this paper, in addition to a dedicated section on temperature and precipitation.

The spatio-temporal regime of lightning strokes over the broader Greek area is studied using 15-day ZEUS cloud-to-ground lightning data for a 10-year period (2005–14). The multivariate statistical method Factor Analysis (S-mode and T-mode) is applied on the mean intra-annual variation of lightning in order to reveal: i) the main modes of intra-annual variation of lighting activity that correspond to specific sub-regions of the Greek area and ii) the main characteristic patterns of strokes prevailing during specific periods of the year (“seasons”). According to the results there are three main modes of intra-annual variation: a) the continental one with maximum lightning activity in early summer, b) the Ionian Sea one with maximum in early autumn and c) the Aegean Sea one with maxima in middle autumn and late May. Also, according to the patterns of strokes, three main “seasons” are found: a) the summer one (early May–middle August) with highest lightning activity over the continental regions of the country due to the atmospheric instability associated with the intense land heating and the persistence of cool upper air masses, b) the winter one (middle December–middle February) with the maximum of lightning activity over the sea where instability is high due to the frequent prevalence of cold air masses over the warm sea-surface and the associated passages of Mediterranean depressions and c) the autumn one (early September–middle October) with maximum over northwestern Greece associated with the early cold season low pressure systems, the warm sea body of Ionian Sea and the essential role of topography.

Mediterranean thunderstorms climatology is analyzed for a 10-year period from 2005 to 2014. For this analysis, we applied a clustering method to the cloud-to-ground lightning stroke dataset, derived from the ZEUS detection system, operated by the National Observatory of Athens. Results showed that the annual days of thunderstorm occurrence are dependent on the diurnal cycle of insolation and on the underlying topographic features. Therefore, the majority of thunderstorms occur over land in spring and summer, while winter and autumn thunderstorms tend to occur over the sea. On average, winter thunderstorms are less frequent and of lower intensity than the ones of summer, but last longer, tend to attain larger sizes and are associated with significantly higher velocity. Regardless the season, all thunderstorms tend to propagate towards the east, while their speed is higher in winter and autumn. In particular, for thunderstorms occurring over land, we found that the higher the duration of the lifetime of a thunderstorm, the earlier in the day it is expected to occur. Mediterranean thunderstorms size and duration have been found to be correlated, while a linear relation was determined between thunderstorms intensity and rainfall over the Mediterranean region, where the rain yield was estimated to be about 1.8 108 kg of rainfall per CG lightning.

An intense and fast moving convective line that crossed Massif Central/Cevennes-Vivarais area (south France) during the field campaign of Hydrological Cycle in Mediterranean Experiment (HyMeX) — Special Observing Period 1 (SOP1) is examined. The mesoscale analysis demonstrates a complex convective system with a V-shape in the Infrared (IR) satellite imagery and a squall line pattern on the radar imagery. Ground stations observed up to 60 mm h− 1 of rain accumulation, while the lightning activity, as observed by 4 detection networks, was also exceptionally high. The Weather Research and Forecasting (WRF) model was used to simulate this convective episode and sensitivity tests were performed with various microphysics and convective parameterization schemes. Satellite data from Meteosat SEVIRI Rapid Scanning Service were used in conjunction with radar, lightning and rain gauge data to conclude on the best simulation for which WRF model exhibits a rather precise and realistic distribution and evolution of the precipitation patterns. Finally, a study of the microphysics was performed indicating the interconnection of graupel with lightning activity, confirming recent results, compared against a sophisticated hydrometeor classification radar algorithm and lightning data.

The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite has been utilized for the first time in a study regarding lightning activity modulation due to aerosols. Lightning activity observations, obtained by the ZEUS long range Lightning Detection Network, European Centre for Medium range Weather Forecasts (ECMWF) Convective Available Potential Energy (CAPE) data and Cloud Fraction (CF) retrieved by MODIS on board Aqua satellite have been combined with CALIPSO CALIOP data over the Mediterranean basin and for the period March to November, from 2007 to 2014. The results indicate that lightning activity is enhanced during days characterized by higher Aerosol Optical Depth (AOD) values, compared to days with no lightning. This study contributes to existing studies on the link between lightning activity and aerosols, which have been based just on columnar AOD satellite retrievals, by performing a deeper analysis into the effect of aerosol profiles and aerosol types. Correlation coefficients of R = 0.73 between the CALIPSO AOD and the number of lightning strikes detected by ZEUS and of R = 0.93 between ECMWF CAPE and lightning activity are obtained. The analysis of extinction coefficient values at 532 nm indicates that at an altitudinal range exists, between 1.1 km and 2.9 km, where the values for extinction coefficient of lightning-active and non-lightning-active cases are statistically significantly different. Finally, based on the CALIPSO aerosol subtype classification, we have investigated the aerosol conditions of lightning-active and non-lightning-active cases. According to the results polluted dust aerosols are more frequently observed during non-lightning-active days, while dust and smoke aerosols are more abundant in the atmosphere during the lightning-active days.

The Axios Valley, located in central-northern Greece, is surrounded by complex topography that plays a significant role in the modification of wind flow, both in terms of speed and direction. The characteristic wind regime of this valley is Vardaris, a northwesterly wind that prevails in this region, especially during the cold period of the year. Vardaris is well known for its consistent direction and high intensity, as well as for the effective advection of cold and dry air, often resulting to significant damages in local infrastructures and agriculture. A field campaign under the name AXIOS took place during the period from November 2007 through May 2008 in order to examine this particular wind flow. The analysis of the in situ observational data, which was funded by the research program THESPIA-KRIPIS, showed that topography plays a key role in intensifying Vardaris, generating gusts that approximated 30 m s− 1 during the experimental period. The air temperature and humidity fields were also found to be significantly influenced. In addition to the observational study, an intense Vardaris episode was simulated with the Weather Research and Forecasting (WRF) model at high horizontal resolution. Results revealed that the model was able to reproduce the favorable environmental conditions that lead to Vardaris occurrence, providing a useful insight on the physical mechanisms explaining its structure.

In the framework of this study, the effect of aerosols on lightning activity has been investigated for the first time over the broader Mediterranean Sea. Atmospheric optical depth data retrieved by MODIS on board Aqua satellite and cloud to ground lightning activity data provided by ZEUS network operated by the National Observatory of Athens were analyzed for a time period spanning from 01/01/2005 up to 31/12/2013. The results indicate the importance of aerosols in lightning modulation. The mean aerosol optical depth (AOD) values of the days with lightning activity were found to be higher than the mean seasonal AOD in 90% of the under study domain. Furthermore, the increasing rate of lightning activity with increasing aerosol loading was found to be more pronounced during summertime and for AOD values up to 0.4. Additionally, the spatial analysis showed that the percentage of days with lightning activity during summertime is increasing with increasing AOD. Finally, time series showed similar temporal behavior between AOD seasonal anomalies and days with lightning activity differences. Both the spatial and temporal analysis showed that lightning activity is correlated to AOD, a characteristic consistent for all seasons.

The development and efficiency assessment of a lightning activity nowcasting tool is presented. The tool employs MSG IR imagery and real-time lightning data provided by ZEUS network to nowcast the manifestation of lightning activity over the Greek mainland for a time span of 1 h. The efficiency of the tool is assessed for 20 days with widespread lightning activity observed during the warm period of the year through a verification procedure that computes a collection of appropriate statistics for selected areas. The analysis of these statistics shows that the tool estimates successfully almost 80% of the upcoming activity. The false alarm rate is close to 40%, while a small overestimation is evident. Since the adverse effects of a case of missed activity are much more than that of a false alarm, the tool is considered successful and fit for operational use.

Lightning activity is analysed in a sector covering the Eastern Mediterranean (16–32°E, 34–46°N) for the years 2005–2014. The study is based on the use of cloud-to-ground (CG) lightning activity data retrieved from ZEUS system, operated by the National Observatory of Athens, and examines the seasonal, diurnal and spatial variability of the lightning activity. The effect of elevation, terrain slope, vegetation cover and convective available potential energy (CAPE) on the distribution of the CG lightning strokes is also investigated. Lightning is modulated by the diurnal cycle of insolation and the underlying topographic features of the region. The lightning strokes are more numerous over the mainland than over the sea. CG lightning activity is dominant over the land and the coastal areas during spring and summer while during the coldest period of the year it is dominant over the sea. The maximum value of lightning activity is observed in June and mostly in the afternoon. The orography and the terrain slope affect the distribution of lightning. During the warmest period of the year, the forested areas have an increased “lightning yield”. The number of CG lightning strokes increases with increasing CAPE and the high correlation between them indicates that CAPE values could be used as a proxy for the presence of lightning activity, at least over the eastern Mediterranean region.

In recent years, significant progress has been made in the development and implementation of parameterizations for the prediction of lightning. In the present study, the commonly used Price and Rind lightning parameterization is evaluated. This parameterization has been recently introduced in the state-of-the-art Weather Research and Forecasting (WRF) model, allowing for the online simulation of lightning activity. The evaluation of the parameterization is conducted for ten different single-day events that took place in Greece during the period of years from 2010 to 2013. Results show that the WRF model could be used for real-time lightning prediction applications, given that the lightning parameterization is properly adapted. In particular, the analysis revealed that model-resolved variables related to the microphysics and thermodynamics are necessary for controlling the parameterization of lightning, which otherwise results to significant overprediction. The total ice content, the maximum vertical velocity and the convective available potential energy were found to be the storm parameters that, when combined together, improve the ability of the model to correctly predict lightning, significantly restricting false alarms. This was further highlighted by separately examining two example case studies, for which the numerical simulations successfully reproduced the spatial and temporal characteristics of lightning activity.

In this work the impact of topography on the distribution and the amount of precipitation in Epirus, NW Greece is investigated, during the cold period of the year. A precipitation event with typical wintertime characteristics is considered. In the selected case, extended and heavy precipitation over Epirus is caused by a low pressure system centred northwest of Epirus and moving southeastwards, generating southerly winds over the Ionian Sea. The case is simulated by applying the numerical meteorological model MM5 in a high resolution grid (2 × 2km). Various sensitivity tests are performed, searching for the model initialization time and the convective parameterization scheme (CPS) leading to the best simulation of the observed event. Then, the model is applied again incorporating modified data of topography, in order to study the differentiations in the fields of the most important meteorological parameters, which are directly related with precipitation. The topography is modified with two ways: either by reducing the height of the mountains (type RT) or by elevating the terrain height (type ET). The results show that in the RT type, the precipitation field appears more homogeneous, with weaker maxima on the windward sides and above the mountain ridges. As a consequence, less rain falls in these areas and humid air passes easier to the lee sides reducing rain shadows. In the ET type, rain increases in the windward coastal zone of Epirus while it is reduced in the inland areas where less humid air is available. Generally, the modification of topography changes the flows near the surface, causing a displacement of the convergence zones and thus a new distribution of vertical velocities, which, combined with the new water vapour field, has a direct impact on precipitation.

The lightning activity and precipitation at 3-hour time intervals in grid boxes of 0.25 × 0.25° over East and Central Mediterranean during the summer of 2005 and 2006 are analyzed. The results show that i) the peak of flash density over the continental and coastal areas is in the afternoon hours, while over the maritime areas the peak is in the morning hours; ii) the number of cases with 3-hour accumulated precipitation when lightning flashes are detected is smaller than the number of no-flash cases over all three types of areas — maritime, coastal and continental; iii) the frequency distribution of the precipitation amount is shifted towards larger values for the cases with flashes as compared to the cases without flashes. The analysis also reveals that the average precipitation amount per case is larger when lightning occurred than when it was absent. However the area-average precipitation amount is not always larger for the cases with flashes than the no-flash cases: It depends on the type of the surface (maritime, coastal and continental) and on the period of the day: morning or afternoon. Area-average precipitation amount is smaller when lightning occurred than when it was absent for the afternoon hours over the maritime area and for the morning hours over the continental area. High correlation coefficients R (between 0.70 and 0.98) are found between rain rate and average flash density when flash density is averaged in logarithmic bins of rain-rate.

In the frame of this paper a heat wave that resulted in record high temperatures in Athens Greece is analysed. Namely the analysis focuses in the ability of three widely used planetary boundary layer parameterisations to reproduce the heat wave temperatures. The simulations were performed with the Pennsylvania State University – National Center for Atmospheric Research MM5 model that is also used for operational weather forecasting at the National Observatory of Athens since 2002. The 2-m temperature at a grid increment of 1-km over the highly complex terrain of the Athens Area is statistically verified against the available surface station observations. The results of the analysis showed that the two nonlocal schemes, namely Blackadar and MRF succeeded much better than the local scheme ETA to reproduce the heat wave 2-m temperature although they considerably underestimated the maximum observed temperatures. In addition, it was found that the model grid points characterised by “urban” land-use provided better statistical verification results, with reduced cold bias. Further, in order to study the role of the initial skin temperature conditions on the 2-m temperature forecasts, satellite observed skin temperatures have been used to initialise the high resolution simulations. This sensitivity test showed that when using the satellite observed skin temperature in the model initial conditions, the simulation of the 2-m temperature is positively affected during the early stages of the simulation while later on the model physical parameterisations are decisive for the time evolution of temperature.

A combination of satellite imagery and meteorological data were used to explore the spatial and temporal evolution of a heat wave that resulted in record high temperatures in Athens Greece. This work analyses Land Surface Temperature (LST) generated by the Satellite Application Facility Surface Analysis (Land SAF) product and Moderate Resolution Imaging Spectroradiometer (MODIS) data onboard TERRA and AQUA satellites. The temporal evolution of the heat wave was considered using MODIS LST data, while the surface temperature anomaly was also estimated. MODIS and Land SAF data were verified against the surface air temperature over the Athens area obtained from a network of available surface observation stations, showing significant correlation. Intercomparison of MODIS and Land SAF LST products was considered at common satellite overpasses; a significant correlation was derived. On average the observed daytime differences vary between 2.5 to 5.8 degrees. Finally, Land SAF data were also tested against soil temperature data from a surface observation station located in the centre of Athens. The results extracted depict the role of using a variety of remote sensed and meteorological data for studying such events.

The sensitivity of numerical model quantitative precipitation forecasts to the choice of the convective parameterization scheme (CPS) is examined for twenty selected cases characterized by intense convective activity and widespread precipitation over Greece, during the warm period of 2005–2007. Namely, the study is conducted using MM5 model and the following three different CPSs: Kain–Fritsch, Grell and Betts–Miller–Janjic. Sixty numerical simulations were carried out on two nested domains, with horizontal grid increments of 24 and 8 km respectively. The simulated precipitation from the 8-km grid was verified against raingauge measurements and lightning data provided by the ZEUS long-range lightning detection system, operated by the National Observatory of Athens. Verification results showed that for all three schemes the model presented a tendency to overestimate light to moderate rain while in general it underestimated the high precipitation amounts. The validation against both sources of data showed that among the three CPSs, the more consistent behavior in quantitative precipitation forecasting was obtained by the Kain–Fritsch scheme that provided the best statistical scores. However, the differences of the results of statistical analysis between the Kain–Fritsch and Grell schemes were not large.

The lightning activity and precipitation in two 3-hour time intervals in the grid boxes of 0.25 × 0.25° over East and Central Mediterranean during the summer of 2005 and 2006 are analysed. The results show that the frequency distribution of the precipitation amount is shifted towards larger values for the cases with lightning as compared with the cases without lightning. It was found that the number of cases with 3-hour accumulated rainfall greater than 10 mm was bigger when lightning occurred (65%) than when it was absent (35%). Investigation of diurnal and spatial distributions of lightning shows that the afternoon flash density peak is associated mainly with lightning over the land, which is in accordance with the results of earlier works. The early morning flash density peak is associated mainly with flashes over the sea. High correlation coefficients (0.89 during the morning hours and 0.98 during afternoon) were found between rain rate (mm/h) and average flash density (fl/km 2 ) when flash density is averaged in logarithmic intervals of rain rate.

In the frame of this study, the rain and lightning activity produced by rainfall events in the central and eastern Mediterranean has been studied. The analysis focuses on the study of the rainfall events that occurred during a period of 6 months spanning from October 2003 up to March 2004. The data used consist of 6-h accumulated precipitation amounts reported by an average number of 220 rain gauges of the surface network and the lightning strokes detected by the ATD system operated by the UK Meteorological Office. It was found that for almost 50% of the cases when rain was recorded but in absence of lightnings, the 6-h accumulated precipitation was smaller or equal to 1 mm. This percentage drops to about 15% for the cases in presence of lightnings. The distribution of 6-h accumulated precipitation without lightnings presents a clear maximum at 1 mm while the distribution in presence of lightnings presents a clear maximum at 10 mm and an important swift towards larger precipitation amounts.