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In this work we present downscaling experiments with the Weather Research and Forecasting model (WRF) to test the sensitivity to resolving aerosol–radiation and aerosol–cloud interactions on simulated regional climate for the EURO-CORDEX domain. The sensitivities mainly focus on the aerosol–radiation interactions (direct and semi-direct effects) with four different aerosol optical depth datasets (Tegen, MAC-v1, MACC, GOCART) being used and changes to the aerosol absorptivity (single scattering albedo) being examined. Moreover, part of the sensitivities also investigates aerosol–cloud interactions (indirect effect). Simulations have a resolution of 0.44∘ and are forced by the ERA-Interim reanalysis. A basic evaluation is performed in the context of seasonal-mean comparisons to ground-based (E-OBS) and satellite-based (CM SAF SARAH, CLARA) benchmark observational datasets. The impact of aerosols is calculated by comparing it against a simulation that has no aerosol effects. The implementation of aerosol–radiation interactions reduces the direct component of the incoming surface solar radiation by 20 %–30 % in all seasons, due to enhanced aerosol scattering and absorption. Moreover the aerosol–radiation interactions increase the diffuse component of surface solar radiation in both summer (30 %–40 %) and winter (5 %–8 %), whereas the overall downward solar radiation at the surface is attenuated by 3 %–8 %. The resulting aerosol radiative effect is negative and is comprised of the net effect from the combination of the highly negative direct aerosol effect (−17 to −5 W m−2) and the small positive changes in the cloud radiative effect (+5 W m−2), attributed to the semi-direct effect. The aerosol radiative effect is also stronger in summer (−12 W m−2) than in winter (−2 W m−2). We also show that modelling aerosol–radiation and aerosol–cloud interactions can lead to small changes in cloudiness, mainly regarding low-level clouds, and circulation anomalies in the lower and mid-troposphere, which in some cases, mainly close to the Black Sea in autumn, can be of statistical significance. Precipitation is not affected in a consistent pattern throughout the year by the aerosol implementation, and changes do not exceed ±5 % except for the case of unrealistically absorbing aerosol. Temperature, on the other hand, systematically decreases by −0.1 to −0.5 ∘C due to aerosol–radiation interactions with regional changes that can be up to −1.5 ∘C.

This study explores the direct and semi-direct radiative effect of North African dust in the present and future climate using the regional climate model RegCM4. The simulations cover a historical decade extending from December 1999 to November 2009 and a future decade that spans from December 2089 to November 2099 under the Representative Concentration Pathway 4.5 (RCP4.5), without considering land-cover/land-use changes. For each time-slice a set of two experiments was conducted, namely the “Control”, in which dust is radiatively inactive and the “Feedback”, in which dust interacts with shortwave and longwave radiation. The impact of North African dust on the regional radiative balance is assessed by comparing the “Feedback” and the “Control” experiments during the historical period. The results indicate that the combined effect of dust Direct + Semi-direct Radiative Effect (DSRE) on the shortwave is − 13.8 W m−2 and − 10.7 W m−2 over the Sahel and the Sahara, respectively. The Direct Radiative Effect (DRE) dominates over the Semi-direct Radiative Effect (SRE) in both winter and summer, although during summer over some parts of the desert the SRE in the longwave spectrum accounts for almost 50% of the DSRE. Part of this is due to a noteworthy statistically significant increase of clouds that reaches values up to 3% and stretches across the eastern and western Sahara desert. Dust DSRE intensifies moderately in the future period (− 15.8 W m−2 and − 11.0 W m−2), while its spatial distribution remains the same, suggesting that the effect of climate change in the atmosphere will not alter the radiative effect of dust over North Africa considerably. When taking into account the dust radiative feedback in regional climate simulations the maximum temperature is altered by − 0.2/− 0.2 °C and − 0.3/− 0.6 °C over the Sahel and Sahara regions, respectively, during the summer/winter period, mainly as a result of changes in the shortwave radiative balance. On the contrary, the minimum temperature increases, since it is mostly controlled by the longwave radiation emitted from the Earth’s surface. In the future period the near surface air temperature increases by 1.5–2.5 °C and the fine dust column burden increases by + 4% to + 8% in comparison to the historical period, mainly due to the RCP4.5 forcing. When the dust feedback on climate is active in future simulations it can decrease the summer daily maximum temperature by 0.3 °C over Sahel, and decrease or increase it locally in Sahara by up to 0.2 °C. Prior to the Feedback-Control analysis an extensive evaluation has been conducted for dust optical depth, dust extinction, near surface air temperature and cloud fraction cover using the LIVAS, CRU and CM SAF datasets.

This study investigates an intense precipitation event and its lightning activity that affected northern Greece and primarily Thessaloniki on 15 July 2014. The precipitation measurement of 98.5 mm in 15 h at the Aristotle University of Thessaloniki set a new absolute record maximum. The thermodynamic analysis indicated that the event took place in an environment that could support deep thunderstorm activity. The development of this intense event was associated with significant low-level convergence and upper-level divergence even before its triggering and a positive vertical gradient of relative vorticity advection. The high resolution (1.667 km × 1.667 km) non-hydrostatic WRF-ARW numerical weather prediction model was used to simulate this intense precipitation event, while the Lightning Potential Index was utilized to calculate the potential for lightning activity. Sensitivity experiments suggested that although the strong synoptic forcing assumed primary role in the occurrence of intense precipitation and lightning activity, their spatiotemporal variability was affected by topography. The application of the very fine resolution topography of NASA Shuttle Radar Topographic Mission improved the simulated precipitation and the calculated lightning potential.

In this work the sensitivity of the Weather Research and Forecasting (WRF) model to a choice of parameterization modulating the cloud-radiation feedbacks is assessed. Two 5-year (1990–1994) climate simulations covering Europe were conducted using WRF-ARW v3.6.1 with 0.44° spatial resolution. In the base-case, WRF does not consider cumulus cloud feedbacks to radiation, while in the modified case the subgrid-scale cloudiness associated with convection, is taken into account. The results are compared with the base case and the differences for shortwave downward radiation at the surface, total cloud cover and convective available potential energy (CAPE) are discussed. Our analysis indicates that including subgrid-scale cumulus clouds in the radiation scheme leads to a decrease of shortwave radiation at the surface and convective precipitation. The behavior of CAPE is more complex and can be either decreasing (southern Europe) or increasing (northern Europe).

In this study six EURO-CORDEX regional climate simulations forced by reanalysis data (ERA-interim) are evaluated for summer temperature and precipitation. The simulations cover the region of Europe from 1990 to 2008. The aim of this study is to demonstrate the degree to which the characteristics of past climate are reproduced and to highlight systematic errors in the simulations. Results indicate that temperature is slightly overestimated by the ensemble (+0.3 °C), but for individual members the mean biases may be either negative or positive, ranging from −0.6 to +1.6 °C. All models are warmer than the E-OBS climatology in southern Europe, whereas the northern regions are mostly colder. Regarding summer precipitation, the ensemble mean relative bias is very small (+3 %), while biases of individual ensemble members can be positive or negative, reaching up to ±20 %. The spatial pattern of relative bias generally consists of wet northern and drier central European areas. For south Europe some models present clear underestimation of precipitation, while others have areas of positive relative bias.

In the current work we present six hindcast Weather Research and Forecasting (WRF) simulations for the EURO-CORDEX domain with different configurations in microphysics, convection and radiation for the time period 1990–2008. All regional model simulations are forced by the ERA-Interim reanalysis and have the same spatial resolution (0.44°). These simulations are evaluated for surface temperature, precipitation, short- and longwave downward radiation at the surface and total cloud cover. The analysis of the WRF ensemble indicates systematic biases in both temperature and precipitation linked to different physical mechanisms for the summer and winter season. Overestimation of total cloud cover and underestimation of downward shortwave radiation at the surface, mostly when using Grell–Devenyi convection and the CAM radiation scheme, intensifies the negative summer temperature bias in northern Europe (max −2.5 °C). Conversely, a strong positive downward shortwave summer bias in central (40–60%) and southern Europe mitigates the systematic cold bias in WRF over these regions, signifying a typical case of error compensation. Maximum winter cold bias is over north-eastern Europe (−2.8 °C); this location is indicative of land–atmosphere rather than cloud-radiation interactions. Precipitation is systematically overestimated in summer by all model configurations, especially the higher quantiles, which are associated with summertime deep cumulus convection. The Kain–Fritsch convection scheme produces the larger summertime precipitation biases over the Mediterranean. Winter precipitation is reproduced with lower biases by all model configurations (15–30%). The results of this study indicate the importance of evaluating not only the basic climatic parameters of interest for climate change applications (temperature-precipitation), but also other components of the energy and water cycle, in order to identify the sources of systematic biases, possible compensatory or masking mechanisms and suggest methodologies for model improvement.

Individual storm measurements are obtained from the C-band weather radar, located close to the area of interest, using the cell tracker TITAN (thunderstorm identification, tracking, analysis, and nowcasting). The study aims in analyzing certain storm characteristics over central Greece, as a tool for investigating possible rain enhancement feasibility potential. The objective on this study is two folded: firstly, to analyze and describe storm characteristics and secondly, to identify differences in storm characteristics, structure and their behavior, among distinct synoptic situation patterns. The radar-based storm characteristics are extracted using certain thresholds. Storm characteristics include: storm initiation time, storm duration, storm motion, cloud top, reflectivity, storm volume and storm area. Frequency analyses of the aforementioned findings bring out evidences of the storm characteristics, their extends and limitations, providing thus an integrated view of the experienced isolated storms over the examined area. Moreover, the classification of the storm days reveals certain differences in several storm properties, indicating how isolated storms over central Greece vary under different synoptic situation patterns.

A three-dimensional cloud resolving model is used to study the 10th of August intensive storm. This convective case occurred over north-central Greece and resembled similar characteristics of a cell merger, causing heavy rainfall, hailfall and high-frequency lightning. Three distinct numerical experiments have been performed. In the unseeded case, the structural and evolutionary properties of the reflectivity are analyzed, horizontally and vertically, in different simulation times. The 3-d numerical simulations suggest that the merger process occurred from two or three isolated single-cells, formed during their SW-NE motion. The merging process apparently alters the dynamical and microphysical properties through low and middle level forcing, increasing cloud diameters and cloud depths, enhancing convection, producing more graupel and ice particles and increasing radar reflectivity values. The resolved TITAN radar imageries depict a similar view of the storm structure, evolution and interactions of such merging processes. The model calculated maximum radar reflectivity values coincide with the recorded ones. For these specific cell mergers, two distinct seeding experiments were conducted, to find out the optimal seeding parameters, related to seeding criteria. Specific storm characteristics are demonstrated for the stages: before and after seeding, and before and after merging.

A particulate air pollution episode of relatively long duration was recorded in the industrial basin of Western Macedonia, Greece, during November of 2009. A case study of the meteorological aspects of this episode is presented in this paper, based on the combined utilization of PM10 concentration measurements, surface and lower atmosphere parameters and simulations. An Omega blocking pattern which gradually transformed to a high-over-low pattern prevailed over central and southern Europe during the episode’s period. The absence of significant convection in the middle and lower troposphere combined with a weak horizontal wind field near the surface and the reduced mixing height, resulted in the pollutants’ trapping and the increased PM10 concentrations. The change of the synoptic setting marked the end of the episode.

A modeling system based on the air quality model CAMx driven off-line by the regional climate model RegCM3 is used for assessing the impact of lateral boundary conditions and anthropogenic emissions on tropospheric ozone over Europe for the period 1996–2006. The RegCM3 and CAMx simulations were performed on a 50 km × 50 km grid over Europe with RegCM3 driven by NCEP reanalysis fields. Average monthly concentration values obtained from the global chemistry climate model ECHAM5-MOZ were used as chemical boundary conditions for the CAMx simulations. The present period (1996–2006) was simulated four times. The first run was forced with time and space invariable lateral chemical boundary conditions and EMEP emissions based on the year 1996. The second decadal simulation was based on ECHAM5-MOZ chemical boundary conditions and emissions both fixed for the year 1996. The third decadal simulation was based on ECHAM5-MOZ chemical boundary conditions with interannual variation but fixed emissions from the year 1996. Finally, the fourth decadal simulation was based on ECHAM5-MOZ chemical boundary conditions and emissions, both having interannual variation. Simulated ozone concentrations are compared against measurements from the EMEP network in order to evaluate the modeling system.

This paper presents a case study of an episode of Vardaris that occurred on 10 November 2007 and caused widespread problems in Central Macedonia. It uses surface data including four stations along Axios valley (Evzonoi, Aksioupoli, Akropotamos, and Kymina), analyses and the nonhydrostatic Weather Research and Forecasting model. The episode was associated with the combination of a transient depression over Greece and an anticyclone in Western Europe. Maximum sustained wind speeds of 20.5 m/s with gusts up to 28.2 m/s appeared along Axios and at Thessaloniki airport (24 m/s with gusts up to 31.9 m/s). The model simulations with a horizontal resolution of 1 km were in good agreement with observations. The strongest winds, in excess of 30–35 m/s, were simulated between 750 and 900 hPa in the form of a jet streak. A numerical experiment showed that the occurrence of the episode was determined by the synoptic scale flow, but its maximum intensity was specified by the local conditions due to channeling.

The aim of this paper is to analyze the heat wave events during the decade 2001–2010 over Thessaloniki and the determination of their basic characteristics. Since extreme weather events are accompanied by serious pollution episodes, the potential linkages of heat wave events with high particulate matter (PM10) concentrations are investigated. In order to describe the heat wave events we use the Temperature Humidity Index (THI) and we classify accordingly the days fulfilling the heatwave criteria. PM10 measurements at two monitoring stations in Thessaloniki are used in order to investigate their potential linkage to the extreme meteorological events. In the urban background station of Panorama the distribution of PM10 is shifted to higher size bins and the mean of the distribution increases by 6 μg/m3 during the heatwave days. At the urban station of Agia Sofia, the impact of heatwave on PM10 pollution on the PM10 distribution is weaker, due to masking effects of the local (traffic) emission sources.

The synoptic, dynamic and thermodynamic characteristics of the intensive storm that occurred over the north-central Greece on the 10th August 2008 are studied. The event started as two individual cells that formed over the Vermion mountainous area moved northeastwards and eventually merged into one cell, growing spontaneously and developing very strong storm characteristics. Both cells were seeded before and after merging. The study is based on data derived through: ECMWF gridded analyses, synoptic charts, radiosonde, and mostly on measurements recorded and analyzed by the cell tracker TITAN. Synoptic, dynamic and thermodynamic analysis revealed favourable conditions for storm development over the area. The analysis of storm characteristics obtained from the weather radar depicted in a realistic way an integrated view of the storm triggering, development and dissipation pointing out the importance of weather radars and the powerful tool of the cell tracker TITAN.

The objective of this study is the analysis of the mesoscale thermodynamic characteristics of the hailstorms, which occurred over the region of Central Macedonia, where the National Hail Suppression Program is applied, during the period 1998–2008, except for the year 2003. The data used are derived through the C-band weather radar at Filyro and from the representative soundings of the synoptic station of Thessaloniki. The frequency of hailstorms’ occurrence, their movement, intensity and the maximum development are studied. The radar data are analyzed and studied through the software TITAN (Thunderstorm Identification, Tracking, Analysis, and Nowcasting), for the period 2006–2008.