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Dr. George Varlas
Hellenic Centre for Marine Research - Institute of Marine Biological Resources and Inland Waters

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Research Keywords & Expertise

0 Atmosphere
0 Atmospheric Modeling
0 Meteorology
0 Numerical Weather Prediction
0 air-sea interaction

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Atmosphere
air-sea interaction
Wave modeling
Numerical Weather Prediction

Honors and Awards

EMS Young Scientist Award 2018

Award for the significant contribution to the field of numerical weather prediction by developing a new integrated modelling system to simulate air-ocean wave interactions and their impact on cyclone formation. The two-way coupled atmospheric and ocean wave modelling system simulates air-sea interactions such as the modification to the air-sea fluxes and the characteristics of the atmospheric flow over the sea. More at: https://www.emetsoc.org/ems-young-scientist-award-2018-for-georgios-varlas/

European Meteorological Society




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Journal article
Published: 13 July 2021 in Atmosphere
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A hydrometeorological forecasting system has been operating at the Institute of Marine Biological Resources and Inland Waters (IMBRIW) of the Hellenic Centre for Marine Research (HCMR) since September 2015. The system consists of the Advanced Weather Research and Forecasting (WRF-ARW) model, the WRF-Hydro hydrological model, and the HEC-RAS hydraulic–hydrodynamic model. The system provides daily 120 h weather forecasts focusing on Greece (4 km horizontal resolution) and hydrological forecasts for the Spercheios and Evrotas rivers in Greece (100 m horizontal resolution), also providing flash flood inundation forecasts when needed (5 m horizontal resolution). The main aim of this study is to evaluate precipitation forecasts produced in a 4-year period (September 2015–August 2019) using measurements from meteorological stations across Greece. Water level forecasts for the Evrotas and Spercheios rivers were also evaluated using measurements from hydrological stations operated by the IMBRIW. Moreover, the forecast skill of the chained meteorological–hydrological–hydraulic operation of the system was investigated during a catastrophic flash flood in the Evrotas river. The results indicated that the system provided skillful precipitation and water level forecasts. The best evaluation results were yielded during rainy periods. They also demonstrated that timely flash flood forecasting products could benefit flood warning and emergency responses due to their efficiency and increased lead time.

ACS Style

George Varlas; Anastasios Papadopoulos; George Papaioannou; Elias Dimitriou. Evaluating the Forecast Skill of a Hydrometeorological Modelling System in Greece. Atmosphere 2021, 12, 902 .

AMA Style

George Varlas, Anastasios Papadopoulos, George Papaioannou, Elias Dimitriou. Evaluating the Forecast Skill of a Hydrometeorological Modelling System in Greece. Atmosphere. 2021; 12 (7):902.

Chicago/Turabian Style

George Varlas; Anastasios Papadopoulos; George Papaioannou; Elias Dimitriou. 2021. "Evaluating the Forecast Skill of a Hydrometeorological Modelling System in Greece." Atmosphere 12, no. 7: 902.

Journal article
Published: 31 May 2021 in Hydrology
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Water resources, especially riverine ecosystems, are globally under qualitative and quantitative degradation due to human-imposed pressures. High-temporal-resolution data obtained from automatic stations can provide insights into the processes that link catchment hydrology and streamwater chemistry. The scope of this paper was to investigate the statistical behavior of high-frequency measurements at sites with known hydromorphological and pollution pressures. For this purpose, hourly time series of water levels and key water quality indicators (temperature, electric conductivity, and dissolved oxygen concentrations) collected from four automatic monitoring stations under different hydromorphological conditions and pollution pressures were statistically elaborated. Based on the results, the hydromorphological conditions and pollution pressures of each station were confirmed to be reflected in the results of the statistical analysis performed. It was proven that the comparative use of the statistics and patterns of the water level and quality high-frequency time series could be used in the interpretation of the current site status as well as allowing the detection of possible changes. This approach can be used as a tool for the definition of thresholds, and will contribute to the design of management and restoration measures for the most impacted areas.

ACS Style

Angeliki Mentzafou; George Varlas; Anastasios Papadopoulos; Georgios Poulis; Elias Dimitriou. Assessment of Automatically Monitored Water Levels and Water Quality Indicators in Rivers with Different Hydromorphological Conditions and Pollution Levels in Greece. Hydrology 2021, 8, 86 .

AMA Style

Angeliki Mentzafou, George Varlas, Anastasios Papadopoulos, Georgios Poulis, Elias Dimitriou. Assessment of Automatically Monitored Water Levels and Water Quality Indicators in Rivers with Different Hydromorphological Conditions and Pollution Levels in Greece. Hydrology. 2021; 8 (2):86.

Chicago/Turabian Style

Angeliki Mentzafou; George Varlas; Anastasios Papadopoulos; Georgios Poulis; Elias Dimitriou. 2021. "Assessment of Automatically Monitored Water Levels and Water Quality Indicators in Rivers with Different Hydromorphological Conditions and Pollution Levels in Greece." Hydrology 8, no. 2: 86.

Journal article
Published: 15 April 2021 in Resources
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The adoption of Nature-Based Solutions (NBSs) represents a novel means to mitigate natural hazards. In the framework of the OPERANDUM project, this study introduces a methodology to assess the efficiency of the NBSs and a series of Open-Air Laboratories (OALs) regarded as a proof-of-concept for the wider uptake of NBSs. The OALs are located in Finland, Greece, UK, Italy, and Ireland. The methodology is based on a wide modeling activity, incorporated in the context of future climate scenarios. Herein, we present a series of models’ chains able to estimate the efficiency of the NBSs. While the presented models are mainly well-established, their coupling represents a first fundamental step in the study of the long-term efficacy and impact of the NBSs. In the selected sites, NBSs are utilized to cope with distinct natural hazards: floods, droughts, landslides, salt intrusion, and nutrient and sediment loading. The study of the efficacy of NBSs to mitigate these hazards belongs to a series of works devoted to the implementation of NBSs for environmental purposes. Our findings prove that land management plays a crucial role in the process. Specifically, the selected NBSs include intensive forestry; the conversion of urban areas to grassland; dunes; marine seagrass; water retention ponds; live cribwalls; and high-density plantations of woody vegetation and deep-rooted herbaceous vegetation. The management of natural resources should eventually consider the effect of NBSs on urban and rural areas, as their employment is becoming widespread.

ACS Style

Glauco Gallotti; Marco Santo; Ilektra Apostolidou; Jacopo Alessandri; Alberto Armigliato; Bidroha Basu; Sisay Debele; Alessio Domeneghetti; Alejandro Gonzalez-Ollauri; Prashant Kumar; Angeliki Mentzafou; Francesco Pilla; Beatrice Pulvirenti; Paolo Ruggieri; Jeetendra Sahani; Aura Salmivaara; Arunima Basu; Christos Spyrou; Nadia Pinardi; Elena Toth; Silvia Unguendoli; Umesh Pillai; Andrea Valentini; George Varlas; Giorgia Verri; Filippo Zaniboni; Silvana Di Sabatino. On the Management of Nature-Based Solutions in Open-Air Laboratories: New Insights and Future Perspectives. Resources 2021, 10, 36 .

AMA Style

Glauco Gallotti, Marco Santo, Ilektra Apostolidou, Jacopo Alessandri, Alberto Armigliato, Bidroha Basu, Sisay Debele, Alessio Domeneghetti, Alejandro Gonzalez-Ollauri, Prashant Kumar, Angeliki Mentzafou, Francesco Pilla, Beatrice Pulvirenti, Paolo Ruggieri, Jeetendra Sahani, Aura Salmivaara, Arunima Basu, Christos Spyrou, Nadia Pinardi, Elena Toth, Silvia Unguendoli, Umesh Pillai, Andrea Valentini, George Varlas, Giorgia Verri, Filippo Zaniboni, Silvana Di Sabatino. On the Management of Nature-Based Solutions in Open-Air Laboratories: New Insights and Future Perspectives. Resources. 2021; 10 (4):36.

Chicago/Turabian Style

Glauco Gallotti; Marco Santo; Ilektra Apostolidou; Jacopo Alessandri; Alberto Armigliato; Bidroha Basu; Sisay Debele; Alessio Domeneghetti; Alejandro Gonzalez-Ollauri; Prashant Kumar; Angeliki Mentzafou; Francesco Pilla; Beatrice Pulvirenti; Paolo Ruggieri; Jeetendra Sahani; Aura Salmivaara; Arunima Basu; Christos Spyrou; Nadia Pinardi; Elena Toth; Silvia Unguendoli; Umesh Pillai; Andrea Valentini; George Varlas; Giorgia Verri; Filippo Zaniboni; Silvana Di Sabatino. 2021. "On the Management of Nature-Based Solutions in Open-Air Laboratories: New Insights and Future Perspectives." Resources 10, no. 4: 36.

Journal article
Published: 01 April 2021 in Sustainability
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Nature-based solutions (NBS) are being deployed around the world in order to address hydrometeorological hazards, including flooding, droughts, landslides and many others. The term refers to techniques inspired, supported and copied from nature, avoiding large constructions and other harmful interventions. In this work the development and evaluation of an NBS applied to the Spercheios river basin in Central Greece is presented. The river is susceptible to heavy rainfall and bank overflow, therefore the intervention selected is a natural water retention measure that aims to moderate the impact of flooding and drought in the area. After the deployment of the NBS, we examine the benefits under current and future climate conditions, using various climate change scenarios. Even though the NBS deployed is small compared to the rest of the river, its presence leads to a decrease in the maximum depth of flooding, maximum velocity and smaller flooded areas. Regarding the subsurface/groundwater storage under current and future climate change and weather conditions, the NBS construction seems to favor long-term groundwater recharge.

ACS Style

Christos Spyrou; Michael Loupis; Νikos Charizopoulos; Ilektra Apostolidou; Angeliki Mentzafou; George Varlas; Anastasios Papadopoulos; Elias Dimitriou; Depy Panga; Lamprini Gkeka; Paul Bowyer; Susanne Pfeifer; Sisay Debele; Prashant Kumar. Evaluating Nature-Based Solution for Flood Reduction in Spercheios River Basin under Current and Future Climate Conditions. Sustainability 2021, 13, 3885 .

AMA Style

Christos Spyrou, Michael Loupis, Νikos Charizopoulos, Ilektra Apostolidou, Angeliki Mentzafou, George Varlas, Anastasios Papadopoulos, Elias Dimitriou, Depy Panga, Lamprini Gkeka, Paul Bowyer, Susanne Pfeifer, Sisay Debele, Prashant Kumar. Evaluating Nature-Based Solution for Flood Reduction in Spercheios River Basin under Current and Future Climate Conditions. Sustainability. 2021; 13 (7):3885.

Chicago/Turabian Style

Christos Spyrou; Michael Loupis; Νikos Charizopoulos; Ilektra Apostolidou; Angeliki Mentzafou; George Varlas; Anastasios Papadopoulos; Elias Dimitriou; Depy Panga; Lamprini Gkeka; Paul Bowyer; Susanne Pfeifer; Sisay Debele; Prashant Kumar. 2021. "Evaluating Nature-Based Solution for Flood Reduction in Spercheios River Basin under Current and Future Climate Conditions." Sustainability 13, no. 7: 3885.

Special issue paper
Published: 29 March 2021 in Hydrological Processes
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A common source of uncertainty in flood inundation forecasting is the hydrograph used. Given the role of sea‐air‐hydro‐land chain processes on the water cycle, flood hydrographs in coastal areas can be indirectly affected by sea state. This study investigates sea‐state effects on precipitation, discharge, and flood inundation forecasting implementing atmospheric, ocean wave, hydrological, and hydraulic‐hydrodynamic coupled models. The Chemical Hydrological Atmospheric Ocean wave System (CHAOS) was used for coupled hydro‐meteorological‐wave simulations “accounting” or “not accounting” the impact of sea state on precipitation and, subsequently, on flood hydrograph. CHAOS includes the WRF‐Hydro hydrological model and the WRF‐ARW meteorological model two‐way coupled with the WAM wave model through the OASIS3‐MCT coupler. Subsequently, the 2D HEC‐RAS hydraulic‐hydrodynamic model was forced by the flood hydrographs and map the inundated areas. A flash flood event occurred on 15 November 2017 in Mandra, Attica, Greece, causing 24 fatalities, and damages was selected as case study. The calibration of models was performed exploiting historical flood records and previous studies. Human interventions such as hydraulic works and the urban areas were included in the hydraulic modeling geometry domain. The representation of the resistance caused by buildings was based on Unmanned Aerial System (UAS) data while the local elevation rise method was used in the urban‐flood simulation. The flood extent results were assessed using the Critical Success Index (CSI), and CSI penalize. Integrating sea‐state affected the forecast of precipitation and discharge peaks, causing up to +24% and from ‐8% to +36% differences, respectively, improving inundation forecast by 4.5% and flooding additional ~70 building blocks. The precipitation forcing time step was also highlighted as significant factor in such a small‐scale flash flood. The integrated multidisciplinary methodological approach could be adopted in operational forecasting for civil protection applications facilitating the protection of socio‐economic activities and human lives during similar future events.

ACS Style

George Papaioannou; George Varlas; Anastasios Papadopoulos; Athanasios Loukas; Petros Katsafados; Elias Dimitriou. Investigating sea‐state effects on flash flood hydrograph and inundation forecasting. Hydrological Processes 2021, 35, 1 .

AMA Style

George Papaioannou, George Varlas, Anastasios Papadopoulos, Athanasios Loukas, Petros Katsafados, Elias Dimitriou. Investigating sea‐state effects on flash flood hydrograph and inundation forecasting. Hydrological Processes. 2021; 35 (4):1.

Chicago/Turabian Style

George Papaioannou; George Varlas; Anastasios Papadopoulos; Athanasios Loukas; Petros Katsafados; Elias Dimitriou. 2021. "Investigating sea‐state effects on flash flood hydrograph and inundation forecasting." Hydrological Processes 35, no. 4: 1.

Journal article
Published: 15 March 2021 in Water Research
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Understanding the climatic drivers of eutrophication is critical for lake management under the prism of the global change. Yet the complex interplay between climatic variables and lake processes makes prediction of phytoplankton biomass a rather difficult task. Quantifying the relative influence of climate-related variables on the regulation of phytoplankton biomass requires modelling approaches that use extensive field measurements paired with accurate meteorological observations. In this study we used climate and lake related variables obtained from the ERA5-Land reanalysis dataset combined with a large dataset of in-situ measurements of chlorophyll-a and phytoplankton biomass from 50 water bodies to develop models of phytoplankton related responses as functions of the climate reanalysis data. We used chlorophyll-a and phytoplankton biomass as response metrics of phytoplankton growth and we employed two different modelling techniques, boosted regression trees (BRT) and generalized additive models for location scale and shape (GAMLSS). According to our results, the fitted models had a relatively high explanatory power and predictive performance. Boosted regression trees had a high pseudo R2 with the type of the lake, the total layer temperature, and the mix-layer depth being the three predictors with the higher relative influence. The best GAMLSS model retained mix-layer depth, mix-layer temperature, total layer temperature, total runoff and 10-m wind speed as significant predictors (p<0.001). Regarding the phytoplankton biomass both modelling approaches had less explanatory power than those for chlorophyll-a. Concerning the predictive performance of the models both the BRT and GAMLSS models for chlorophyll-a outperformed those for phytoplankton biomass. Overall, we consider these findings promising for future limnological studies as they bring forth new perspectives in modelling ecosystem responses to a wide range of climate and lake variables. As a concluding remark, climate reanalysis can be an extremely useful asset for lake research and management.

ACS Style

Konstantinos Stefanidis; George Varlas; Aikaterini Vourka; Anastasios Papadopoulos; Elias Dimitriou. Delineating the relative contribution of climate related variables to chlorophyll-a and phytoplankton biomass in lakes using the ERA5-Land climate reanalysis data. Water Research 2021, 196, 117053 .

AMA Style

Konstantinos Stefanidis, George Varlas, Aikaterini Vourka, Anastasios Papadopoulos, Elias Dimitriou. Delineating the relative contribution of climate related variables to chlorophyll-a and phytoplankton biomass in lakes using the ERA5-Land climate reanalysis data. Water Research. 2021; 196 ():117053.

Chicago/Turabian Style

Konstantinos Stefanidis; George Varlas; Aikaterini Vourka; Anastasios Papadopoulos; Elias Dimitriou. 2021. "Delineating the relative contribution of climate related variables to chlorophyll-a and phytoplankton biomass in lakes using the ERA5-Land climate reanalysis data." Water Research 196, no. : 117053.

Journal article
Published: 09 February 2021 in Remote Sensing
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Atmospheric-chemical coupled models usually parameterize sea-salt aerosol (SSA) emissions using whitecap fraction estimated considering only wind speed and ignoring sea state. This approach may introduce inaccuracies in SSA simulation. This study aims to assess the impact of sea state on SSA modeling, applying a new parameterization for whitecap fraction estimation based on wave age, calculated by the ratio between wave phase velocity and wind speed. To this end, the new parameterization was incorporated in the coupled Chemical Hydrological Atmospheric Ocean wave modeling System (CHAOS). CHAOS encompasses the wave model (WAM) two-way coupled through the OASIS3-MCT coupler with the Advanced Weather Research and Forecasting model coupled with Chemistry (WRF-ARW-Chem) and, thus, enabling the concurrent simulation of SSAs, wind speed and wave phase velocity. The simulation results were evaluated against in-situ and lidar measurements at 2 stations in Greece (Finokalia on 4 and 15 July 2014 and Antikythera-PANGEA on 15 September 2018). The results reveal significant differences between the parameterizations with the new one offering a more realistic representation of SSA levels in some layers of the lower atmosphere. This is attributed to the enhancement of the bubble-bursting mechanism representation with air-sea processes controlling whitecap fraction. Our findings also highlight the contribution of fresh wind-generated waves to SSA modeling.

ACS Style

George Varlas; Eleni Marinou; Anna Gialitaki; Nikolaos Siomos; Konstantinos Tsarpalis; Nikolaos Kalivitis; Stavros Solomos; Alexandra Tsekeri; Christos Spyrou; Maria Tsichla; Anna Kampouri; Vassilis Vervatis; Elina Giannakaki; Vassilis Amiridis; Nikolaos Mihalopoulos; Anastasios Papadopoulos; Petros Katsafados. Assessing Sea-State Effects on Sea-Salt Aerosol Modeling in the Lower Atmosphere Using Lidar and In-Situ Measurements. Remote Sensing 2021, 13, 614 .

AMA Style

George Varlas, Eleni Marinou, Anna Gialitaki, Nikolaos Siomos, Konstantinos Tsarpalis, Nikolaos Kalivitis, Stavros Solomos, Alexandra Tsekeri, Christos Spyrou, Maria Tsichla, Anna Kampouri, Vassilis Vervatis, Elina Giannakaki, Vassilis Amiridis, Nikolaos Mihalopoulos, Anastasios Papadopoulos, Petros Katsafados. Assessing Sea-State Effects on Sea-Salt Aerosol Modeling in the Lower Atmosphere Using Lidar and In-Situ Measurements. Remote Sensing. 2021; 13 (4):614.

Chicago/Turabian Style

George Varlas; Eleni Marinou; Anna Gialitaki; Nikolaos Siomos; Konstantinos Tsarpalis; Nikolaos Kalivitis; Stavros Solomos; Alexandra Tsekeri; Christos Spyrou; Maria Tsichla; Anna Kampouri; Vassilis Vervatis; Elina Giannakaki; Vassilis Amiridis; Nikolaos Mihalopoulos; Anastasios Papadopoulos; Petros Katsafados. 2021. "Assessing Sea-State Effects on Sea-Salt Aerosol Modeling in the Lower Atmosphere Using Lidar and In-Situ Measurements." Remote Sensing 13, no. 4: 614.

Journal article
Published: 15 December 2020 in Dynamics of Atmospheres and Oceans
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The formation of cyclones over the sea surface is driven by air-sea interaction with feedbacks on wave generation, thermohaline structures, and biochemical properties of upper-ocean layers. In the Mediterranean basin, strong cyclonic systems having tropical-like characteristics are called “medicanes”. The consequences of such events are usually assessed over the land, however, hydrographic effects are particularly difficult to be quantified mostly due to the rarity of oceanographic in-situ monitoring systems. In this study, the hydrographic effects of a high-impact medicane, crossing the central-eastern Mediterranean in late September 2018 are investigated mainly based on Argo float measurements. We traced its hydrographic fingerprint and examined its effects on the upper-layer physical properties by analyzing temperature and salinity profiles from floats that overlapped with its track. Float data is supported by satellite sea surface temperature reanalysis data and meteorological records. The synergistic effects of intense evaporation and vertical mixing mechanisms triggered by the medicane, resulted in abrupt surface cooling, especially in the Ionian and Aegean Seas. A relatively homogenous decrease of temperature in the upper 50-m-depth layer followed, along with the deepening of the mixed-layer depth, corresponding to an estimated per profile average deficit of (-2.72 ± 1.23) x 108 J m−2 in the ocean heat content. Simultaneously, significant upper-layer freshening occurred because of vertical mixing and heavy rainfall. However, a salinity increase in the subsurface waters was observed after the medicane event, which is associated with both horizontal advection and vertical mixing which followed the weakening of the Atlantic Water signal and the dominance of the Levantine Surface Water in this zone. Our findings highlight strong, short-scale hydrographic alterations made available due to the expansion of the marginal seas operational oceanographic network.

ACS Style

Dimitris Kassis; George Varlas. Hydrographic effects of an intense “medicane” over the central-eastern Mediterranean Sea in 2018. Dynamics of Atmospheres and Oceans 2020, 93, 101185 .

AMA Style

Dimitris Kassis, George Varlas. Hydrographic effects of an intense “medicane” over the central-eastern Mediterranean Sea in 2018. Dynamics of Atmospheres and Oceans. 2020; 93 ():101185.

Chicago/Turabian Style

Dimitris Kassis; George Varlas. 2020. "Hydrographic effects of an intense “medicane” over the central-eastern Mediterranean Sea in 2018." Dynamics of Atmospheres and Oceans 93, no. : 101185.

Chapter
Published: 12 September 2020 in The Handbook of Environmental Chemistry
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The meteorological conditions over the Aegean Sea are subjected to strong influences from the large-scale patterns of the general atmospheric circulation, the surrounding continents, the almost enclosed Mediterranean Sea, the complex topography and coastline, as well as the mesoscale and local phenomena imposed on the synoptic-scale atmospheric circulation. This chapter discusses the most important circulation patterns that affect the area determining not only the weather conditions but also the climatic characteristics. The discussion unravels seasonal and spatial characteristics of the weather and climate patterns giving also a particular emphasis on the formation of cyclones (cyclogenesis). The role of upper-air conditions and air-sea interaction is highlighted, especially demonstrating the remarkable patterns of explosive cyclones and Mediterranean tropical-like cyclones (medicanes). This chapter also provides a preliminary analysis of air temperature and precipitation over the Aegean Sea unveiling spatiotemporal variabilities and trends.

ACS Style

Anastasios Papadopoulos; George Varlas. Weather Systems Affecting the Meteorological Conditions over the Aegean Sea. The Handbook of Environmental Chemistry 2020, 1 -25.

AMA Style

Anastasios Papadopoulos, George Varlas. Weather Systems Affecting the Meteorological Conditions over the Aegean Sea. The Handbook of Environmental Chemistry. 2020; ():1-25.

Chicago/Turabian Style

Anastasios Papadopoulos; George Varlas. 2020. "Weather Systems Affecting the Meteorological Conditions over the Aegean Sea." The Handbook of Environmental Chemistry , no. : 1-25.

Journal article
Published: 27 August 2020 in Remote Sensing
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Severe hydrometeorological hazards such as floods, droughts, and thunderstorms are expected to increase in the future due to climate change. Due to the significant impacts of these phenomena, it is essential to develop new and advanced early warning systems for advance preparation of the population and local authorities (civil protection, government agencies, etc.). Therefore, reliable forecasts of extreme events, with high spatial and temporal resolution and a very short time horizon are needed, due to the very fast development and localized nature of these events. In very short time-periods (up to 6 h), small-scale phenomena can be described accurately by adopting a “nowcasting” approach, providing reliable short-term forecasts and warnings. To this end, a novel nowcasting system was developed and presented in this study, combining a data assimilation system (LAPS), a large amount of observed data, including XPOL radar precipitation measurements, the Chemical Hydrological Atmospheric Ocean wave System (CHAOS), and the WRF-Hydro model. The system was evaluated on the catastrophic flash flood event that occurred in the sub-urban area of Mandra in Western Attica, Greece, on 15 November 2017. The event was one of the most catastrophic flash floods with human fatalities (24 people died) and extensive infrastructure damage. The update of the simulations with assimilated radar data improved the initial precipitation description and led to an improved simulation of the evolution of the phenomenon. Statistical evaluation and comparison with flood data from the FloodHub showed that the nowcasting system could have provided reliable early warning of the flood event 1, 2, and even to 3 h in advance, giving vital time to the local authorities to mobilize and even prevent fatalities and injuries to the local population.

ACS Style

Christos Spyrou; George Varlas; Aikaterini Pappa; Angeliki Mentzafou; Petros Katsafados; Anastasios Papadopoulos; Marios Anagnostou; John Kalogiros. Implementation of a Nowcasting Hydrometeorological System for Studying Flash Flood Events: The Case of Mandra, Greece. Remote Sensing 2020, 12, 2784 .

AMA Style

Christos Spyrou, George Varlas, Aikaterini Pappa, Angeliki Mentzafou, Petros Katsafados, Anastasios Papadopoulos, Marios Anagnostou, John Kalogiros. Implementation of a Nowcasting Hydrometeorological System for Studying Flash Flood Events: The Case of Mandra, Greece. Remote Sensing. 2020; 12 (17):2784.

Chicago/Turabian Style

Christos Spyrou; George Varlas; Aikaterini Pappa; Angeliki Mentzafou; Petros Katsafados; Anastasios Papadopoulos; Marios Anagnostou; John Kalogiros. 2020. "Implementation of a Nowcasting Hydrometeorological System for Studying Flash Flood Events: The Case of Mandra, Greece." Remote Sensing 12, no. 17: 2784.

Journal article
Published: 18 July 2020 in Ocean Modelling
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Understanding the governing mechanisms of atmosphere–wave–ocean​ interactions is critical for unravelling the formation and evolution mechanisms of severe weather phenomena. This study aims at investigating the effects of atmosphere–wave–ocean​ feedbacks on a Mediterranean tropical-like cyclone (medicane), occurred on 27–30 September 2018 at the central-eastern Mediterranean Sea and characterized by severe environmental and socioeconomic impact. To unveil the interactions across the air–sea interface, the medicane was simulated by an integrated modelling system consisting of the Chemical Hydrological Atmospheric Ocean wave System (CHAOS), upgraded by embedding to it the Nucleus for European Modelling of the Ocean (NEMO) as ocean circulation component. Coupled simulations revealed that air–seaheat transfer and Ekman pumping, bringing sub-surface cold waters in upper ocean layers (upwelling), caused SST cooling (∼2–3 °C). SST cooling triggered a negative feedback loop procedure tending to balance between atmospheric and ocean processes. It also attenuated the cyclone and, subsequently, reduced the atmospheric energy embedded in ocean through the upper ocean vertical stratification weakening, thus, upper ocean vertical mixing, upwelling and SST cooling. The waves adjusted this feedback loop making the system more resistant in air–sea flux variations. Waves additionally weakened the cyclone not only due to the kinetic energy loss in the lower-atmosphere but also due to the enhancement of SST cooling which is attributed to the strengthening of Ekman pumping and vertical mixing, forced by wind stress increase. Nevertheless, waves partially balanced the air–wave–sea exchanges through the slight enthalpy flux gain under high wind conditions which is explained by considering the increase of enthalpy transfer coefficient in rougher sea areas.

ACS Style

G. Varlas; V. Vervatis; Christos Spyrou; E. Papadopoulou; A. Papadopoulos; P. Katsafados. Investigating the impact of atmosphere–wave–ocean interactions on a Mediterranean tropical-like cyclone. Ocean Modelling 2020, 153, 101675 .

AMA Style

G. Varlas, V. Vervatis, Christos Spyrou, E. Papadopoulou, A. Papadopoulos, P. Katsafados. Investigating the impact of atmosphere–wave–ocean interactions on a Mediterranean tropical-like cyclone. Ocean Modelling. 2020; 153 ():101675.

Chicago/Turabian Style

G. Varlas; V. Vervatis; Christos Spyrou; E. Papadopoulou; A. Papadopoulos; P. Katsafados. 2020. "Investigating the impact of atmosphere–wave–ocean interactions on a Mediterranean tropical-like cyclone." Ocean Modelling 153, no. : 101675.

Journal article
Published: 28 March 2020 in Mediterranean Marine Science
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Nowadays, it has become clear that atmosphere and ocean should be simulated by integrated modeling systems resolving interconnected physical factors which determine Earth’s energy balance. Waves play a key role on the interfacial interaction between atmosphere and ocean regulating momentum, heat and moisture exchange. This study aims to evaluate the two-way coupled atmosphere-ocean wave system CHAOS (Chemical Hydrological Atmosphere Ocean wave System) over the Mediterranean and Black Seas. The evaluation is performed against in-situ and remote sensing data for the period from 1 December 2013 to 1 December 2014. CHAOS includes the Weather Research Forecasting (WRF) model version 3.8 as atmospheric component and the Wave model (WAM) cycle 4.5.4 as ocean wave component, coupled through the OASIS3-MCT coupler version 3.0. In order to assess the impact of the atmosphere-ocean waves coupling, two approaches of continuous model simulations are followed. In the first approach (1-way coupling mode) the ocean wave component uses the winds produced by the atmospheric component while in the second approach (2-way coupling mode) the atmospheric component additionally uses the sea state information estimated by the ocean wave component through wave-dependent Charnock parameter computations. In the 2-way coupling mode, the attenuation of the atmospheric flow has a damping effect on wind-generated waves. The simulations in 2-way coupling mode produce more realistic results yielding statistical improvements. Against buoys observations, 2-way reduces root mean square error (RMSE) per 1.2% and 6.3% for wind speed and significant wave height, respectively, while against Jason-2 satellite retrievals per 0.5% and 2.4%, respectively. Additionally, the 2-way outperforms the 1-way coupling mode under intense wind and wave conditions during this one-year period.

ACS Style

George Varlas; Christos Spyrou; Anastasios Papadopoulos; Gerasimos Korres; Petros Katsafados. One-year assessment of the two-way coupled atmosphere-ocean wave modeling system CHAOS over the Mediterranean and Black Seas. Mediterranean Marine Science 2020, 1 .

AMA Style

George Varlas, Christos Spyrou, Anastasios Papadopoulos, Gerasimos Korres, Petros Katsafados. One-year assessment of the two-way coupled atmosphere-ocean wave modeling system CHAOS over the Mediterranean and Black Seas. Mediterranean Marine Science. 2020; ():1.

Chicago/Turabian Style

George Varlas; Christos Spyrou; Anastasios Papadopoulos; Gerasimos Korres; Petros Katsafados. 2020. "One-year assessment of the two-way coupled atmosphere-ocean wave modeling system CHAOS over the Mediterranean and Black Seas." Mediterranean Marine Science , no. : 1.

Journal article
Published: 05 February 2020 in Atmosphere
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The necessity to reduce C O 2 emissions in combination with the rising energy demand worldwide makes the extensive use of renewable energy sources increasingly important. To that end, countries with long coastlines, such as Norway, can exploit ocean wave energy to produce large amounts of power. In order to facilitate these efforts as well as to provide quantitative data on the wave energy potential of a specific area, it is essential to analyze the weather and climatic conditions detecting any variabilities. The complex physical processes and the atmosphere-wave synergetic effects make the investigation of temporal variability of wave energy a challenging issue. This work aims to shed new light on potential wave energy mapping, presenting a spatio-temporal assessment of swell- and wind-sea-induced energy flux in the Nordic Seas with a focus on the Norwegian coastline using the NORA10 hindcast for the period 1958–2017 (59 years). The results indicate high spatial and seasonal variability of the wave energy flux along the coast. The maximum wave energy flux is observed during winter, while the minimum is observed during summer. The highest coastal wave energy flux is observed in the Norwegian Sea. The majority of areas with dominant swell conditions (i.e., in the Norwegian Sea) are characterized by the highest coastal wave energy flux. The maximum values of wave energy flux in the North Sea are denoted in its northern parts in the intersection with the Norwegian Sea. In contrast to the Norwegian Sea, areas located in the North Sea and the Barents Sea show that wind sea is contributing more than swell to the total wave energy flux.

ACS Style

Konstantinos Christakos; George Varlas; Ioannis Cheliotis; Christos Spyrou; Ole Johan Aarnes; Birgitte Rugaard Furevik. Characterization of Wind-Sea- and Swell-Induced Wave Energy along the Norwegian Coast. Atmosphere 2020, 11, 166 .

AMA Style

Konstantinos Christakos, George Varlas, Ioannis Cheliotis, Christos Spyrou, Ole Johan Aarnes, Birgitte Rugaard Furevik. Characterization of Wind-Sea- and Swell-Induced Wave Energy along the Norwegian Coast. Atmosphere. 2020; 11 (2):166.

Chicago/Turabian Style

Konstantinos Christakos; George Varlas; Ioannis Cheliotis; Christos Spyrou; Ole Johan Aarnes; Birgitte Rugaard Furevik. 2020. "Characterization of Wind-Sea- and Swell-Induced Wave Energy along the Norwegian Coast." Atmosphere 11, no. 2: 166.

Journal article
Published: 07 November 2019 in Climate
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In this study, the physically-based hydrological model MIKE SHE was employed to investigate the effects of anthropogenic land cover changes to the hydrological cycle components of a regional watershed in Central Greece. Three case studies based on the land cover of the years 1960, 1990, and 2018 were examined. Copernicus Climate Change Service E-OBS gridded meteorological data for 45 hydrological years were used as forcing for the model. Evaluation against observational data yielded sufficient quality for daily air temperature and precipitation. Simulation results demonstrated that the climatic variabilities primarily in precipitation and secondarily in air temperature affected basin-averaged annual actual evapotranspiration and average annual river discharge. Nevertheless, land cover effects can locally outflank the impact of climatic variability as indicated by the low interannual variabilities of differences in annual actual evapotranspiration among case studies. The transition from forest to pastures or agricultural land reduced annual actual evapotranspiration and increased average annual river discharge while intensifying the vulnerability to hydrometeorological-related hazards such as droughts or floods. Hence, the quantitative assessment of land cover effects presented in this study can contribute to the design and implementation of successful land cover and climate change mitigation and adaptation policies.

ACS Style

Angeliki Mentzafou; George Varlas; Elias Dimitriou; Anastasios Papadopoulos; Ioannis Pytharoulis; Petros Katsafados. Modeling the Effects of Anthropogenic Land Cover Changes to the Main Hydrometeorological Factors in a Regional Watershed, Central Greece. Climate 2019, 7, 129 .

AMA Style

Angeliki Mentzafou, George Varlas, Elias Dimitriou, Anastasios Papadopoulos, Ioannis Pytharoulis, Petros Katsafados. Modeling the Effects of Anthropogenic Land Cover Changes to the Main Hydrometeorological Factors in a Regional Watershed, Central Greece. Climate. 2019; 7 (11):129.

Chicago/Turabian Style

Angeliki Mentzafou; George Varlas; Elias Dimitriou; Anastasios Papadopoulos; Ioannis Pytharoulis; Petros Katsafados. 2019. "Modeling the Effects of Anthropogenic Land Cover Changes to the Main Hydrometeorological Factors in a Regional Watershed, Central Greece." Climate 7, no. 11: 129.

Journal article
Published: 07 November 2019 in Water
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Nowadays, as extreme weather increasingly threatens human health and economy, early warning system approaches are critical for timely preparedness and response. Towards the implementation of a multi-model forecasting system for flood hazards, this study presents a coupled application of three (3) models: The WRF-ARW weather model, the WRF-Hydro hydrological model, and the HEC-RAS 2D hydraulic model. A flash flood event that occurred on 9 October 2006 in Volos city, Greece, is used as a case study to assess the accuracy of the integrated modelling approach to simulate the flood hydrograph and flood extent in Xerias ungauged catchment. The hydrometeorological simulation results indicated a severe persistent storm over Pelion mountain at the northeast of Volos, as the main factor of the major flash flood and extensive impacts. Historical flood records retrieved by several conventional and non-conventional sources are used to validate the flooded area. Compared to the collected data and prior studies, the generated inundation map of Xerias river is found to realistically capture highly impacted areas which experienced infrastructure damage and human rescues from inundated roads and buildings. Results from our analyses show that the proposed physically-based modelling approach can give reliable inputs into flood risk management.

ACS Style

George Papaioannou; George Varlas; Galateia Terti; Anastasios Papadopoulos; Athanasios Loukas; Yiannis Panagopoulos; Elias Dimitriou. Flood Inundation Mapping at Ungauged Basins Using Coupled Hydrometeorological–Hydraulic Modelling: The Catastrophic Case of the 2006 Flash Flood in Volos City, Greece. Water 2019, 11, 2328 .

AMA Style

George Papaioannou, George Varlas, Galateia Terti, Anastasios Papadopoulos, Athanasios Loukas, Yiannis Panagopoulos, Elias Dimitriou. Flood Inundation Mapping at Ungauged Basins Using Coupled Hydrometeorological–Hydraulic Modelling: The Catastrophic Case of the 2006 Flash Flood in Volos City, Greece. Water. 2019; 11 (11):2328.

Chicago/Turabian Style

George Papaioannou; George Varlas; Galateia Terti; Anastasios Papadopoulos; Athanasios Loukas; Yiannis Panagopoulos; Elias Dimitriou. 2019. "Flood Inundation Mapping at Ungauged Basins Using Coupled Hydrometeorological–Hydraulic Modelling: The Catastrophic Case of the 2006 Flash Flood in Volos City, Greece." Water 11, no. 11: 2328.

Journal article
Published: 17 May 2019 in Mediterranean Marine Science
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The forecast skill of numerical weather prediction (NWP) models relies, among other factors such as the prediction itself and the assimilation scheme, on the accuracy of the observations utilized in the assimilation systems for the production of initial and boundary conditions. One of the most crucial parameters in weather forecasting is the sea surface temperature (SST). In the majority of NWP models, the initial and lower boundary conditions involve gridded (SST) analyses which consist of data obtained by buoys, ships and satellites. The main aim of this study is to integrate Argo temperature measurements in gridded SST analyses and to assess their impact on the forecast skill of a limited area atmospheric model. Argo floats are “state-of-the-art” oceanographic instruments producing high-quality temperature profiles for the ice-free ocean. In this study, Argo temperatures are incorporated into gridded SST fields without applying any smoothing method in order to directly assess the impact of Argo temperatures on numerical weather prediction. Their impact is assessed under intense weather cyclonic conditions at the Mediterranean Sea by performing two sensitivity simulations either incorporating or not Argo temperatures into gridded SST fields used in the generation of the initial and lower boundary conditions. The results indicate that the inclusion of Argo-measured near-surface temperatures in the lower boundary condition modifies the surface heat fluxes, thus affecting mean sea level pressure and precipitation. In particular, an overall improvement of the precipitation forecast skill up to 3% has been demonstrated. Moreover, the incorporation of Argo temperatures affects the simulated track and intensity of the cyclone over the Balkan Peninsula.

ACS Style

George Varlas; Petros Katsafados; Gerasimos Korres; Anastasios Papadopoulos. Assessing the impact of Argo floats temperature measurements on the numerical weather prediction forecast skill. Mediterranean Marine Science 2019, 20, 331 -341.

AMA Style

George Varlas, Petros Katsafados, Gerasimos Korres, Anastasios Papadopoulos. Assessing the impact of Argo floats temperature measurements on the numerical weather prediction forecast skill. Mediterranean Marine Science. 2019; 20 (2):331-341.

Chicago/Turabian Style

George Varlas; Petros Katsafados; Gerasimos Korres; Anastasios Papadopoulos. 2019. "Assessing the impact of Argo floats temperature measurements on the numerical weather prediction forecast skill." Mediterranean Marine Science 20, no. 2: 331-341.

Journal article
Published: 28 December 2018 in Remote Sensing
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Urban areas often experience high precipitation rates and heights associated with flash flood events. Atmospheric and hydrological models in combination with remote-sensing and surface observations are used to analyze these phenomena. This study aims to conduct a hydrometeorological analysis of a flash flood event that took place in the sub-urban area of Mandra, western Attica, Greece, using remote-sensing observations and the Chemical Hydrological Atmospheric Ocean Wave System (CHAOS) modeling system that includes the Advanced Weather Research Forecasting (WRF-ARW) model and the hydrological model (WRF-Hydro). The flash flood was caused by a severe storm during the morning of 15 November 2017 around Mandra area resulting in extensive damages and 24 fatalities. The X-band dual-polarization (XPOL) weather radar of the National Observatory of Athens (NOA) observed precipitation rates reaching 140 mm/h in the core of the storm. CHAOS simulation unveils the persistent orographic convergence of humid southeasterly airflow over Pateras mountain as the dominant parameter for the evolution of the storm. WRF-Hydro simulated the flood using three different precipitation estimations as forcing data, obtained from the CHAOS simulation (CHAOS-hydro), the XPOL weather radar (XPOL-hydro) and the Global Precipitation Measurement (GMP)/Integrated Multi-satellitE Retrievals for GPM (IMERG) satellite dataset (GPM/IMERG-hydro). The findings indicate that GPM/IMERG-hydro underestimated the flood magnitude. On the other hand, XPOL-hydro simulation resulted to discharge about 115 m3/s and water level exceeding 3 m in Soures and Agia Aikaterini streams, which finally inundated. CHAOS-hydro estimated approximately the half water level and even lower discharge compared to XPOL-hydro simulation. Comparing site-detailed post-surveys of flood extent, XPOL-hydro is characterized by overestimation while CHAOS-hydro and GPM/IMERG-hydro present underestimation. However, CHAOS-hydro shows enough skill to simulate the flooded areas despite the forecast inaccuracies of numerical weather prediction. Overall, the simulation results demonstrate the potential benefit of using high-resolution observations from a X-band dual-polarization radar as an additional forcing component in model precipitation simulations.

ACS Style

George Varlas; Marios N. Anagnostou; Christos Spyrou; Anastasios Papadopoulos; John Kalogiros; Angeliki Mentzafou; Silas Michaelides; Evangelos Baltas; Efthimios Karymbalis; Petros Katsafados. A Multi-Platform Hydrometeorological Analysis of the Flash Flood Event of 15 November 2017 in Attica, Greece. Remote Sensing 2018, 11, 45 .

AMA Style

George Varlas, Marios N. Anagnostou, Christos Spyrou, Anastasios Papadopoulos, John Kalogiros, Angeliki Mentzafou, Silas Michaelides, Evangelos Baltas, Efthimios Karymbalis, Petros Katsafados. A Multi-Platform Hydrometeorological Analysis of the Flash Flood Event of 15 November 2017 in Attica, Greece. Remote Sensing. 2018; 11 (1):45.

Chicago/Turabian Style

George Varlas; Marios N. Anagnostou; Christos Spyrou; Anastasios Papadopoulos; John Kalogiros; Angeliki Mentzafou; Silas Michaelides; Evangelos Baltas; Efthimios Karymbalis; Petros Katsafados. 2018. "A Multi-Platform Hydrometeorological Analysis of the Flash Flood Event of 15 November 2017 in Attica, Greece." Remote Sensing 11, no. 1: 45.

Research letter
Published: 07 November 2018 in Geophysical Research Letters
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One of the most understudied and less understood phenomenon in air‐sea interaction studies is the effect of rain on the sea state and in turn the atmosphere's response to this surface forcing. In order to examine the entire process, a new parameterization scheme for rain‐induced sea surface roughness was incorporated in the two‐way atmosphere‐ocean wave coupled system CHAOS. The simulation of the late October 2012 hurricane Sandy indicates that the complex rain‐on‐ocean effect primarily modifies sea surface roughness and attenuates both the atmospheric flow and the ocean wave growth. The heterogeneity of roughness variations modulates the asymmetry of the hurricane and affects its central pressure and its track as well. Comparing with in‐situ and remotely sensed data, the consideration of rain‐on‐ocean effect reduces the root mean square error up to 8% and 11% for wind speed and significant wave height, respectively, and finally improves Sandy's track offshore eastern US coastline.

ACS Style

P. Katsafados; G. Varlas; A. Papadopoulos; C. Spyrou; G. Korres. Assessing the Implicit Rain Impact on Sea State During Hurricane Sandy (2012). Geophysical Research Letters 2018, 45, 12,015 -12,022.

AMA Style

P. Katsafados, G. Varlas, A. Papadopoulos, C. Spyrou, G. Korres. Assessing the Implicit Rain Impact on Sea State During Hurricane Sandy (2012). Geophysical Research Letters. 2018; 45 (21):12,015-12,022.

Chicago/Turabian Style

P. Katsafados; G. Varlas; A. Papadopoulos; C. Spyrou; G. Korres. 2018. "Assessing the Implicit Rain Impact on Sea State During Hurricane Sandy (2012)." Geophysical Research Letters 45, no. 21: 12,015-12,022.

Journal article
Published: 01 August 2018 in Atmospheric Research
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ACS Style

George Varlas; Petros Katsafados; Anastasios Papadopoulos; Gerasimos Korres. Implementation of a two-way coupled atmosphere-ocean wave modeling system for assessing air-sea interaction over the Mediterranean Sea. Atmospheric Research 2018, 208, 201 -217.

AMA Style

George Varlas, Petros Katsafados, Anastasios Papadopoulos, Gerasimos Korres. Implementation of a two-way coupled atmosphere-ocean wave modeling system for assessing air-sea interaction over the Mediterranean Sea. Atmospheric Research. 2018; 208 ():201-217.

Chicago/Turabian Style

George Varlas; Petros Katsafados; Anastasios Papadopoulos; Gerasimos Korres. 2018. "Implementation of a two-way coupled atmosphere-ocean wave modeling system for assessing air-sea interaction over the Mediterranean Sea." Atmospheric Research 208, no. : 201-217.

Original paper
Published: 31 January 2018 in Theoretical and Applied Climatology
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Hurricane Sandy affected the Caribbean Islands and the Northeastern United States in October 2012 and caused 233 fatalities, severe rainfalls, floods, electricity blackouts, and 75 billion U.S. dollars in damages. In this study, the synoptic and dynamical characteristics that led to the formation of the hurricane are investigated. The system was driven by the interaction between the polar jet displacement and the subtropical jet stream. In particular, Sandy was initially formed as a tropical depression system over the Caribbean Sea and the unusually warm sea drove its intensification. The interaction between a rapidly approaching trough from the northwest and the stagnant ridge over the Atlantic Ocean drove Sandy to the northeast coast of United States. To better understand the dynamical characteristics and the mechanisms that triggered Sandy, a non-hydrostatic mesoscale model has been used. Model results indicate that the surface heat fluxes and the moisture advection enhanced the convective available potential energy, increased the low-level convective instability, and finally deepened the hurricane. Moreover, the upper air conditions triggered the low-level frontogenesis and increased the asymmetry of the system which finally affected its trajectory.

ACS Style

George Varlas; Anastasios Papadopoulos; Petros Katsafados. An analysis of the synoptic and dynamical characteristics of hurricane Sandy (2012). Theoretical and Applied Climatology 2018, 131, 443 -453.

AMA Style

George Varlas, Anastasios Papadopoulos, Petros Katsafados. An analysis of the synoptic and dynamical characteristics of hurricane Sandy (2012). Theoretical and Applied Climatology. 2018; 131 (3):443-453.

Chicago/Turabian Style

George Varlas; Anastasios Papadopoulos; Petros Katsafados. 2018. "An analysis of the synoptic and dynamical characteristics of hurricane Sandy (2012)." Theoretical and Applied Climatology 131, no. 3: 443-453.