I. Tegoulias

Recent past and future patterns of the Etesian winds based on regional scale climate model simulations

The aim of this work is to investigate the recent past and future patterns of the Etesian winds, one of the most persistent localized wind systems in the world, which dominates the wind regime during warm period over the Aegean Sea and eastern Mediterranean. An objective classification method, the Two Step Cluster Analysis (TSCA), is applied on daily data from regional climate model simulations carried out with RegCM3 for the recent past (1961–1990) and future periods (2021–2050 and 2071–2100) constrained at lateral boundaries either by ERA-40 reanalysis fields or the global circulation model (GCM) ECHAM5. Three distinct Etesian patterns are identified by TSCA with the location and strength of the anticyclonic action center dominating the differences among the patterns. In case of the first Etesian pattern there is a ridge located over western and central Europe while for the other two Etesian patterns the location of the ridge moves eastward indicating a strong anticyclonic center over the Balkans. The horizontal and vertical spatial structure of geopotential height and the vertical velocity indicates that in all three Etesian patterns the anticyclonic action center over central Europe or Balkan Peninsula cannot be considered as an extension of the Azores high. The future projections for the late 21st century under SRES A1B scenario indicate a strengthening of the Etesian winds associated with the strengthening of the anticyclonic action center, and the deepening of Asian thermal Low over eastern Mediterranean. Furthermore the future projections indicate a weakening of the subsidence over eastern Mediterranean which is rather controlled by the deepening of the south Asian thermal Low in line with the projected in future weakening of South Asian monsoon and Hadley cell circulations.

Climate change effects on the marine characteristics of the Aegean and Ionian Seas

This paper addresses the effects of estimated climate change on the sea-surface dynamics of the Aegean and Ionian Seas (AIS). The main aim is the identification of climate change impacts on the severity and frequency of extreme storm surges and waves in areas of the AIS prone to flooding. An attempt is made to define design levels for future research on coastal protection in Greece. Extreme value analysis is implemented through a nonstationary generalized extreme value distribution function, incorporating time harmonics in its parameters, by means of statistically defined criteria. A 50-year time span analysis is adopted and changes of means and extremes are determined. A Regional Climate Model (RegCM3) is implemented with dynamical downscaling, forced by ECHAM5 fields under 20C3M historical data for the twentieth century and the SRES-A1B scenario for the twenty-first century. Storm surge and wave models (GreCSSM and SWAN, respectively) are used for marine climate simulations. Comparisons of model results with reanalysis and field data of atmospheric and hydrodynamic characteristics, respectively, are in good agreement. Our findings indicate that the dynamically downscaled RegCM3 simulation adequately reproduces the present general circulation patterns over the Mediterranean and Greece. Future changes in sea level pressure and mean wind fields are estimated to be small, yet significant for marine extremes. In general, we estimate a projected intensification of severe wave and storm surge events during the first half of the twenty-first century and a subsequent storminess attenuation leading to the resettlement of milder extreme marine events with increased prediction uncertainty in the second half of the twenty-first century.

Numerical Study of an Intense Episode of Vardaris Wind

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.

A Three-Dimensional Simulation of the 10th August 2008 Storm Occurred Over Greece: AgI Seeding of Cell Merger by Using a Cloud Resolving Model

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.

Comparison of an automated classification system with an empirical classification of circulation patterns over the Pannonian basin, Central Europe

The aim of the study is to compare the performance of the two classification methods, based on the atmospheric circulation types over the Pannonian basin in Central Europe. Moreover, relationships including seasonal occurrences and correlation coefficients, as well as comparative diagrams of the seasonal occurrences of the circulation types of the two classification systems are presented. When comparing of the automated (objective) and empirical (subjective) classification methods, it was found that the frequency of the empirical anticyclonic (cyclonic) types is much higher (lower) than that of the automated anticyclonic (cyclonic) types both on an annual and seasonal basis. The highest and statistically significant correlations between the circulation types of the two classification systems, as well as those between the cumulated seasonal anticyclonic and cyclonic types occur in winter for both classifications, since the weather-influencing effect of the atmospheric circulation in this season is the most prevalent. Precipitation amounts in Budapest display a decreasing trend in accordance with the decrease in the occurrence of the automated cyclonic types. In contrast, the occurrence of the empirical cyclonic types displays an increasing trend. There occur types in a given classification that are usually accompanied by high ratios of certain types in the other classification.

Regional Air Quality Simulations Over Europe in Present and Future Climate: Evaluation and Climate Change Impacts on Near Surface Ozone

Regional climate-air quality simulations were carried out over Europe with the RegCM3/CAMx modelling system for two future decades, 2041–2050 and 2091–2100 under IPCC A1B scenario and the control decade 1991–2000. The RegCM3/CAMx simulations for the present decade were driven either by ERA-40 reanalysis or the global circulation model (GCM) ECHAM5 while the simulations for the two future decades were driven by ECHAM. The simulations serve as a theoretical experiment to investigate the impact of changing climate on near surface ozone. The simulated ozone was evaluated with respect to near surface ozone measurements from the EMEP database for the control decade 1991–2000 indicating a relatively good performance of the RegCM3/CAMx modelling system in simulating near-surface ozone levels over Europe. Our projections in future suggest that the changes in near surface ozone imposed by climate change are small over the near future decade (2041–2050) but become significant over the end of twenty-first century. Specifically the ozone change over the future decade 2091–2100 is more intense over south-west Europe during summer, increasing by 6.2 ppbv as a result of the combination of relatively more stagnant conditions, increased temperatures and solar radiation within an anticyclonic anomaly.

An Objective Classification of Synoptic Types over Europe

A synoptic classification scheme is developed for Europe based on an automated two-step cluster analysis. It employs daily NCEP-NCAR reanalysis data over 62 years (1948–2009) in creating synoptic types from surface and upper air (1,000, 850, 700 and 500 hPa) temperature and humidity data as well as geopotential height and winds aloft. The synoptic types that have been created exhibit distinct seasonal preferences.

Air Quality Simulations Over Europe for the Period 1996–2006 with Emphasis on Tropospheric Ozone

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.