K. Lagouvardos

HYMEX , a 10-year Multidisciplinary Program on the mediterranean water cycle.

The Mediterranean countries are experiencing important challenges related to the water cycle, including water shortages and floods, extreme winds, and ice/snow storms, that impact critically the socioeconomic vitality in the area (causing damage to property, threatening lives, affecting the energy and transportation sectors, etc.). There are gaps in our understanding of the Mediterranean water cycle and its dynamics that include the variability of the Mediterranean Sea water budget and its feedback on the variability of the continental precipitation through air–sea interactions, the impact of precipitation variability on aquifer recharge, river discharge, and soil water content and vegetation characteristics specific to the Mediterranean basin and the mechanisms that control the location and intensity of heavy precipitating systems that often produce floods. The Hydrological Cycle in Mediterranean Experiment (HyMeX) program is a 10-yr concerted experimental effort at the international level that aims to advance the scientific knowledge of the water cycle variability in all compartments (land, sea, and atmosphere) and at various time and spatial scales. It also aims to improve the processes-based models needed for forecasting hydrometeorological extremes and the models of the regional climate system for predicting regional climate variability and evolution. Finally, it aims to assess the social and economic vulnerability to hydrometeorological natural hazards in the Mediterranean and the adaptation capacity of the territories and populations therein to provide support to policy makers to cope with water-related problems under the influence of climate change, by linking scientific outcomes with related policy requirements.

On the Long‐Range transport of air pollutants from Europe to Africa

The general climatic conditions and the physiographic characteristics of the area around the Mediterranean Sea, result in the formation of a flow pattern which is from North to South during all seasons and mainly during summer. This flow transports polluted air masses from southern Europe towards Africa. This transport is being investigated with the combined use of an atmospheric and a Lagrangian dispersion model. Air pollutants released from sources located in southern Europe were found in the entire tropospheric region over North Africa. The time scales for such a transport were found to be four to six days. This kind of transport can have several implications ranging from degradation of the air quality in North African cities to the water budget and regional climatic change.

Observations and model simulations of a winter sub-synoptic vortex over the central Mediterranean

A sub-synoptic vortex with characteristics of a tropical storm developed over the area between Italy and Greece, during the period 14–18 January 1995. Satellite imagery revealed that this vortex was associated with spirally distributed bands and a clearly defined ‘eye’. Ship reports and SSM/I retrieved winds show the existence of strong surface winds around the vortex. Simulation of the genesis and life-cycle of the vortex using the Regional Atmospheric Modelling System and the Eta model successfully reproduced the vortex formation, its strength and its path towards the northern African coast. The model results showed the importance of the surface fluxes (sensible and latent heat) in the development of the vortex on the rear side of a parent low-pressure system which was advected towards the east. During the mature stage of the vortex, latent-heat release within the convective motions was the dominant mechanism which sustained the vortex until its landfall. Copyright

Numerical Simulations of the Meteorological and Dispersion Conditions during an Air Pollution Episode over Athens, Greece

In this study a summer air pollution episode from 6 to 8 August 1994 over Athens, Greece, is investigated through advanced atmospheric modeling. This episode was reported from the air quality monitoring network, as well as from research aircraft measurements performed during this period for the Transport and Transformation of Air Pollutants from Europe to the East Mediterranean region project. The meteorological conditions prevailing during the period 6‐8 July 1994 are analyzed based on simulations performed with the Colorado State University‐ Regional Atmospheric Modeling System and on the available surface and upper-air observations. Indeed, the synoptic settings induced favorable conditions for the development of local-scale circulations, which defined the poor dispersion conditions over the area. The dispersion of the urban plume of Athens is studied with the use of the Hybrid Particle and Concentration Transport package model. The urban plume of Athens is tracked down the Saronic Gulf and the eastern coast of Peloponnisos, more than 200 km southward from the Athens Basin in good agreement with the research aircraft observations.

Observational evidence of an ozone episode over the Greater Athens Area

Research flights have been performed over the Greater Athens Area (GAA) and southwards over the Island of Aegina and east of Peloponnisos in order to investigate the evolution of an ozone episode over GAA and the transportation of the urban pollution plume southwards from the Athens region. During the 3 day period of 6 July to 8 July 1994, the GAA was under the influence of an ozone episode with ground-level noontime concentrations of more than 120 ppbv. Upper-air ozone concentrations measured during the flights were as high as 135 ppbv. The interaction of the weak synoptic conditions over the area along with the development of a mesoscale thermal circulation created poor dispersion conditions during the period of interest and resulted in elevated ozone levels. The primary pollutants emitted in the GAA during the night and early morning hours, were funneled out to the Saronic Gulf and southwards along the southwestern Aegean Sea, near the coast of east Peloponnisos. Under the influence of strong sunlight these primary pollutants continued to undergo photochemical reaction giving rise to elevated ozone levels tens of kilometers downwind of the pollution emission sources. Further evidence of the photochemically-aged air masses was the high correlation (R2 = 0.8) observed between NOy, and ozone. The ozone production efficiency in these transported air masses reached a value of close to six.

The storm of October 21–22, 1994, over Greece: Observations and model results

During October 21, 1994, a cold front passed over Greece. This frontal passage provoked catastrophic floods and there were many casualties. Eleven deaths were reported during this event, nine of them inside the Greater Athens Area. Significant damages occurred in transportation telecommunication and energy supply networks, especially in the eastern part of the country. This paper reports on the simulations of the observed storm conducted by two numerical models : the Colorado State University-Regional Atmospheric Modelling System (CSU-RAMS) and the η-cta/National Meteorological Center (ETA/NMC) model. The intercomparison of results between a regional research-oriented model (RAMS) with an operational model (ETA/NMC) penrmitted to explore the capabilities and limitations of each one of them. RAMS was operated in a nonhydrostatic mode using explicit microphysics and grid nesting (two nests with 40- and 10-km horizontal grid interval) and provided results which compare favorably with observations, suggesting that the model can adequately represent the mesoscale structure of the system. ETA/NMC is a hydrostatic limited-area model using parameterization of large-scale and convective precipitation. It was operated with 25-km horizontal resolution and it forecasted successfully the major characteristics of the system but failed in reproducing quantitatively the precipitation pattern at the mesoscale.

Flash flood occurrence and relation to the rainfall hazard in a highly urbanized area

The paper examines the flash flood events that occurred over a decade in the Attica prefecture, the most urbanized region of Greece, with the aim of identifying triggering rainfall thresholds, as well as assessing the effect of rainfall upon the magnitude of the induced damages. The analysis incorporates rainfall records from a network of 28 surface meteorological stations and information on the spatial distribution of the flash flood events that is derived from the active database of damaging weather events maintained by the atmospheric modelling group of the National Observatory of Athens. The main findings concern firstly the relation between the flash flood impact, as measured by the fire service operations in flooded properties, and precipitation in various time intervals. In the period 2005–2014, 48 damaging flash flood events occurred in the target area and caused more than 3500 fire service operations in flooded properties. Most of the events are associated with maximum accumulated rainfall of more than 20 mm in 24 h and 3 mm in 10 min. However, the flash flood impact intensity, as measured by the number of the fire service operations per event, increases significantly above the levels of 60 mm in 24 h and 10 mm in 10 min. Secondly, graphs of rainfall intensity versus duration are developed for 15 sub-areas of Attica in order to define rainfall intensity thresholds for flood triggering at a more local level. It is shown that conclusions regarding the reliability of the estimated thresholds should take into account the representativity of the rain gauges, which is determined by the local network’s density, the gauges’ location and record length.

Intra-annual variation of atmospheric static stability in the Mediterranean region: a 60-year climatology

The seasonal characteristics of atmospheric static stability in the Mediterranean region are examined, for the 60-year period 1948–2007 and for the four 15-year sub-periods 1948–1962, 1963–1977, 1978–1992 and 1993–2007. S-Mode and T-Mode Factor Analysis are applied to the mean 5-day values of K static stability index over the Mediterranean region. Three dominant modes are revealed for both, the intra-annual variation and the spatial distribution of K-index. It is found that these modes are connected to the seasonal characteristics of the main atmospheric circulation systems affecting the region and the thermal properties of the Earth’s surface (land or sea). The differences among the results of the four sub-periods partially reflect the inter-decadal variations of the strength of the above factors.

Temporal and Spatial Scales for Transport and Transformation Processes in the Eastern Mediterranean

In several studies during the past, the urban plumes have been extensively considered. In these studies, the spatial and temporal scales of episodic conditions have been described and emphasis was given to the formation and evolution of air pollution episodes within city limits (or in an area covering a few tens of kilometers around the city) and for a time period of one to two days. Moreover, the weather phenomena exhibiting strong diurnal variations (e. g. sea/land-breezes, upslope/downslope and drainage flows, orographic effects, heat islands etc.) were emphasized. The influence of the regional scale phenomena in such cases was not considered on a systematic manner. Actually, the role of phenomena with wavelengths larger than a few tens of kilometers was considered as not important for the formation of a specific air quality over the city of consideration. During the last few years, the influence of regional scale forcing on the formation of specific air quality conditions was found to be important. Kallos et al. (1993) reported that the regional scale phenomena should contribute significantly in the formation of specific air quality conditions in the Greater Athens Area (GAA). Luria et al. (1996) showed that significant degradation of the air quality in some areas should be attributed to regional scale transport phenomena. While the physicochemical properties of various urban plumes have been described at the urban scale with the aid of organized experimental campaigns and/or mesoscale and photochemical modeling (e. g. Ziomas, 1996), not enough attention was paid to the properties of the urban plume as it is passing to areas relatively far from its origin. Consequently, the urban plume impact on remote locations has not been extensively studied. Such phenomena should be considered as very important in some cases, especially in areas with specific characteristics like the Mediterranean Region.

Use of METEOSAT water-vapour images for the diagnosis of a vigorous stratospheric intrusion over the central Mediterranean

The diagnosis of a vigorous dry intrusion over the central Mediterranean is performed using water-vapour images from METEOSAT. This dry intrusion was located on the rear side of a cold front (propagating from Italy to Greece) and played an important role in the onset of thunderstorms over the western Greek coasts. A combination of satellite imagery and potential vorticity analyses showed that the dry air originated in the lower-stratospheric and higher-tropospheric layers. The interaction of the dry air with the moist air masses within the warm conveyor belt ahead of the cold front (overrun of warm air by low equivalent potential temperature air) produced a potentially unstable region over the area of reported thunderstorms. Copyright