Vassiliki Kotroni

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.

Lightning occurrence in relation with elevation, terrain slope, and vegetation cover in the Mediterranean

[1]xa0In the frame of this paper, 1-year of lightning data from the experimental network ZEUS operated by National Observatory of Athens is analyzed. The area of interest is the Mediterranean and surrounding countries. At a first stage, the ZEUS data are compared with the data provided by the UK Met Office long-range lightning detection network for the warm period of the year. It was found that although ZEUS system underestimates the nighttime and early morning activity, during the rest of the day ZEUS system detects increased number of lightning compared to ATD that suffers from saturation when the number of flashes exceeds a certain threshold. Furthermore, the possible relationship between lightning and elevation, terrain slope and vegetation is investigated. The analysis showed that during spring and summer there is a positive relationship of lightning activity with elevation, while this feature is not evident during the rest of the year. The lightning activity was found to be positively correlated with the elevation slope throughout the year except winter. As it concerns the vegetation cover, it was found that over bareground the lightning “yield” is low during the whole year, while the inverse is true for woodland areas. During the warm period of the year, due to drying of the Mediterranean surfaces, the forested and wooded areas that keep soil moisture present increased lightning “yield” in contrast with the shrubland areas that, for the same period, present a decreased lightning yield.

Predicting the potential for lightning activity in Mediterranean storms based on the Weather Research and Forecasting (WRF) model dynamic and microphysical fields

[1]xa0A new parameter is introduced: the lightning potential index (LPI), which is a measure of the potential for charge generation and separation that leads to lightning flashes in convective thunderstorms. The LPI is calculated within the charge separation region of clouds between 0°C and −20°C, where the noninductive mechanism involving collisions of ice and graupel particles in the presence of supercooled water is most effective. As shown in several case studies using the Weather Research and Forecasting (WRF) model with explicit microphysics, the LPI is highly correlated with observed lightning. It is suggested that the LPI may be a useful parameter for predicting lightning as well as a tool for improving weather forecasting of convective storms and heavy rainfall.

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

The Meteorological Model BOLAM at the National Observatory of Athens: Assessment of Two-Year Operational Use

Abstract Since November 1999, the hydrostatic meteorological Bologna Limited-Area Model (BOLAM) has been running operationally at the National Observatory of Athens. The assessment of the model forecast skill during the 2-yr period included (a) calculation of the root-mean-square errors (model vs gridded analyses) of geopotential height and temperature at 850 and 500 hPa, (b) evaluation of the models quantitative precipitation forecast skill for the most important events, and (c) evaluation of the model skill in the prediction of surface winds in comparison with buoy observations. Comparison of the verification results with those provided in the literature showed that BOLAM has a high forecast skill, even for precipitation, which is the most difficult parameter to forecast. Especially for precipitation, the comparison between coarse (∼21 km) and fine (∼6.5 km) grid spacing forecasts showed that for the low and medium precipitation amounts, the finer-grid forecasts are not as good as the coarse-grid forec...

Some observational and modeling evidence of long-range transport of air pollutants from Europe toward the Israeli coast

The present paper reports results of a study that attempted to elucidate the factors causing relatively high levels of particulate sulfate that have frequently been observed over central Israel. Aircraft research flights were performed some 70 km west of and parallel to the Israeli coastline during September 1993 and June 1994. Comparison between the two measurement periods revealed a distinctive difference between the two different sampled air masses. While both air masses were nearly homogeneous throughout the measurement period and along the 180 km flight path, the air mass sampled in September 1993 was much cleaner than the air mass sampled during June 1994. The concentrations of the air pollutants measured during the 1993 campaign averaged 0.7 ± 0.4 parts per billion by volume (ppbv) SO 2 , 1.0 ± 0.6 ppbv NO y , 39 ± 7 ppbv O 3 and 38 ± 7 nmol/m 3 particulate sulfate, whereas in the second period the levels averaged 3.0 ± 1.0, 3.9 ± 1.8, 48 ± 9, and 108 ± 63, respectively. These results suggest that the two air masses traveled different paths before reaching the eastern Mediterranean region. Further examination of the air mass sources and transport were performed using the Regional Atmospheric Modeling System for meteorological simulations and the Hybrid Particle and Concentration Transport Package for dispersion modeling. The model simulation showed that during the 1993 measurement period, the pollution sources in southern Europe and the Balkans did not effect the eastern coasts of the Mediterranean, while the synoptic conditions and simulation results for the June 1994 period indicated that the winds over the eastern Mediterranean tended to be northwesterly and thus forcing the polluted air masses toward the coast of Israel.

Statistical evaluation of MPA‐RT high‐resolution precipitation estimates from satellite platforms over the central and eastern Mediterranean

[1]xa0A statistical verification of satellite precipitation estimates has been conducted for a one year period over the Central and Eastern Mediterranean. The NASA real-time Multi-satellite Precipitation Analysis (MPA-RT) data are verified against 73 raingauge data. The verification aims to assess the skill of these satellite estimates to detect rainy areas and to give information on the accumulated precipitation errors. The results show almost unbiased results for the low and medium precipitation thresholds, especially during the wet period of the year. At higher accumulation thresholds, the satellite data overestimate the rain events compared to the raingauges, especially during the dry period of the year, when the major part of precipitation is produced by isolated thunderstorms. Moreover the analysis showed that for the high precipitation amounts and during the whole period the probability of detection is quite low and the false alarm ratio is high (reaching ∼85% during the dry period).

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.