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V. Kotroni and K. Lagouvardos

Abstract

In this paper the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) forecast skill over an area of complex terrain is evaluated. Namely, the model is verified over a period of 1 yr (2002) over the greater area of Athens, Greece, for its near-surface temperature and wind forecasts, at 8- and 2-km grid spacing, but also over a 15-day period for the summer thunderstorm activity forecasts. For the near-surface temperature a cold bias is evident. The model is, in general, unable to reproduce the summer heat waves observed in the area. The increase of the grid resolution, from 8 to 2 km, results in an improvement of the forecast skill. Postprocessing of the forecasts by applying a Kalman-filtering correction method was very effective for both the 8- and the 2-km forecasts. For the forecast skill of wind, the analysis showed that there is not any net increase of the errors with increasing forecast time for the 48-h forecast period, the mean absolute errors, in general, present the lowest values at noontime, and the increase in resolution, from 8 to 2 km, results in a slight decrease of these errors. The analysis of the model skill to accurately forecast summertime precipitation showed that the 2-km simulations, without activation of the convective parameterization scheme, were unable to reproduce the observed thunderstorm activity. Sensitivity tests for the same period with simulations in which the convective parameterization was not activated for both the 8- and the 2-km simulations were still inaccurate, while activation of the convective parameterization scheme at all grids (even at 2 km) considerably increased the precipitation forecast skill.

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K. Lagouvardos and V. Kotroni
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V. Kotroni and C. Amory-Mazaudier

Abstract

Sodar friction velocities, obtained during frontal events traversing areas characterized by different orgraphic and canopy conditions (flat, bare ground, small hills and valleys with agricultural crops and trees, agricultural crops and forest on a flat ground, bare ground on the side of a mountain), are compared in order to identity the influence of topography on this parameter. For some case studies, sounding and sodar data are combined in order to provide a relation between the friction velocity and the low-level jet presence. For the cases analyzed in this paper, the following results are obtained: the frontal passage is associated with a decrease of the horizontal wind speed (about 50% in magnitude) in the surface layer, and an increase of the friction velocity before the frontal passage followed by a decrease just at the time of the frontal passage or with a little delay. Friction velocity is more intense in the cold side of the low-level jet and its maximum represents 2% of the low-level jet maximum magnitude. As it concerns the influence of the terrain conditions on friction velocity, mountain effects yield to more intense friction-velocity values and to a superposition of an oscillating behavior on the time variation of friction velocity, while forests effects induce a shift of the frontal signature an the time variation of friction velocity at higher height levels.

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V. Kotroni, G. Kallos, K. Lagouvardos, M. Varinou, and R. Walko

Abstract

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.

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D. K. Katsanos, K. Lagouvardos, V. Kotroni, and A. A. Argiriou

Abstract

In this paper, the relationship of lightning activity in the central and eastern Mediterranean with the 85-GHz polarization-corrected temperature (PCT) and radar reflectivity provided by the Tropical Rainfall Measuring Mission (TRMM) satellite is investigated. Lightning observations were mainly provided by the Met Office’s Arrival Time Difference system as well as by the TRMM Lightning Imaging Sensor. The studied period spans from September 2003 to April 2004 and focuses on the events with the most important lightning activity. It was found that 50% of the cases with flashes have PCTs lower than 225 K, while only 3% of the “no lightning” cases have PCTs below this value. Further, if PCT is used as a proxy for the presence of lightning, the value of 217 K gives the best statistical scores for the presence of at least one observed flash. In addition, the ratio of cloud-to-ground lightning to total lightning activity has higher values for the “colder” PCT values and decreases as PCT increases. In addition, the mean and maximum reflectivity profiles with collocated lightning are from 3 to 10 dB and from 6 to 15 dB, respectively, higher than that without lightning. Further, a reflectivity profile with values greater than 53 dBZ in the low levels (below 3 km), of ∼45 dBZ at 5 km and 40 dBZ at 7 km is associated with a probability of 80% for lightning occurrence.

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K. Lagouvardos, V. Kotroni, T. M. Giannaros, and S. Dafis

Abstract

On 23 July 2018, Attica, Greece, was impacted by a major wildfire that took place in a wildland–urban interface area and exhibited extreme fire behavior, characterized by a very high rate of spread. One-hundred civilian fatalities were registered, establishing this wildfire as the second-deadliest weather-related natural disaster in Greece, following the heat wave of July 1987. On the day of the deadly wildfire, a very strong westerly flow was blowing for more than 10 h over Attica. Wind gusts up to 30–34 m s−1 occurred over the mountainous areas of Attica, with 20–25 m s−1 in the city of Athens and surrounding suburban areas. This strong westerly flow interacted with the local topography and acted as downslope flow over the eastern part of Attica, with temperatures rising up to 39°C and relative humidity dropping to 19% prior to the onset of the wildfire. These weather elements are widely acknowledged as the major contributing factors to extreme fire behavior. WRF-SFIRE correctly predicted the spatiotemporal distribution of the fire spread and demonstrated its utility for fire spread warning purposes.

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N. Mazarakis, V. Kotroni, K. Lagouvardos, and A. A. Argiriou

Abstract

Lightning activity over Greece during the warm season (May–September) of the years 2003–06 is investigated in relation to the synoptic meteorological conditions that prevailed in the region. The study is based on the use of cloud-to-ground lightning activity data from the Met Office Arrival Time Difference system and upper-air analyses from the European Centre for Medium-Range Weather Forecasts. Analysis of the spatial variability of lightning shows that the highest “relative” flash densities are observed in northern and western Greece and in central and western Peloponnissos. The relative flash density is correlated with elevation: it increases with elevation along the slopes of terrain features. The study of the synoptic patterns related to lightning is based on the analysis of 60 active days and 60 inactive days in terms of lightning activity over Greece. The days with high lightning activity are characterized by a short-wave trough at the 500-hPa level over the Ionian Sea. On the other hand, during the days with no lightning, a northwest flow prevails over Greece. It was also found that high lightning activity is related to high values of absolute vorticity, equivalent potential temperature, and convective available potential energy.

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K. Lagouvardos, A. Karagiannidis, S. Dafis, A. Kalimeris, and V. Kotroni

Abstract

During 15–21 September 2020, an intense medicane, named Ianos, formed over the warm Mediterranean Sea. Following a path of approximately 1,900 km, Medicane Ianos affected Greece resulting in four casualties and devastating damage in the western and central parts of Greece. Persistent gale force 1-min winds up to 44 m s−1 and wind gusts up to 54 m s−1 were recorded in Cephalonia Island (Ionian Sea), while record-breaking amounts of accumulated rainfall have been recorded in several Ionian islands, as well as in parts of central Greece. Analysis of the available observations showed that Ianos was the most intense medicane ever recorded in the Mediterranean. This paper aims at investigating the genesis and evolution of the medicane, based on in situ observations, satellite measurements, and model analyses. Toward that objective, Meteosat Second Generation (MSG) SEVIRI imagery, combined with lightning data permitted to follow the evolution of convective activity during the various phases of Ianos. This investigation is complemented with upper-air model analyses in order to evaluate the synoptic environment within which Ianos had formed and was sustained over 7 days. Finally, the Global Precipitation Measurement Core Observatory (GPM CO) satellite overpasses over Medicane Ianos provided invaluable information about its 3D structure, especially during its most intense phase.

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V. Kotroni, K. Lagouvardos, E. Defer, S. Dietrich, F. Porcù, C. M. Medaglia, and M. Demirtas

Abstract

In the frame of this work, the storm that occurred on 5 December 2002 in Antalya, located on the southwestern Mediterranean Sea coast of Turkey, is analyzed. More than 230 mm of 24-h-accumulated rainfall have been reported during the event that produced floods in the area. The analysis is based on the results of model simulations with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Observational data provided by the Tropical Rainfall Measuring Mission (TRMM) sensors (including the Lightning Imaging Sensor and TRMM Microwave Imager), Special Sensor Microwave Imager (SSM/I), Meteosat-7, and Met Office Arrival Time Difference (ATD) lightning network are used for both the comparison with the model results and also for the characterization of the storm. The synergetic use of all of this information was crucial for the description of the event. The maximum of precipitation was associated with the warm and moist air masses driven by a low-level jet over the area and impinging over the orographic barriers. The improvement of representation of the humidity field in the model initial conditions, through a simple technique of humidity adjustment based on satellite rainfall estimates, resulted in an improvement of the prediction of the timing and quantity of the precipitation maxima during the event.

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K. Lagouvardos, V. Kotroni, A. Koussis, H. Feidas, A. Buzzi, and P. Malguzzi

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 model's 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 forecasts. For the large precipitation amounts, the calculated statistical scores provide only little support of the idea that the fine-grid forecasts are better than those of the coarse grid because the fine-grid forecasts give better scores only for the quantity bias and the mean absolute error.

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