J. Kouroutzoglou

On Cyclonic Tracks over the Eastern Mediterranean

Abstract In this study, an updated and extended climatology of cyclonic tracks affecting the eastern Mediterranean region is presented, in order to better understand the Mediterranean climate and its changes. This climatology includes intermonthly variations, classification of tracks according to their origin domain, dynamic and kinematic characteristics, and trend analysis. The dataset used is the 1962–2001, 2.5° × 2.5°, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The identification and tracking of the cyclones was performed with the aid of the Melbourne University algorithm. It was verified that considerable intermonthly variations of track density occur in the eastern Mediterranean, consistent with previous studies for the entire Mediterranean, while further interesting new features have been revealed. The classification of the tracks according to their origin domain reveals that the vast majority originate within the examined area itself, mainly in the Cyprus...

The ability of a barotropic model to simulate sea level extremes of meteorological origin in the Mediterranean Sea, including those caused by explosive cyclones

Storm surges are responsible for great damage to coastal property and loss of life every year. Coastal management and adaptation practices are essential to reduce such damage. Numerical models provide a useful tool for informing these practices as they simulate sea level with high spatial resolution. Here we investigate the ability of a barotropic version of the HAMSOM model to simulate sea level extremes of meteorological origin in the Mediterranean Sea, including those caused by explosive cyclones. For this purpose, the output of the model is compared to hourly sea level observations from six tide gauge records (Valencia, Barcelona, Marseille, Civitavecchia, Trieste, and Antalya). It is found that the model underestimates the positive extremes significantly at all stations, in some cases by up to 65%. At Trieste, the model can also sometimes overestimate the extremes significantly. The differences between the model and the residuals are not constant for extremes of a given height, which limits the applicability of the numerical model for storm surge forecasting because calibration is difficult. The 50 and 10 year return levels are reasonably well captured by the model at all stations except Barcelona and Marseille, where they are underestimated by over 30%. The number of exceedances of the 99.9th and 99.95% percentiles over a period of 25 years is severely underestimated by the model at all stations. The skill of the model for predicting the timing and value of the storm surges seems to be higher for the events associated with explosive cyclones at all stations.

Seasonal Aspects of an Objective Climatology of Anticyclones Affecting the Mediterranean

AbstractAn objective climatology of anticyclones over the greater Mediterranean region is presented based on the Interim ECMWF Re-Analysis (ERA-Interim) for a 34-yr period (1979–2012) and the Melbourne University automatic identification and tracking algorithm. The scheme’s robustness and reliability for the transient extratropical propagation of anticyclones, with the appropriate choices of parameter settings, has been established and the results obtained here present new research perspectives on anticyclonic activity affecting the Mediterranean. Properties of Mediterranean anticyclones, such as frequency, generation and dissipation, movement, scale, and depth are investigated. The highest frequency of anticyclones is found over continental areas, while the highest maritime frequency occurs over closed basins exhibiting also maxima of anticyclogenesis. There is a significant seasonality in system density and anticyclogenesis maxima, this being associated with the seasonal variations of the larger-scale a...

On the Dynamics of Mediterranean Explosive Cyclogenesis

In this study, a first attempt is made to examine the mechanisms contributing to the explosive cyclogenesis in the Mediterranean basin during the cold period of the year. Surface explosive cyclones and their vertical characteristics were defined with the aid of the University of Melbourne Cyclone Tracking and Vertical Tracing Software, using the 6-hourly ERA-40 datasets at the resolution of 1° × 1° for a 40-year period (1962–2001), separately for the western (WM), central (CM) and eastern Mediterranean (EM). Composite anomalies of various thermodynamic parameters verified the strong baroclinic character of the phenomenon and the decisive role of the upper level dynamics during the time of explosive cyclogenesis. Moreover, an interesting differentiation characterizes the low level thermodynamic structure of the WM and mainly CM cases in relation to the EM ones, since explosive cyclogenesis in the former areas seem to occur as cold low – level air warms from below, penetrating the Mediterranean from NW, while in the EM cold and high – PV upper level air moves over areas of warm and moist low level environment. Nevertheless, a synergy of the lower and the upper levels is implied for all parts of the Mediterranean basin with different relative importance.

Identification of the development mechanisms of an explosive cyclone in the central Mediterranean with the aid of the MSG satellite images

The development mechanisms of an explosive cyclone over Central Mediterranean are examined, by relating the cloud patterns in the infrared, water vapour and visible channels of the Meteosat Second Generation (MSG) satellite images, to the surface-upper air operational analyses and thermodynamic parameters, including potential and geostrophic vorticity analyses, potential and equivalent potential temperature, static stability and thermal heat fluxes. The specific case study derived from an updated climatology of Mediterranean explosive cyclones for the 2002-2010 period, being performed with the aid of the University of Melbourne Cyclone Tracking scheme (MS algorithm) and ERA-INTERIM datasets. It was found that during the ordinary cyclogenesis the increased mid-upper level relative humidity over Northern Algeria along with the enhanced moisture gradient in the area between Portugal and Spain; indicate the existence of a jet -streak, with the dry zone to be located on the cold side of the jet axis. The descent of the dry air is more pronounced within the left exit region of the jet streak in the poleward portion of an upper confluence zone. Six hours later, the eastward movement of the cloud pattern in the Northern African coasts significantly resembles the structure of a baroclinic leaf, being related to the early stages of surface frontogenesis, due to the deformation process within a strong wind field. During the time of rapid deepening, when the surface cyclone propagated from the Northern African coasts towards the area of Sicily - Gulf of Taranto, the development is characterised by the transition from the baroclinic leaf structure to the comma cloud one and finally the formation of a bent-back occlusion.

Assessing the Sensitivity of COSMO/GR Atmospheric Model to Effectively Simulate the Influence of Diabatic Heating on Eastern Mediterranean Explosive Cyclogenesis Under Different Parameterizations of the Model Physics

Mediterranean explosive cyclogenesis is the result of the interaction between upper level baroclinic and low level diabatic processes. This interaction presents significant differences in specific Mediterranean sub-areas and in different cases of rapid deepening, especially regarding the evolution of the diabatic processes. In this study, the regional atmospheric model COSMO-GR is used for the evaluation of the spatial and temporal variations of low level diabatic heating in a case of explosive cyclogenesis in the north Aegean Sea. Model runs were performed for a series of different values of the model parameter sea roughness, which has proved to significantly affect the simulation of the diabatic processes. It was found that smaller values of the above parameter compared to the default parameterization, lead to significant enhancement of the model calculated surface turbulent fluxes and the respective surface deepening rates during the explosive deepening period and thus to lower minimum pressures, which are closer to ERA-Interim reanalysis values. Moreover, the intrusion of the diabatic heating in the middle troposphere due to latent heat release was also examined, demonstrating the necessity for the determination of the most suitable parameterization of the model physics.

Comparison of the Synoptic Environment and Mechanisms of Two Cases of Surface Cyclogenesis in Greek Area Associated to Strong Low-Level Cold Surges

In this study the synoptic environment and thermodynamic mechanisms of two cases of surface cyclogenesis in the south Ionian and Aegean Seas, associated to strong low-level cold surges, are analyzed. The cases are characteristic examples of two different synoptic categories, affecting the weather in the Greek area. The analysis was performed using the ERA-Interim dataset on a 0.5° × 0.5° regular latitude-longitude grid. In the first case, the passage of a frontal depression from North Greece caused the extension of a cold anticyclone from NW Europe towards Greece. Then, the formation of a surface frontal depression is triggered over the south Ionian Sea. In the second case, the eastward movement of a strong frontal depression from North Europe along with a secondary frontogenesis in East Europe favored surface cyclogenesis over the north Aegean Sea. The depression propagated towards the south Aegean Sea, combined with the extension of a cold anticyclone over the Balkans. Although upper-level baroclinic processes significantly affected the formation of the surface depressions in both cases, the two cases are characterized by differences at the low levels, i.e. strengthening of low-level baroclinicity and convergence in the first case and low-level cold advection from the north in the second.

Study of Cold Anticyclones Generating in the Mediterranean

A comprehensive climatology of anticyclones affecting the Mediterranean has been assembled employing the objective finding and tracking scheme of the University of Melbourne (MS). The identified anticyclones have been characterized as cold or warm on the basis of their thermal structure at the lower isobaric levels with the aid of an extension module of MS that enables the examination of the vertical thermal extent of synoptic systems. The aim of the present study is to get a perspective of the processes involved in the formation of surface anticyclones. The mean and anomaly fields and the system-centered composites of several atmospheric variables are studied throughout the troposphere, taking under consideration the position of the systems and the season of the year at the time of formation. The present analysis is confined to cold-core systems and focuses on the areas of anticyclogenetic maxima in order to ensure the statistical robustness. The synergy of the low-level cooling and the upper-level dynamics is confirmed for the northern examined areas for both seasons, while anticyclogenesis over central and eastern African areas seems to be mainly governed by thermal processes.