Mediterranean cyclones and windstorms in a changing climate

Changes in the frequency and intensity of cyclones and associated windstorms affecting the Medi-terranean region simulated under enhanced Greenhouse Gas forcing conditions are investigated. The analysis is based on 7 climate model integrations performed with two coupled global models (ECHAM5 MPIOM and INGV CMCC), comparing the end of the twentieth century and at least the first half of the twenty-first century. As one of the models has a considerably enhanced resolution of the atmosphere and the ocean, it is also investigated whether the climate change signals are influenced by the model resolution. While the higher resolved simulation is closer to reanalysis climatology, both in terms of cyclones and windstorm distributions, there is no evidence for an influence of the resolution on the sign of the climate change signal. All model simulations show a reduction in the total number of cyclones crossing the Mediterranean region under climate change conditions. Exceptions are Morocco and the Levant region, where the models predict an increase in the number of cyclones. The reduction is especially strong for intense cyclones in terms of their Laplacian of pressure. The influence of the simulated positive shift in the NAO Index on the cyclone decrease is restricted to the Western Mediterranean region, where it explains 10–50 % of the simulated trend, depending on the individual simulation. With respect to windstorms, decreases are simulated over most of the Mediterranean basin. This overall reduction is due to a decrease in the number of events associated with local cyclones, while the number of events associated with cyclones outside of the Mediterranean region slightly increases. These systems are, however, less intense in terms of their integrated severity over the Mediterranean area, as they mostly affect the fringes of the region. In spite of the general reduction in total numbers, several cyclones and windstorms of intensity unknown under current climate conditions are identified for the scenario simulations. For these events, no common trend exists in the individual simulations. Thus, they may rather be attributed to long-term (e.g. decadal) variability than to the Greenhouse Gas forcing. Nevertheless, the result indicates that high-impact weather systems will remain an important risk in the Mediterranean Basin

Upper-tropospheric downstream development leading to surface cyclogenesis in the central Mediterranean

In this study an attempt is made to investigate the upper-tropospheric downstream development over north-west Europe, which leads to surface cyclogenesis in the central Mediterranean. A case study is analysed to demonstrate that the upper-tropospheric downstream development could be closely related to the upper-tropospheric frontogenesis that appears upon the north-eastern flank of a blocking high. The frontogenesis is characterised by a jet streak within a strongly baroclinic zone and a tropopause folding associated with cold stratospheric air intrusion into the troposphere. According to this interpretation, the eddy ageostrophic divergence of eddy geopotential fluxes (dispersion and spreading of eddy kinetic energy), other than friction dissipation and barotropic conversion to the mean flow, is mainly responsible for the loss of Kinetic energy from a decaying depression of synoptic scale that has passed the mature stage. This dispersed eddy Kinetic energy accumulates in the vicinity of the aforementioned jet streak where it is transferred downstream and further triggers the generation or rejuvenation of a new disturbance

Quasi-Lagrangian energetics of an intense Mediterranean cyclone

Studies of the energetics of synoptic-scale systems and similar kinds of investigation have traditionally used a Eulerian framework. In this study, the energetics of a synoptic-scale system have been considered using a quasi-Lagrangian method, in order to isolate the disturbance under consideration within a volume which moves together with the system at each stage of its development. Applying a Lagrangian framework implies that the dimensions of the computational area can be modified on the basis of predetermined criteria. In this study, an area surrounding a depression as shown on the surface analysis, has been selected. This area moves together with the centre of the depression. The energetics results obtained using such a quasi-Lagrangian scheme are compared to those obtained by using a Eulerian framework. The synoptic-scale system studied here is a wintertime frontal depression, the greatest development of which occurred in the central Mediterranean on 7 December 1991. This depression moved east accompanied by significant temperature changes, heavy precipitation and gale force winds

A study of frost events in areas characterised by the absence of observations

Serious events of radiative frost were reported in an important agricultural area of central Macedonia, the valley of Lagadas, at the end of March 1994. Since the phenomenon has a strong local character and no surface observations are available, a simulation is performed using a three dimensional numerical model in order to investigate the horizontal and vertical thermal structure of the lower troposphere during the frost event. The synoptic analysis reveals that the background flaw was weak, associated with anticyclonic circulation and insignificant pressure gradient in the lower troposphere. Under these synoptic conditions, it was demonstrated that the meteorological model represents successfully the temperature and wind fields over the valley of Lagadas during the frost event. It appears that the model can serve as an alternative method to analyse the characteristics of the radiative frost that occurs very frequently over the examined area associated with substantial economic repercussion

Relationship of air quality indicators and synoptic scale circulation at 850 hPa over Athens during 1983-1995

The daily relationship between synoptic circulation in the lower troposphere and air quality conditions is demonstrated over the Athens Metropolitan Area (AMA) for the period between 1983 and 1995. This approach utilises eight distinct synoptic categories at the isobaric level of 850 hPa and air quality indicators with respect to O-3, NO2, CO, SO2 and black smoke (BS). It is demonstrated that high concentrations of photochemical pollutants and SO2 are closely related to the synoptic scale circulation while CO and BS show a weaker correlation. The most favourable synoptic pattern for the accumulation of high concentrations is the open anticyclonic circulation and for the occurrence of extreme events the closed anticyclone. NO2 and BS present the highest frequency of severe events. The seasonal analysis reveals that the bad and extreme air quality conditions due to O-3 and NO2 prevail in the warm season while during the cold season CO and SO2 events prevail. The synoptic category representing a combination of high and low pressure systems, commonly associated with an intense wind flow over the Aegean Sea in the warm period, seems to favour the accumulation of photochemical pollutants, especially O-3, over the AMA. The converse relationship was found for the SO2 case. During cold periods, the synoptic category characterised by ‘zonal flow’ seems to favour the development of bad conditions for NO2 and particularly CO

Seasonal Aspects of an Objective Climatology of Anticyclones Affecting the Mediterranean

Abstract An 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 atmospheric circulation that affect the greater Mediterranean region