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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