Dissertation committee:
Έλενα Φλόκα Αναπλ. Καθηγήτρια (Επιβλέπουσα) , Γεώργιος Θεοχαράτος Αναπλ. Καθηγητής, Χαρά Μιχαλοπούλου Επίκ. Καθηγήτρια
Summary:
In this dissertation explosive cyclogenesis in Mediterranean was studied for
the first time on a climatological basis with the use of objective methods for
the specification of the surface tracks and their vertical extension in the
middle and upper troposphere. It was examined the spatial and temporal
distribution of the frequency and other diagnostic parameters of the explosive
cyclones and their trends, as well. It was also studied the kinematic
characteristics of these depressions from cyclogenesis until their cyclolysis.
Moreover it was examined the effect of the spatial resolution of mean sea level
pressure in the characteristics of the surface explosive cyclones and compared
with the respective of the ordinary depressions in Mediterranean. Finally it
was examined the dynamic and thermodynamic mechanisms in the lower and upper
troposphere which favor the appearance of the phenomenon analyzing the
interaction between the baroclinic and the respective diabatic processes.
In this dissertation it was used the algorithm for the detection of the
cyclonic centers and their tracks and the respective algorithm of the detection
of the vertical structure of the depressions, which were developed at the
University of Melbourne and were properly modified for Mediterranean, which
constitutes a closed maritime basin with complex topography. As input data were
used the ERA40 files of the European Centre for Medium Range Weather Forecasts
(ECMWF) for the 1962-2001 time period, with spatial resolution of 2.5x2.5 and
1x1, respectively.
Explosive cyclones in Mediterranean represent a relatively rare phenomenon
which appears mainly over maritime surfaces during winter in the Western and
Central Mediterranean, with maximum during December, without statistically
significant trend of their frequency for the period of study. The areas of
maximum frequency include the northern strongly baroclinic maritime – coastal
regions of Mediterranean from the Balearic Islands up to the Adriatic and the
Tyrrhenian Sea, while secondary maxima appear in the Ionian and the Aegean Seas
as well as in the area of NW. Africa. Explosive cyclones of greater spatial
scale and depth appear in the region between Sicily, Tyrrhenian and Ionian Sea
and the area of Syrte, while the depressions of smaller spatial scale and
Laplacian appear in the northern maritime - coastal regions of Mediterranean,
with the effect of orography to play a significant role in the generation and
identification from datasets of high spatial resolution, of such strong
cyclones of smaller scale in the above regions.
The comparison of the explosive cyclone samples based on data of higher and
lower spatial resolutions from the same assimilation model and specifically the
ERA-40 model, proved the clear effect of the spatial resolution in the
detection of the explosive cyclone tracks and the necessity for the application
of high resolution datasets in areas with complex topography and appearance of
smaller spatial and temporal scale cyclones, like the Mediterranean basin.
The comparison of the explosive towards the ordinary cyclogenesis in
Mediterranean showed that, contrary to the explosive cyclones, the frequency of
the ordinary cyclones is maximized during spring, while present smaller
duration of lifetime and track lengths, as well as smaller mean deepening
rates.
It was proved that the explosive cyclones constitute cyclonic systems with a
well-organized vertical structure, while their baroclinic character is
verified. It was also proved that for the majority of the surface cyclones, the
500hPa cyclones were formed before the surface cyclones, while about 80% of the
explosive cyclone track steps extended to the lower troposphere, implying that
the upper level dynamics contributes to the explosive cyclogenesis.
Nevertheless, a significant percentage of the surface cases were formed before
the respective at 500hPa, implying a tendency for antecedent surface
cyclogenesis and stressing at the same time the low level dynamics, such as the
surface thermal fluxes, the diabatic heating, the low static stability and the
orography.
The extended analysis of the dynamic and thermodynamic characteristics of the
low and upper troposphere and the comparison of these characteristics between
the Mediterranean subregions showed a tendency for stronger upper level
baroclinic structure in Western and Central Mediterranean areas, along with the
existence of a jet-streak. The upper level baroclinicity is verified for the
cases in EM too, although weaker than the two other areas. The intrusion of dry
and cold stratospheric air in the upper troposphere was verified for all
regions. Moreover, it was observed the existence of cold environment in the low
troposphere in the Western and Central Mediterranean, while on the contrary in
EM the pre-existed environment is warm and moist. Nevertheless, in all areas
was shown the effect of latent heat release due to water vapor condensation,
which for the Western and Central Mediterranean constitutes a factor which
warms and makes dynamically unstable the relatively colder environment in the
lower troposphere.
Enhancement of sensible and latent heat release observed in almost all the
Mediterranean subareas during the time of explosive cyclogenesis, while this
enhancement was more intense in the area of Eastern Mediterranean.
The approach of the relationship between the surface explosive cyclogenesis in
Mediterranean with the appearance of the 500hPa blocking situations, showed
that a ridge of a blocking form is located mainly to the west of the respective
surface cyclones position during the time of explosive cyclogenesis, proving
the upper level dynamics on the basis of the generation of upper level cyclonic
downstream development on the eastern flanks of the ridge. Finally it is
concluded that explosive cyclogenesis in Mediterranean results from the
interaction between baroclinic and diabatic processes.
Keywords:
Explosive cyclogenesis, Diabatic processes, Potential vorticity, Surface rapid deepening, Upper level dynamics