The northwest african depressions affecting the south balkans

Prezerakos, N. G.

doi:10.1002/joc.3370050606

Cited by 37 publications

(38 citation statements)

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“…According to the study, winter tracks are oriented northeast to north, characterised by cyclone protrusion to the Mediterranean Sea over the Tunisia area, while spring tracks tend to have a more pronounced eastern advection component. Probably the most extensive work on the description of synoptic conditions associated with NWA depressions was done by PREZERAKOS (1985PREZERAKOS ( , 1990 and PREZERAKOS et al (1990). In these studies, a detailed classification of different types of NWA depressions was established and their main characteristics discussed.…”

Section: Introductionmentioning

confidence: 99%

A numerical study of the first phase of a deep Mediterranean cyclone: Cyclogenesis in the lee of the Atlas Mountains

Horvath¹,

Fita²,

Romero³

et al. 2006

metz

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The initiation of a deep and severe impact Mediterranean cyclone in the lee of the Atlas Mountains is investigated by a series of numerical experiments using the MM5 forecast model. The roles of orography, surface sensible heat flux and upper-level potential vorticity anomaly are identified using the factor separation method. In addition, a sensitivity experiment addressing the role of a thermal anomaly in the lee of the Atlas is performed. The results of model simulations show that orography blocking is responsible for the generation of a low-level shallow vortex in the first phase of the lee development. An upper-level potential vorticity anomaly is the principal ingredient of this event, responsible for a dominant deepening effect in the later stage of lee formation. The analysis of the cyclone paths shows that orography tends to keep the cyclone stationary, while upper-level dynamical factors prove crucial for the advection of the system to the Mediterranean Sea. The most noteworthy influence of surface sensible heat flux is identified as an afternoon destruction of the surface baroclinic zone and the associated weaker cyclogenesis. Furthermore, it is shown that the thermal anomaly in the lee of the Atlas builds up rather quickly and tends to be responsible for the cyclone initiation positioning in the mountain lee. Zusammenfassung Die Anfangsphase einer großen Mittelmeerzyklone mit starkem Einfluss im Lee des Atlasgebirges wird in einer Reihe numerischer Experimente untersucht. Dabei wird das prognostische Modell MM5 angewendet. Die Rolle der Orographie, des Oberflächenflusses sensibler Wärme und der Anomalie der potenziellen vorticity wurde mithilfe der Factor Separations Methode ermittelt. Außerdem wurde ein Sensitivitätstest durchgeführt, um die Rolle einer thermischen Anomalie im Lee des Atlasgebirges festzustellen. Die Resultate der Modellsimulationen zeigen, dass die orographische Blockierung für das Entstehen eines flachen Bodenwirbels in der ersten Phase der Entwicklung einer Leezyklone verantwortlich ist. Eine Anomalie der potenziellen vorticity ist der Hauptbestandteil dieses Prozesses und verantwortlich für den dominanten Vertiefungseffekt in der späteren Phase der Leeentwicklung. Die Analyse der Trajektorie der Zyklone zeigt, dass die Orographie eine Tendenz dazu hat, die Zyklone stationär zu halten, während die dynamischen Höhen-faktoren für die Advektion des Systems zum Mittelmeer hin ausschlaggebend sind. Als wichtigster Einfluss des Oberflächenflusses sensibler Wärme wurde der nachmittägliche Abbau der baroklinen Bodenzone und die dazugehörige schwächere Zyklogenese identifiziert. Weiterhin wird gezeigt, dass sich die thermische Anomalie im Lee des Atlasgebirges ziemlich schnell entwickelt und für das anfängliche Positionieren der Zyklone im Lee des Gebirges verantwortlich ist.

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Section: Introductionmentioning

confidence: 99%

A numerical study of the first phase of a deep Mediterranean cyclone: Cyclogenesis in the lee of the Atlas Mountains

Horvath¹,

Fita²,

Romero³

et al. 2006

metz

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“…These patterns start 72 h and end 24 h before the first appearance of the depressions on the 0000 or 12.00 GMT surface charts. The first paper (Prezerakos, 1985b) dealt with the basic characteristics of northwest African depressions, from the time of their generation onward. In that paper, attention was drawn to the following features:…”

Section: Introductionmentioning

confidence: 99%

Synoptic flow patterns leading to the generation of north‐west African depressions

Prezerakos¹

1990

Intl Journal of Climatology

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This paper reports a study and analysis of the synoptic evolution of the atmospheric flow patterns that cause the formation of north-west African depressions. The study uses mean 500 and 850 hPa heights and their anomalies, associated with the subcategories into which the north-west African depressions have been classified. The time period under consideration covers the consecutive years from 1970 to 1974.To study better the synoptic evolution and the separation of existing centres of action for each subcategory, charts of the mean 500 and 850 hPa heights and temperatures for 72,48, and 24 h before the first appearance of a depression have been prepared. For the mean 500 hPa height field and for each subcategory, anomalies based on differences from the longterm (1950-1973) average have been calculated (at the 0.01 statistical level of significance) for each grid point of the 'anomaly' fields. From the study as a whole it can be deduced that the synoptic flow patterns, which cause the generation of north-west African depressions, are unstable baroclinic waves, at the synoptic scale, initiated in the middle troposphere over geographical regions far from the Atlas Mountains. These waves can be seen on the 500 hPa charts at least 72 h before the first appearance of the depressions on the mean sea-level (MSL) charts for north-west Africa. It also seems that the intensive low-level tropospheric baroclinicity, which is most pronounced along the north-west African coast, plays an important role in the initiation and self-maintenance of the surface lows. Finally, the key atmospheric circulation system that causes these baroclinic waves to form must be a large-scale ridge, often of a blocking type, located in the North Atlantic Ocean or north-west Europe.

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“…As is known (Meteorological Office, 1962;Karoulias, 1976;Prezerakos, 1985;Flokas, 1988;Georgiev and Sharov, 1990;Prezerakos, 1991), these depressions appear mainly in spring and affect mostly the weather of the Balkans. The two October cases occurred in second half of October, whereas the August case occurred mid-month as the occurrence date shows.…”

Section: Statistical Characteristicsmentioning

confidence: 96%

CLimatological characteristics of very low humidity in Northern Crete due to Downslope southerly winds

Prezerakos

1994

Intl Journal of Climatology

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In this paper an attempt is made to study the climatological characteristics of very low relative humidity (below or equal to 10 per cent) in northern Crete, especially at Heraklio, by utilizing data from a 34-year period (19561989). Specifically, after selecting the very low humidity days based on a previously determined definition and the synoptic situation with which they are associated, the following aspects were examined: (i) the interannual and monthly variation, (ii) the mean duration of very low humidity occurrence on each day at Heraklio and its monthly frequency, (iii) the mean values of the meteorological elements (temperature, wind, wind gusts, pressure, total and low-cloud amounts, and visibility) for the time of synoptic observations reporting very low humidity at Heraklio and for the corresponding times of the day gust before it for both Heraklio and Tibaki, that is, on the northern and southern coasts of Crete. Also by comparing Heraklio's and Tibaki's mean values of the meteorological elements it was possible to determine the contribution of the Cretan relief characteristics to the very low humidity occurrence at Heraklio. KEY WORDS Very low humidity Fohn effect Downslope winds Crete

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The northwest african depressions affecting the south balkans

Cited by 37 publications

References 1 publication

A numerical study of the first phase of a deep Mediterranean cyclone: Cyclogenesis in the lee of the Atlas Mountains

A numerical study of the first phase of a deep Mediterranean cyclone: Cyclogenesis in the lee of the Atlas Mountains

Synoptic flow patterns leading to the generation of north‐west African depressions

CLimatological characteristics of very low humidity in Northern Crete due to Downslope southerly winds

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