International audienc

Abstract. The general objective of the international MEDiterranean EXperiment (MEDEX) was the better understanding and forecasting of cyclones that produce high impact weather in the Mediterranean. This paper reviews the motivation and foundation of MEDEX, the gestation, history and organisation of the project, as well as the main products and scientific achievements obtained from it. MEDEX obtained the approval of World Meteorological Organisation (WMO) and can be considered as framed within other WMO actions, such as the ALPine EXperiment (ALPEX), the Mediterranean Cyclones Study Project (MCP) and, to a certain extent, THe Observing System Research and Predictability EXperiment (THORPEX) and the HYdrological cycle in Mediterranean EXperiment (HyMeX). Through two phases (2000-2005 and 2006-2010), MEDEX has produced a specific database, with information about cyclones and severe or high impact weather events, several main reports and a specific data targeting system field campaign (DTS-MEDEX-2009). The scientific achievements are significant in fields like climatology, dynamical understanding of the physical processes and social impact of cyclones, as well as in aspects related to the location of sensitive zones for individual cases, the climatology of sensitivity zones and the improvement of the forecasts through innovative methods like mesoscale ensemble prediction systems.

Cyclones that appear in the basin of Adriatic Sea strongly influence the climate and weather conditions in the area. It is therefore crucial to classify the different cyclone types in the area since it enhances the understanding and prediction of the related phenomena. In this study, based on the analysis of four year (2002 -2005) operational ECMWF T511 dataset, we classify various types of cyclone tracks as well as isolate the mesocyclogenesis areas in the vicinity of Adriatic basin. Our analysis indicates that four types of cyclogenesis over the Adriatic Sea can be identified: (1) Type A: cyclones connected with pre-existing Genoa cyclones. Two subcategories are found: (I) continuous track: Genoa cyclones crossing over the Apennines to the Adriatic Sea and (II) discontinuous track: new surface cyclones generated over the Adriatic Sea under the influence of a parent cyclone generated in the Gulf of Genoa (Genoa cyclones) but blocked by the Apennines; (2) Type B: cyclones developed in situ over the Adriatic Sea without any connections with other pre-existing cyclones in the surrounding area; (3) Type AB: mixed types A and B cyclones. In this type of cyclones, two cyclones co-exist and stride over the Apennines (twin or eyeglasses cyclones); and (4) Type C: cyclones moving from Mediterranean Sea, but not from the Gulf of Genoa (non-Genoa cyclones). Two subcategories are found: (I) continuous track: a non-Genoa cyclone is able to cross over the Apennines to the Adriatic Sea continuously and (II) discontinuous track: a non-Genoa cyclone is blocked by the Apennines and a new surface cyclone is generated over the Adriatic Sea.

While statistical analyses and observations show that severe bora with maximum gusts exceeding 40 m s−1 can occur in all parts of the Adriatic, the bora research to date has been mainly focused on the dynamics and structure of severe bora in the northern Adriatic. Examined to a significantly lesser degree is a less predictable counterpart in the southern Adriatic, where the Dinaric Alps are higher, broader, and steeper, and where the upwind bora layer is generally less well defined. Identification of the main differences in the sequence of mesoscale and macroscale events leading to the onset of bora in the northern and southern parts of the eastern Adriatic is of fundamental importance for its forecasting. To this end, presented here is a comparative analysis of the evolution and structure of two typical severe cyclonic bora events—one “northern” (7–8 November 1999) and one “southern” (6–7 May 2005) event. The analysis utilizes airborne, radiosonde, and ground-based observations, as well as the hydrostatic Aire Limitée Adaptation Dynamique Developement International (ALADIN/HR) mesoscale model simulations. It is shown that the development of a severe bora in both the northern and southern Adriatic is critically dependent on the synoptic setting to create an optimal set of environmental conditions. For severe bora in the northern Adriatic, these conditions include a strong forcing of the northeasterly low-level jet and pronounced discontinuities in the upstreamflow structure that promote layering, such as lower- to midtropospheric inversions and environmental critical levels. The development of severe bora in the southern Adriatic is crucially dependent on the establishment of a considerably deeper upstream layer that is able to overcome the strong blocking potential of the southern Dinaric Alps. While the upstream layering is less pronounced, it is closely tied to the presence of a cyclone in the southern Adriatic or over the southern Balkan peninsula. The upstream atmospheric layering is shown to strongly modulate bora behavior, and different phases of severe bora, related to the presence or absence of upstream layering, are shown to occur within a single bora episode. Furthermore, the presence of a mountain-parallel upper-level jet aloft appears to impede severe bora development in both the northern and southern Adriatic.

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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Most bora research, until now, considered the appearance of the severe bora only on the northern Adriatic. However, the important topic of this paper is the internationally less known southern Adriatic bora. Preliminary results, including ours, begin to change the traditional view of bora as a local small-scale phenomenon by suggesting that bora-related airflow has a multiscale nature. Here, we present a rare synoptic situation which was characterised by the strong NE flow through the entire depth of the troposphere, which could be responsible for the appearance of the unusually severe wind over the continental part of Croatia and the bora on the southern Adriatic at 27 /29 December 1992. The results of isentropic diagnostics show that the southern Adriatic bora mechanism could not be explained by 2-D hydraulic theory with simpler upstream conditions as was mainly the case on the northern Adriatic. The downstream effects, due to the isallobaric component of the ageostrophic motion, play an essential role in the southern Adriatic bora genesis. This does not diminish the importance of upstream effects on the bora flow. From the theoretical approach, it is considered that upstream conditions for the southern Adriatic bora should be based on 3-D theories and numerical simulation which could explain the flow splitting, divergence and steep lowering of isentropic surfaces.

Most bora research, until now, considered the appearance of the severe bora only on the northern Adriatic. However, the important topic of this paper is the internationally less known southern Adriatic bora. Preliminary results, including ours, begin to change the traditional view of bora as a local small‐scale phenomenon by suggesting that bora‐related airflow has a multi‐scale nature. Here, we present a rare synoptic situation which was characterized by the strong NE flow through the entire depth of the troposphere, which could be responsible for the appearance of the unusually severe wind over the continental part of Croatia and the bora on the southern Adriatic at 27/29 December 1992. The results of isentropic diagnostics show that the southern Adriatic bora mechanism could not be explained by 2‐D hydraulic theory with simpler upstream conditions as was mainly the case on the northern Adriatic. The downstream effects, due to the isallobaric component of the ageostrophic motion, play an essential rôle in the southern Adriatic bora genesis. This does not diminish the importance of upstream effects on the bora flow. From the theoretical approach, it is considered that upstream conditions for the southern Adriatic bora should be based on 3‐D theories and numerical simulation which could explain the flow splitting, divergence and steep lowering of isentropic surfaces.