Mediterranean Cyclones Research Articles

Local and Time Changes over a 66-Year Period and Annual Relocation of Saudi Arabian Subtropical High Pressure

Saudi Arabian subtropical high pressure is a major system affecting general circulation of the atmosphere of west Asia. Its annual relocation affects the relocation of other systems in the area, such as Mediterranean cyclones, Sudanese low pressure areas, and west wind waves. This system is known to relocate to the south and north in response to outward solar relocation, but the reasons behind its eastern and western relocation have not been studied thoroughly. The present study examined 1000 and 850 HPa levels over the course of 66 years (1948-2015) to determine a pattern of latitudinal and longitudinal relocation of the system using synoptic maps. The research showed that, after 2008, high pressure latitudinal and longitudinal swings were larger than in previous years and the annual high pressure relocation was not in concord with the apparent motion of the sun. At the onset of autumnal moderation, the high pressure core was positioned to the north of Saudi Arabia (22° - 30° north latitude and 42° - 50° south longitude). Southern movement continued until the end of March, when the core again relocated to the north. These relocations first occurred slowly, but the northern relocation occurred very quickly from May to June, such that the core moved northward 22° to 30°. After June, the core did not relocate much until the end of September. After September, it relocated strongly south in all time periods. It was noted that high pressure over Saudi Arabia had two cores from June to September in some years; in others the high pressure core was in southwestern Iran and Iraq. In still others, a southwest to northeast high pressure tab entered Iran from the southwest (Khuzestan) and continued northward with a core forming in the northern Caspian Sea.

Projected changes in medicanes in the HadGEM3 N512 high-resolution global climate model

Medicanes or “Mediterranean hurricanes” represent a rare and physically unique type of Mediterranean mesoscale cyclone. There are similarities with tropical cyclones with regard to their development (based on the thermodynamical disequilibrium between the warm sea and the overlying troposphere) and their kinematic and thermodynamical properties (medicanes are intense vortices with a warm core and even a cloud-free eye). Although medicanes are smaller and their wind speeds are lower than in tropical cyclones, the severity of their winds can cause substantial damage to islands and coastal areas. Concern about how human-induced climate change will affect extreme events is increasing. This includes the future impacts on medicanes due to the warming of the Mediterranean waters and the projected changes in regional atmospheric circulation. However, most global climate models do not have high enough spatial resolution to adequately represent small features such as medicanes. In this study, a cyclone tracking algorithm is applied to high resolution global climate model data with a horizontal grid resolution of approximately 25 km over the Mediterranean region. After a validation of the climatology of general Mediterranean mesoscale cyclones, changes in medicanes are determined using climate model experiments with present and future forcing. The magnitude of the changes in the winds, frequency and location of medicanes is assessed. While no significant changes in the total number of Mediterranean mesoscale cyclones are found, medicanes tend to decrease in number but increase in intensity. The model simulation suggests that medicanes tend to form more frequently in the Gulf of Lion–Genoa and South of Sicily.

Simulation of temperature and precipitation climatology for the CORDEX-MENA/Arab domain using RegCM4

The performance of a regional climate model RegCM4.3.4 (RegCM4) in simulating the climate characteristics of the Middle East and North Africa (MENA) region has been evaluated. The simulations carried out in this study contribute to the joint effort by the international regional downscaling community called Coordinated Regional climate Downscaling Experiment (CORDEX). The model has been forced with the boundary conditions obtained from the global reanalysis dataset ERA-Interim for the period 1979–2010. An east–west cold bias is found in the northern part of the MENA domain in RegCM4 that is absent in the ERA-Interim driving forcings, whereas a large warm bias is found over the southern Arabian Peninsula (Yemen/Oman) for both RegCM4 and ERA-Interim. The possible causes leading to the warm bias over Yemen/Oman in the RegCM4 are discussed. The model performed well in capturing the salient features of precipitation which includes ITCZ, Mediterranean cyclones as well as precipitation minima over the deserts. Moreover, the annual cycles of precipitation and mean temperature over the prominent river basins of the region have been ably captured by the model. Temperature-precipitation relationship revealed that the ERA-Interim driving forcings stay closer to the observations; however, RegCM4 remains competent for most of the Koppen-Geiger climate classification types. Performance of the model in capturing the near surface winds and specific humidity is also presented. Based on the results of this study, it is concluded that RegCM4 is well suited to conduct long-term high-resolution climate change projection for the future period over the CORDEX-MENA/Arab domain.

Processes leading to heavy precipitation associated with two Mediterranean cyclones observed during the HyMeX SOP1

Two deep cyclones occurred in the Mediterranean between 25 and 31 October 2012, during the first Special Observation Period (SOP1) of the Hydrological cycle in the Mediterranean Experiment (HyMeX). Both cyclones were associated with extreme rainfall covering a large part of the western Mediterranean Sea, where 24 h accumulated precipitation measurements exceeded 150 mm. We combine complementary observations from airborne radar and lidar systems, ZEUS lightning detection network and meteorological surface stations along with satellite diagnostics on deep convection, for a detailed microphysics and (thermo‐)dynamical analysis of the two extreme rainfall cases. In addition, we use operational analysis data from the European Centre for Medium‐range Weather Forecasts (ECMWF) for analysing the synoptic conditions and diagnosing strongly ascending air masses in the vicinity of the cyclones, so‐called warm conveyor belts (WCBs). The analysis revealed the different physical characteristics of the two cyclones responsible for the extreme rainfalls. Both cyclones were associated with a WCB and a comma cloud, but deep convection, intense lightning and very cold cloud tops occurred only for the first case while the second cyclone was mostly associated with stratiform rainfall, a strong WCB, and only a few embedded cells of deep convection.

Mediterranean cyclone characteristics related to precipitation occurrence in Crete, Greece

Abstract. The characteristics of the cyclone tracks that caused precipitation events of variable intensity for the period 1979–2011 over the island of Crete are presented. The data set used for cyclone identification is the 0.5° × 0.5°, 30 years European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim mean sea-level pressure. Cyclone characteristics are calculated with the aid of the Melbourne University algorithm (MS scheme). Daily precipitation data from a dense gauging network over the island of Crete are also used for the classification of the precipitation events in terms of rain accumulation (intensity). Daily precipitation is classified in three categories and the associated cyclones are chosen according to their distance from Crete island. The seasonal and annual cycle of the physical characteristics of the cyclone tracks are investigated with respect to the cyclones' relative position to the island of Crete. It was found that cyclones affecting Crete most frequently approach from the western side of the island and the actual cyclone centers associated with precipitation events are usually located northwest and southeast of the Crete domain. Cyclone-induced rainfall increases in function to cyclones' depth, radius and propagation velocity increase as well as cyclones' pressure decrease. Spring cyclones that affect Crete with rainfall present lower pressures and higher cyclone propagation velocity in contrast to the ones associated with winter and autumn precipitation events. The examination of the relation between cyclone characteristics and precipitation occurrence provides valuable information related to forecasting potential and management of the water resources and the rainfall extremes.

Contributing factors to Koshava wind characteristics

ABSTRACTKoshava is a local wind usually observed in the cold part of the year over the large part of Serbia, parts of Romania, parts of Hungary, and east Croatia. The main characteristics of the Koshava wind are its high wind speed, southeasterly direction, persistence, and gustiness. This paper analyses the synoptic [Mediterranean cyclones (MCs) and Eurasian anticyclones (EAs)] as well as the mesoscale contributors to the Koshava wind. Time and spatial features of the pressure systems have been analysed using an automatic cyclone tracking scheme. The results show that the MCs and EAs that generate Koshava are approximately 1000 km away from the Koshava region. It is demonstrated that the strong anticyclones are the main trigger for the Koshava wind. Less than 3% of the Koshava winds occurred without the occurrence of either an anticyclone or a cyclone. The Synoptic Koshava Index (SKI), developed in this study and defined as the difference between the area‐averaged mean sea level pressures in the anticyclone and cyclone regions, has been shown to be a good indicator of the Koshava occurrence. Koshava has also been investigated from the perspective of the gap flow windstorm. The across‐mountain pressure difference, ΔP, is the most important mesoscale contributor to the Koshava characteristics. The across‐mountain potential temperature difference, Δθ, together with ΔP, can be successfully used to predict Koshava's occurrence. A simple (linear) probabilistic model for forecasting the Koshava mean hourly wind speed has been constructed. The results demonstrate that the occurrence of the Koshava wind can be predicted with the significantly higher accuracy than the Koshava mean hourly speed. The correlation between Koshava and winds at higher levels is small.

Saharan dust transport by Mediterranean cyclones causing mud rain in Istanbul

North Africa is the largest dust source on Earth. Mediterranean cyclones occasionally transport Saharan dust to Turkey in late winter and early spring, leading to mud rain events in some regions. Two mud rain events which occurred in Istanbul on 11 February 2010 and 8 March 2010 were simulated using the WRF‐ARW mesoscale model. Met Office analysis charts, backward trajectories, ground based observations, and satellite images from EUMETSAT Meteosat Second Generation were examined to explain the underlying mechanisms.

Cyclone contribution to the Mediterranean Sea water budget

This paper analyzes the impact of cyclones to the atmospheric components on the Mediterranean Sea Water Budget, namely the cyclones contribution to precipitation and evaporation over the Mediterranean Sea. Three regional simulations were performed with the WRF model for the period 1989–2008. The model was run (1) as a standalone model, (2) coupled with the oceanic model NEMO-MED12 and (3) forced by the smoothed Sea Surface Temperature (SST) fields from the second simulation. Cyclones were tracked in all simulations, and their contribution to the total rainfall and evaporation was quantified. Results show that cyclones are mainly associated with extreme precipitation, representing more than 50 % of the annual rainfall over the Mediterranean Sea. On the other hand, we found that cyclone-induced evaporation represents only a small fraction of the annual total, except in winter, when the most intense Mediterranean cyclones take place. Despite the significant contribution of cyclones to rainfall, our results show that there is a balance between cyclone-induced rainfall and evaporation, suggesting a weak net impact of cyclones on the Mediterranean Sea water budget. The sensitivity of our results with respect to rapid SST changes during the development of cyclones was also investigated. Both rainfall and evaporation are affected in correlation with the SST response to the atmosphere. In fact, air feedbacks to the Mediterranean Sea during the cyclones occurrence were shown to cool down the SST and consequently to reduce rainfall and evaporation at the proximity of cyclone centers.

Large‐scale wind and precipitation extremes in the Mediterranean: a climatological analysis for 1979–2012

A new method for identifying high impact large‐scale wind and precipitation events in the extended Mediterranean region is outlined and applied to the European Centre for Medium‐range Weather Forecasts (ECMWF) reanalysis dataset ERA‐Interim for the years 1979–2012. The method highlights large‐scale 10 m gust and precipitation events that classify as extreme if integrated over a spatial scale of 1000 km and a temporal scale of 3 days. The method detects successfully high impact events, and reveals clear seasonal differences among the subregions of the Mediterranean. Western Mediterranean precipitation extremes are more intense, and occur mainly in autumn, while eastern Mediterranean events occur in winter. Composite dynamical analyses of large‐scale wind and precipitation extremes, and a combination of them, highlight coherent dynamical flow structures associated with the extremes in the different subregions of the Mediterranean. Precipitation events are preceded by an upper‐level trough and strong jet on its western flank, followed by cyclogenesis (mainly in the western Mediterranean), and/or a merging of the polar with the subtropical jet over northeastern Africa (in the eastern Mediterranean). Strong surface wind extremes develop around cyclones that intensify south of a deep parent cyclone near the exit of a strong anticyclonically curved jet, propagate eastwards and create a cold and dry northerly wind anomaly at the surface. Furthermore, combined large‐scale wind and precipitation extremes often occur simultaneously near cyclones, either North Atlantic cyclones, which project the wind and precipitation into the western Mediterranean, or Mediterranean cyclones. The latter produce wind extremes over a localized area, which often overlaps entirely with the region that receives extreme precipitation.

A new methodology for identifying daughter cyclogenesis: application for the Mediterranean Basin

ABSTRACTThis study is the first one to objectively identify ‘parent–daughter’ relationships between cyclones and to apply it to Mediterranean cyclones (MCs). The methodology includes cyclone detection, tracking and mapping the area of influence of a cyclone; here daughter cyclones are expected to be found. Cyclone detection and tracking is based on standard algorithms, which were modified to cope with irregularities in intensity and trajectory shapes of MCs. We have developed our own algorithm for mapping cyclones' area of influence. Application of our methodology to the ERA‐Interim data for 33 mid‐winter seasons identified parents for 97.5% of the MCs. While previous studies claimed that most MC parents are situated over Europe, this study found the majority of MCs' parents (56%) positioned over the Mediterranean itself. This disagreement stems from the fine spatial resolution used here (0.75° × 0.75°), which enabled uncovering smaller cyclones than noticed previously, thus revealing earlier stages of cyclogenesis. The parenthood analysis identified two main types of cyclogenesis within the Mediterranean; one driven by parent MCs, the other driven by Atlantic or European cyclones. The latter is manifested either by directly generating a new MC or by intensifying existing weak MCs. Cyclone generations were explored through backtracking chains of parents down to the ninth generation, the longest chain found. The results indicate that the primary source of MCs is the Atlantic (2/3 of the MCs) and then Equatorial Africa and the Red Sea (1/3). The results also imply that although native MCs owe their existence to the Atlantic‐European cyclone track, regional cyclogenetic effects, such as topographic, thermal and baroclinic, are even more influential. The innovative methodology for objective identification of cyclone parents can be used to characterize cyclogenetic scenarios, in particular away from main storm tracks and over complex terrain.

Szlaki niżów śródziemnomorskich nad Europą Środkowo-Wschodnią a opady w Polsce

Szlaki niżów śródziemnomorskich nad Europą Środkowo-Wschodnią a opady w Polsce

The effect of the North Atlantic Oscillation on the Iraqi climate 1982–2000

In this study, we have analyzed the spatial and temporal correlation between the North Atlantic Oscillation (NAO) and the climate in Iraq, with a focus on precipitation, temperature, and number of Mediterranean cyclones. It was found that the influence of the NAO varies per climate indicator. For example, the influence is greater on precipitation than on temperature. One conclusion of the study is that the mean annual precipitation in Iraq increases during a negative phase of the NAO and decreases during a positive phase. In addition, the correlation between NAO and precipitation is more pronounced in the south of the country than in the middle and northern regions.

Extratropical cyclones and the projected decline of winter Mediterranean precipitation in the CMIP5 models

The Mediterranean region has been identified as a climate change “hot-spot” due to a projected reduction in precipitation and fresh water availability which has potentially large socio-economic impacts. To increase confidence in these projections, it is important to physically understand how this precipitation reduction occurs. This study quantifies the impact on winter Mediterranean precipitation due to changes in extratropical cyclones in 17 CMIP5 climate models. In each model, the extratropical cyclones are objectively tracked and a simple approach is applied to identify the precipitation associated to each cyclone. This allows us to decompose the Mediterranean precipitation reduction into a contribution due to changes in the number of cyclones and a contribution due to changes in the amount of precipitation generated by each cyclone. The results show that the projected Mediterranean precipitation reduction in winter is strongly related to a decrease in the number of Mediterranean cyclones. However, the contribution from changes in the amount of precipitation generated by each cyclone are also locally important: in the East Mediterranean they amplify the precipitation trend due to the reduction in the number of cyclones, while in the North Mediterranean they compensate for it. Some of the processes that determine the opposing cyclone precipitation intensity responses in the North and East Mediterranean regions are investigated by exploring the CMIP5 inter-model spread.

Influence of physics parameterization schemes on the simulation of a tropical-like cyclone in the Mediterranean Sea

Numerical experiments are performed using the WRF model to investigate which category of model physics is most critical for a proper simulation of the structure and intensity of a Mediterranean hurricane. Several combinations are used, and the model outputs are compared with the available observations and a reference simulation. The choice of microphysics scheme, and in a minor way, of cumulus parameterization, has the greatest impact on the model results. Boundary layer schemes and land–surface models appear to play only a marginal role.The ability of the model to reproduce the evolution of the cyclone from extra-tropical to tropical-like (TLC) is also explored. Apart from a few experiments differing for the microphysical scheme, all simulations are able to reproduce the cyclone features properly, and identify the presence of tropical characteristics, i.e. a symmetric, deep warm core, in the later stage of the cyclone lifetime, but with duration and intensity depending on the experiment.The motivation for the failure in some simulations is traced back to the incorrect representation of the cyclone location along its transit in the southern Ionian Sea, before its transition to TLC. In this phase, the interaction of the intense easterly flow, developing on the northern side of the cyclone, with the Apennines near the coasts of Calabria is fundamental for the later intensification of the cyclone, due to the severe convection triggered by the orographic uplift. Thus, an even small misplacement of the simulated cyclone during this phase may dramatically affect its following evolution.For the sake of comparison with the multi-physics approach, a time-lagged ensemble, with initial conditions starting at different times, is also considered. The spread in the cyclone depth and track in the latter case is smaller or comparable with that emerging in experiments using different microphysical schemes, but is larger compared with the experiments changing the other categories of parameterization schemes.

The dynamical structure of intense Mediterranean cyclones

This paper presents and analyzes the three-dimensional dynamical structure of intense Mediterranean cyclones. The analysis is based on a composite approach of the 200 most intense cyclones during the period 1989–2008 that have been identified and tracked using the output of a coupled ocean–atmosphere regional simulation with 20 km horizontal grid spacing and 3-hourly output. It is shown that the most intense Mediterranean cyclones have a common baroclinic life cycle with a potential vorticity (PV) streamer associated with an upper-level cyclonic Rossby wave breaking, which precedes cyclogenesis in the region and triggers baroclinic instability. It is argued that this common baroclinic life cycle is due to the strongly horizontally sheared environment in the Mediterranean basin, on the poleward flank of the quasi-persistent subtropical jet. The composite life cycle of the cyclones is further analyzed considering the evolution of key atmospheric elements as potential temperature and PV, as well as the cyclones’ thermodynamic profiles and rainfall. It is shown that most intense Mediterranean cyclones are associated with warm conveyor belts and dry air intrusions, similar to those of other strong extratropical cyclones, but of rather small scale. Before cyclones reach their mature stage, the streamer’s role is crucial to advect moist and warm air towards the cyclones center. These dynamical characteristics, typical for very intense extratropical cyclones in the main storm track regions, are also valid for these Mediterranean cases that have features that are visually similar to tropical cyclones.

Mediterranean Tropical-Like Cyclones in Present and Future Climate

Abstract The Mediterranean has been identified as one of the most responsive regions to climate change. It has been conjectured that one of the effects of a warmer climate could be to make the Mediterranean Sea prone to the formation of hurricanes. Already in the present climate regime, however, a few of the numerous low pressure systems that form in the area develop a dynamical evolution similar to the one of tropical cyclones. Even if their spatial extent is generally smaller and the life cycle shorter compared to tropical cyclones, such storms produce severe damage on the highly populated coastal areas surrounding the Mediterranean Sea. This study, based on the analysis of individual realistically simulated storms in homogeneous long-term and high-resolution data from multiple climate change scenarios, shows that the projected effect of climate change on Mediterranean tropical-like cyclones is decreased frequency and a tendency toward a moderate increase of intensity.

Prolonged dry spells in the Levant region: climatologic-synoptic analysis

Prolonged dry spells (PDSs) during the rainy season have severe environmental implications, including water shortage, damage to agriculture and increased potential for forest fires. This holds in particular for vulnerable regions, such as the Levant, already subjected to decrease in rainfall and lengthening of dry spells, in agreement with predictions of climatic models for the coming decades. This is the first comprehensive study which identifies atmospheric patterns responsible for PDS occurrence on thousands of kilometres scale. A total of 178 PDSs, of >7 days, were found within the 62 seasons studied. A subjective inspection of upper-level geopotential height (GPH), sea-level pressure (SLP) and lower-level temperature anomalies point at three types, each associated with a definite climatic regime. The ‘subtropical’ type is associated with an expansion of the subtropical high over the majority of the Mediterranean, accompanied by northward migration of the Mediterranean cyclone track. The ‘baroclinic’, the most frequent type, is induced by a pronounced stagnant ridge over the eastern Mediterranean, being a part of Rossby wave, accompanied by a pronounced trough/cut-off low over the western Mediterranean. The ‘polar’ type results from intrusion of lower-level continental polar air associated with upper-level trough east of the Levant and blocking high over central Europe. Quantitative indices were derived for objective classification of the types, based on the climatic regimes defined subjectively, and the centers of action representing each. Composite maps for each type indicate substantial differences in the synoptic configuration and the factors explaining absence of rain. For the subtropical type, the dynamic factor of subsidence is dominant. For the polar, the thermodynamic factor of continental dry advection is dominant and for the baroclinic, both dynamic and thermodynamic factors are important. Classification of PDSs according to synoptic scenarios enables analysis of future changes in the occurrence and duration pattern of PDSs, using output of climate models.

Medicanes in an ocean–atmosphere coupled regional climate model

Abstract. So-called medicanes (Mediterranean hurricanes) are meso-scale, marine, and warm-core Mediterranean cyclones that exhibit some similarities to tropical cyclones. The strong cyclonic winds associated with medicanes threaten the highly populated coastal areas around the Mediterranean basin. To reduce the risk of casualties and overall negative impacts, it is important to improve the understanding of medicanes with the use of numerical models. In this study, we employ an atmospheric limited-area model (COSMO-CLM) coupled with a one-dimensional ocean model (1-D NEMO-MED12) to simulate medicanes. The aim of this study is to assess the robustness of the coupled model in simulating these extreme events. For this purpose, 11 historical medicane events are simulated using the atmosphere-only model, COSMO-CLM, and coupled model, with different setups (horizontal atmospheric grid spacings of 0.44, 0.22, and 0.08°; with/without spectral nudging, and an ocean grid spacing of 1/12°). The results show that at high resolution, the coupled model is able to not only simulate most of medicane events but also improve the track length, core temperature, and wind speed of simulated medicanes compared to the atmosphere-only simulations. The results suggest that the coupled model is more proficient for systemic and detailed studies of historical medicane events, and that this model can be an effective tool for future projections.

MEDEX: a general overview

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.

Synoptic and thermodynamic characteristics of 30 March–2 April 2009 heavy rainfall event in Iran

A heavy rainfall event during the period from 30th of March to 2nd of April 2009 has been studied using upper air and surface data as well as NOAA HYSPLIT model. This observational study attempts to determine factors responsible for the occurrence of heavy rainfall over Iran induced by Mediterranean cyclone, a western severe sub-tropical storm that made rainfall on most regions of the country. On the surface chart, cyclones, anticyclones and weather fronts were identified. The positions of the cold and warm fronts, which extended from a two-core low pressure center, were quite in good agreements with directions of winds i.e., westerly, southerly and easterly flows as well as the regions of precipitation. The heavy rain event occurred due to a Mediterranean cyclone’s activity over the study area, while other conditions were also responsible for this event such as an unstable atmosphere condition with abundant low-level moisture, which the warm and moist air parcels were brought by the southwesterly low-level jet into the country from Persian Gulf, Oman Sea, Indian Ocean and Caspian Sea at lower levels as well as Mediterranean Sea, Red Sea and Persian Gulf at upper levels over the examined period. A strong low-level convergence zone was observed along the wind-shift line between the southwesterly flow because of the low-level jet and the northeasterly flow due to the Russian high pressure. The amount of precipitable water varied between 20 and 24 kg m−2, surface moisture convergence exceeded 2.5 g kg−1 s−1 and the highest CAPE value in the sounding profiles was observed in Birjand site with 921 J kg−1 during the study period. The HYSPLIT model outputs confirmed the observed synoptic features for the examined system over the country.