Mediterranean Cyclones Research Articles

A method for quantifying the impacts and interactions of potential‐vorticity anomalies in extratropical cyclones

AbstractThe theory of potential vorticity (PV) allows us to describe the life cycle of mid‐latitude baroclinic systems in terms of the individual impacts and interactions of distinct PV anomalies embedded in the background flow. PV anomalies associated with the undulating tropopause, the low‐level thermal field, and key diabatic processes such as the latent‐heat release within the cloudy systems of the cyclone, are often considered as evolving features regulated by their lateral and vertical mutual interactions and their interaction with the stratospheric high‐latitude PV reservoir. Under some balance‐flow assumptions, PV inversion can be used to quantify the contribution of the PV anomalies to the cyclone depth (or other attributes) at various stages of its life cycle (astaticapproach); but it is less clear how to diagnose with similar quantitative detail the various types of time‐dependent interactions among the anomalies and mean flow that govern the processes of cyclogenesis and cyclolysis (adynamicapproach).This paper presents a method that implements the concepts of ‘PV thinking’ quantitatively. The method first applies a piecewise PV‐inversion scheme formulated according to the Charney nonlinear mass–wind‐balance approximation. Then it combines a prognostic system of balance equations that are consistent with the applied inversion scheme with a factor‐separation technique. By switching on and off the PV anomalies of interest, various flow configurations are generated, and the corresponding solutions to the prognostic equations can be algebraically combined to isolate the magnitude of both the individual and the synergistic effects of the PV anomalies on the spatial pattern of geopotential‐height tendency (and vertical motion) around the cyclone, with low computational cost.The potential of the method to elucidate the relative importance of physically‐meaningful PV anomalies for the growth or decay of baroclinic systems is illustrated for the intense Mediterranean cyclone of 10–12 November 2001, using the NCEP meteorological grid analyses at 12 h intervals.The upper‐level PV anomalies contributed very significantly during the whole life cycle of this Mediterranean cyclone. Surface thermal anomalies were fundamental during the development period, and induced the general northeastward movement of the system during the later stages. During the mature stage of the cyclone, the interaction between the two types of anomalies became the dominant effect. All other contributions, including the individual and synergistic effects of diabatically‐generated PV, were generally most relevant during this mature stage. Copyright © 2008 Royal Meteorological Society

Synoptic climatological analysis of Red Sea Trough and non-Red Sea Trough rain situations over Israel

Abstract. Winter (October to April) rainfall in Israel is caused mostly by migrating Mediterranean cyclones but certain rain situations are accompanied by a low pressure trough extending northward from the southern Red Sea towards the Eastern Mediterranean (EM) and the Levant. This system, the Red Sea Trough (RST) is one of the most frequent surface atmospheric circulation patterns over the southeastern Mediterranean, but most of the RSTs are not accompanied by rain. This paper presents a synoptic climatological comparative analysis between rain situations associated with RSTs, and those not connected with RSTs (non-RST rain, NRR). The RST situations were identified objectively with the aid of a new algorithm applied to the GEOS-1 reanalysis gridded data set from NASA, for the period of March 1985–November 1995. Results show that RST rain is accompanied by a relatively narrow 500-hPa trough, located west of the Israeli coast-line and characterized by a deep southward penetration, while Non-RST-associated rain (NRR) is accompanied by a wider upper trough, located over the Israeli coast-line with a shallower southward penetration. We found a south-southwesterly wind vector anomaly at 200 hPa over Israel during RST rains, while during NRRs a similar wind vector anomaly pattern is observed east of Israel. There is a divergence center over, or a few degrees east of Israel during RST rains, while NRR is associated with a divergence value of nearly zero over Israel and a maximum divergence center located east of Israel. The moisture flux during NRR at 700 and 900 hPa is from the Mediterranean, while during RST rain there is a south-westerly moisture flux at 700 hPa from equatorial Africa to Israel and vicinity. A steeper temperature lapse-rate between 950–500 hPa was found during RST rain compared with NRR, resulting from a combination of cooling aloft together with heating near the surface.

Towards a systematic climatology of sensitivities of Mediterranean high impact weather: a contribution based on intense cyclones

Abstract. One of the multiple approaches currently explored to mitigate the effects of hydro-meteorological hazardous events aims at improving the numerical weather forecasts. Under an ever increasing societal demand for cost cuts and more precise forecasts, targeted observations are currently receiving great attention within the operational weather community. The MEDEX project (http://medex.inm.uib.es) is aimed at improving the forecasts of high impact weather (HIW) in the Mediterranean and, in particular, proposes the creation of a climatology of sensitivities of such episodes. The construction of a comprehensive climatology of sensitivities is hampered by the lack of an exhaustive collection of Mediterranean HIW events. In this study we contribute with a systematic climatology of Mediterranean intense cyclones. We perform an objective cluster analysis of intense cyclones detected from the ECMWF ERA40 reanalysis using a k-means algorithm and compute the sensitivities for each of the resulting classes. For each cluster, a representative sensitivity field is computed using the MM5 Adjoint Modeling system. The results show that although the sensitive areas for intense Mediterranean cyclones are not particularly confined, it is remarkable how areas poorly sampled by the regular observing networks, such as North Africa, the Mediterranean Sea and the eastern North-Atlantic, are highlighted in the prototype sensitivity maps.

Scenarios in the development of Mediterranean cyclones

Abstract. The Mediterranean is one of the most cyclogenetic regions in the world. The cyclones are concentrated along its northern coasts and their tracks are oriented more or less west-east, with several secondary tracks connecting them to Europe and to North Africa. The aim of this study is to examine scenarios in the development of Mediterranean cyclones, based on five selected winter seasons (October–March). We detected the cyclones subjectively using 6-hourly Sea-Level Pressure maps, based on the NCAR/NCEP reanalysis archive. HMSO (1962) has shown that most Mediterranean cyclones (58%) enter the Mediterranean from the Atlantic Ocean (through Biscay and Gibraltar), and from the south-west, the Sahara Desert, while the rest are formed in the Mediterranean Basin itself. Our study revealed that only 13% of the cyclones entered the Mediterranean, while 87% were generated in the Mediterranean Basin. The entering cyclones originate in three different regions: the Sahara Desert (6%), the Atlantic Ocean (4%), and Western Europe (3%). The cyclones formed within the Mediterranean Basin were found to generate under the influence of external cyclonic systems, i.e. as "daughter cyclones" to "parent cyclones" or troughs. These parent systems are located in three regions: Europe (61%), North Africa and the Red Sea (34.5%) and the Mediterranean Basin itself (4.5%). The study presents scenarios in the development of Mediterranean cyclones during the winter season, emphasizing the cyclogenesis under the influence of various external forcing. The large difference with respect to the findings of HMSO (1962) is partly explained by the dominance of spring cyclones generating in the Sahara Desert, especially in April and May that were not included in our study period.

Probability estimates of heavy precipitation events in a flood-prone central-European region with enhanced influence of Mediterranean cyclones

Abstract. Due to synoptic-climatological reasons as well as a specific configuration of mountain ranges, the northeast part of the Czech Republic is an area with an enhanced influence of low-pressure systems of the Mediterranean origin. They are associated with an upper-level advection of warm and moist air and often lead to heavy precipitation events. Particularities of this area are evaluated using a regional frequency analysis. The northeast region is identified as a homogeneous one according to tests on statistical characteristics of precipitation extremes (annual maxima of 1- to 7-day amounts), and observed distributions follow a different model compared to the surrounding area. Noteworthy is the heavy tail of distributions of multi-day events, reflected also in inapplicability of the L-moment estimators for the general 4-parameter kappa distribution utilized in Monte Carlo simulations in regional homogeneity and goodness-of-fit tests. We overcome this issue by using the maximum likelihood estimation. The Generalized Logistic distribution is identified as the most suitable one for modelling annual maxima; advantages of the regional over local approach to the frequency analysis consist mainly in reduced uncertainty of the growth curves and design value estimates. The regional growth curves are used to derive probabilities of recurrence of recent heavy precipitation events associated with major floods in the Odra river basin.

Sensitivities of a Flash Flood Event over Catalonia: A Numerical Analysis

Abstract On 9 and 10 June 2000, the northeastern part of the Iberian Peninsula was affected by heavy rains that produced severe floods over densely populated areas. The zones most affected were the provinces of Tarragona and Barcelona, located in the region of Catalonia. Five people were killed, 500 were evacuated, and the property losses were estimated to exceed 65 million euros. The episode was characterized by the entrance of an Atlantic low-level cold front and an upper-level trough that contributed to the generation of a mesoscale cyclone in the Mediterranean Sea east of mainland Spain. The circulation associated with this mesoscale cyclone advected warm and moist air toward Catalonia from the Mediterranean Sea. The convergence zone between the easterly flow and the Atlantic front, as well as the complex orography of the region, are shown to be involved in the triggering and organization of the convective systems. Radar shows the development of two long-lived mesoscale convective systems that merged and remained quasi-stationary nearby the city of Barcelona for nearly 2 h. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) short-range numerical simulations of the episode succeed reasonably well in capturing the accumulated rainfall patterns and important mesoscale aspects of the event. The role of the orography and latent heat release in generating and sustaining the quasi-stationary precipitating systems is assessed through numerical sensitivity analysis. A piecewise potential vorticity inversion technique, assisted by sensitivity fields derived from an adjoint model, provides a test bed to investigate the predictability of the damaging rains given uncertainties in the initial fields. The adjoint results highlight the importance of the details associated with the upper-level precursor trough. Therefore, the effects of small perturbations to the trough intensity and location are also investigated. Interestingly, despite the orography’s active role in the generation of the Mediterranean mesoscale cyclone and, hence, on the location of the precipitation maxima, the predictability of the spatial and temporal distribution and amounts of precipitation is shorter than might be expected. This reduced predictability of the rainfall field is attributed to the high sensitivity of the location and depth of the mesoscale cyclone identified in the Mediterranean Sea east of Catalonia to perturbations; that is, to the location and intensity of the precursor upper-level trough, compatible with realistic analysis errors.

Unsatisfying forecast of a Mediterranean cyclone: a verification study employing state-of-the-art techniques

Abstract. On 16–17 November 2000, a relatively intense precipitation event on the north-western Italy was heavily underestimated, mainly due to shifting error, by three operational 10-km limited area models (LAMs) which differ about basic equations, domain size, and parameterisation schemes. The scope of the work is to investigate possible common error-sources independent from the single model, in particular the effect of initialisation. Thus, the complex evolution over the western Mediterranean Sea of the cyclone responsible for the event was investigated. Several objective and subjective verification techniques have been employed to check one of the LAMs' forecast against the available observations (precipitation from rain gauge and retrieved from ground-based radar, and satellite-retrieved atmospheric humidity patterns). Despite a clear statement is not achieved, results indicate that high sensitivity to the initial conditions, and the inadequacy of the observational network on the southern Mediterranean area, can play a major role in producing the forecast shifting error on the target area.

A climatic study of severe storms over Bulgaria produced by Mediterranean cyclones in 1990−2001 period

Over south-eastern Europe, severe weather events are often associated with Mediterranean cyclones. This paper presents a climatic study of severe storms over Bulgaria produced by synoptic-scale Mediterranean cyclones, which are the main high-impact weather systems for the region during the winter season. The study is based on a synoptically oriented data set that contains systematic information about the pronounced Mediterranean cyclones including their life and trajectories over the Mediterranean area and the severe storms over Bulgaria produced by them. The definition of a severe storm is a storm in which the wind speed and precipitation exceed thresholds of 15 m/s and 30 mm/24 h, respectively. The observed severe storms were split into two groups by the number of districts where they have caused damages. During the last years a trend of decreasing numbers of initiating synoptic-scale Mediterranean cyclones has been observed. However, the number of those producing a high-impact weather phenomenon over Bulgaria has increased. In these high-impact cases, the observed paths of the cyclones are connected to the specific circulation conditions over the region. In the most severe cases, almost 80% of the cyclones move through the southernmost parts of Balkan Peninsula and for a large portion of them, this motion is associated with a blocking regime in the mid-level mass field. The development of such Mediterranean cyclone cases has been studied and results are presented here. The results illustrate the synoptic-scale mechanisms for intensification of a feeding flow of Mediterranean air towards the area affected by extremely severe weather.

Cyclogenesis in the lee of the Atlas Mountains: a factor separation numerical study

Abstract. The initiation of a deep and severe impact Mediterranean cyclone in the lee of Atlas Mountains is investigated by a series of numerical experiments using the MM5 forecast model. Roles of orography, surface sensible heat flux and an upper-level potential vorticity anomaly are identified using factor separation method. Results of model simulations show that orography blocking is responsible for generation of the low-level shallow vortex in the first phase of lee development. Upper-level potential vorticity is a principal ingredient of this event, responsible for a dominant deepening effect in the later stage of lee formation. Analysis of cyclone paths shows that orography tends to keep the cyclone stationary, while upper-level dynamical factors are crucial for advection of the system to the Mediterranean Sea. The most noteworthy influence of surface sensible heat flux is identified as an afternoon destruction of a surface baroclinic zone and associated weaker cyclogenesis.

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

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.

Intense storms in the Mediterranean: a first description from the ERA-40 perspective

Abstract. The study of the Mediterranean cyclones from a climatological point of view has been one of the objectives of the first phase of WMO WWRP MEDEX (MEDiterranean EXperiment) project. It has been revealing itself like a good procedure for extracting conclusions about their characteristics and behaviour. The implementation on ECMWF ERA-40 reanalysis of the method for detecting and tracking the cyclones developed at the Centro Meteorológico Territorial (CMT) in Illes Balears of the Instituto Nacional de Meteorología (INM, Spain) has allowed the selection of the most intense storms occurred for the last 45 years. This selection has been based on the value of the cyclones' circulation. The areas of maximum intensification for those cyclones have been obtained and the existence of preferential zones of intensification has been verified. Finally a first sight of those cases has allowed to initiate the check up of the ability of the ERA-40 re-analyses for reproducing these intense Mediterranean storms.

Three‐dimensional structure of western Mediterranean cyclones

AbstractAs many studies reveal, the western Mediterranean exhibits a high frequency of cyclone centres. Most of them are small and weak, but in some cases they are related to heavy rain and/or strong wind events. The climatological study of Mediterranean cyclones is the first crucial step to the better understanding and forecasting of such events.In the present paper, a method to objectively detect and track mean sea‐level (MSL) cyclones for the western Mediterranean is described. Furthermore, the three‐dimensional characterization of each MSL cyclone is performed by means of several parameters. This includes the vorticity, thermal and humidity fields at different pressure levels where the cyclone is detected, as well as the wind speed profile and the moist stability over the MSL cyclone. Both methodologies are illustrated and validated by a real case: an intense event of the well‐known Genoa cyclone. Detection, tracking and three‐dimensional characterization are applied to an 8‐year (from June 1995 to May 2003) database of numerical analyses. The result is a MSL cyclone database for the western Mediterranean with a description of their three‐dimensional structure. For a better analysis, cyclones are grouped in three different ways: by thickness, season and region of detection. Results show that western Mediterranean cyclones are located in specific geographical regions and that their location depends on the season. Several cyclones are weak and shallow, mainly because of thermal and/or orographic causes. On the contrary, other cyclones are large and intense and extend throughout the whole troposphere. Differences in cyclone structure depending on the location and season are also discussed. Copyright © 2006 Royal Meteorological Society.

Genesis and maintenance of "Mediterranean hurricanes"

Abstract. Cyclonic storms that closely resemble tropical cyclones in satellite images occasionally form over the Mediterranean Sea. Synoptic and mesoscale analyses of such storms show small, warm-core structure and surface winds sometimes exceeding 25ms-1 over small areas. These analyses, together with numerical simulations, reveal that in their mature stages, such storms intensify and are maintained by a feedback between surface enthalpy fluxes and wind, and as such are isomorphic with tropical cyclones. In this paper, I demonstrate that a cold, upper low over the Mediterranean can produce strong cyclogenesis in an axisymmetric model, thereby showing that baroclinic instability is not necessary during the mature stages of Mediterranean hurricanes.

Sensitivities of an intense Mediterranean cyclone: Analysis and validation

AbstractOn 10 and 11 November 2001 a deep cyclone moved northward across the western Mediterranean. Severe floods affected Algeria on 10 November and a mesoscale‐sized region of strong damaging winds occurred over the Balearics and eastern Spain during the first hours of 11 November. These large intense cyclones, originating over north Africa and moving northward, are occasionally observed in the region. Numerical simulations of these types of events are potentially hampered by the lack of observations over the Mediterranean Sea, north Africa and the Atlantic Ocean. To evaluate more accurately the regions in which the model simulations are influenced by this lack of data, the MM5 adjoint system is used to determine the most sensitive areas within the initial conditions of the simulation of this 10–11 November event. Limitations of available adjoint models, such as their linear character, suggest that a test of the applicability of MM5 to the case under analysis is needed. In this study, the evaluation is performed by means of the tangent linear model and, despite finding that the adjoint has an acceptable accuracy, important nonlinear effects are found and attributed to the moist processes. The study tracks backward in time the sensitivities shown at different simulation times using parameters chosen to characterize the cyclone's intensity at 0000 UTC 11 November. Results reveal that the areas that show the largest sensitivities are located over north Africa for the 12 h and 24 h simulations, whereas south‐western and western Europe emerge as areas with important sensitivities for the longer 36 h and 48 h simulations. Subsynoptic details regarding the shape and intensity of an upper‐level trough, as well as a low‐level cold front, are highlighted by the adjoint runs as the structures which influence most strongly the baroclinic development of the intense Mediterranean cyclone and the damaging surface winds it produces. The usefulness of the sensitivity fields in the nonlinear simulations is confirmed by perturbing the control model's initial conditions using the adjoint results as guidance. This analysis helps to quantify the limitations of the linear estimation when applied to the full nonlinear model, focusing on the nonlinearities introduced by the moist physics parametrizations. Copyright © 2004 Royal Meteorological Society

Atmospheric predictors for major floods in the Negev Desert, Israel

AbstractThe study examines the ability of a set of atmospheric variables to predict major floods in the Negev Desert in the southern part of Israel. The two dominant synoptic types, which contributed over 70% of the major floods during 1965–94 in that region, were examined. These are: the active Red Sea trough (ARST), a surface trough along the Red Sea, combined with a pronounced upper trough; and the Syrian low (SL), an intense Mediterranean cyclone centred over Syria.For each synoptic type, a set of atmospheric variables (predictors) was chosen to reflect its unique atmospheric features, and a prediction score was calculated as the ratio between the number of flood‐producing storms and the total number of events in which all the variables exceeded their threshold values.The prediction score for the ARST type is 86%. Moreover, the predictors identify the major flood dates without any ‘false’ date (100%) for 4 of the 5 months in which major floods of this type had occurred. Most of the predictors are found at the 500 hPa level, and the most powerful is the v/u ratio, which represents the southerly wind component and is responsible for the transport of moist tropical air masses (essential for convection) toward the Middle East. The prediction score for the SL type is 73%. The intensity and structure of the surface cyclone are found to be most powerful predictors, although the importance of geopotential height at 500 hPa indicates that these types of flood depend on the combined effect of several factors.Transforming these remarkably high scores into a high‐skill operational forecast of major floods in the Negev requires reliable forecast models to supply the desired variables with reasonable accuracy. It seems that the current operational models, together with our derived predictors, have the potential to yield a successful forecast of major floods 2 days in advance. Copyright © 2004 Royal Meteorological Society

On the mechanism of winter cyclogenesis in relation to vertical axis tilt

¶This work is concerned with the behavior of the vertical trough axis tilt during the development of a Mediterranean cyclone. The period covering stages of the development of the depression is from 1200 UTC 18 January to 1200 UTC 25 January 1981. Two procedures have been used to illustrate the behavior of the vertical axis tilt with the development of our case study. In the first direct procedure we trace the centers of the cyclone for each level for each day and determine its longitude. In the second indirect procedure we compute the values and signs of the product of the perturbation of temperature and meridional velocity over the computational domain. The energetics of the depression is studied and the partition of available potential energy and kinetic energy into zonal and eddy component is adopted. The energy contents and their changes in different atmospheric levels are discussed in the course of the cyclone’s development.

Synoptic climatology of major floods in the Negev Desert, Israel

AbstractThis study examines the extent to which the floods in the Negev Desert, an area that constitutes the southern half of Israel, are not the outcome of purely local weather conditions but are, rather, the result of distinct synoptic‐scale events. This was done through compiling and analysing a hydro‐climatological database of all the major floods in the Negev, and then categorizing them manually into synoptic types that cause the major floods.The type analysis is based on the US National Meteorological Center data sets with 2.5° × 2.5° resolution analysed by GrADS. Data were compiled and studied for 52 floods for the period 1965–94, with peak discharge above the magnitude of 5 year recurrence intervals (RI > 5 years) in at least one drainage basin.Distinct extreme synoptic patterns are indeed associated with 42 of the 52 floods. They were grouped into four synoptic types, two of which were associated with 37 events: (a) an active Red Sea trough, defined as a surface trough extending from East Africa through the Red Sea toward the eastern Mediterranean, accompanied by a pronounced trough at the 500 hPa level over eastern Egypt: (b) a Syrian low, defined as a well‐developed Mediterranean cyclone accompanied by a pronounced upper‐level trough, both located over Syria. Each of the four synoptic types has its own evolution course, and a unique seasonal and spatial distribution of its associated flooded basins.These findings imply that the major floods in the Negev can be considered as signatures of exceptional synoptic‐scale evolutions, and that major floods reflect extreme climatic events. Our results indicate that it is possible to use a set of dynamic and thermodynamic variables for predicting the occurrence and location of major flash floods. Copyright © 2002 Royal Meteorological Society.

Sensitivity of a heavy-rain-producing western Mediterranean cyclone to embedded potential-vorticity anomalies

Sensitivity of a heavy-rain-producing western Mediterranean cyclone to embedded potential-vorticity anomalies

Sensitivity of a heavy‐rain‐producing western Mediterranean cyclone to embedded potential‐vorticity anomalies

AbstractThe development and track of a surface cyclone that produced heavy precipitation in the western Mediterranean region on 28–29 September 1994 is numerically simulated using a mesoscale model. Diagnostic calculations reveal that surface‐pressure falls‐and upward motion‐occur in response to a well‐defined pattern of upward quasi‐geostrophic forcing at all tropospheric levels. In addition, convective instability and low‐tropospheric water vapour flux convergence are significant over the western Mediterranean, and therefore the environment is highly supportive for convection development.The appreciable dynamical forcing at upper levels appears to be associated with two positive potential‐vorticity (PV) anomalies that are embedded within the large‐scale trough and rotate about each other. Motivated by the fact that the small‐scale features of the PV field are more prone to analysis or forecast error than the large‐scale components, a sensitivity study is conducted in order to analyse the dependence of the mesoscale forecast on the initial intensity and position of the two embedded PV centres. This is accomplished by first applying a piecewise PV‐inversion technique which allows the balanced flow associated with each PV centre to be calculated, and then the inverted mass and wind fields are used to modify the model initial conditions. Eight simulations are run after doubling or removing one or both anomalies (sensitivity to intensity), and eight other simulations after displacing the anomalies towards or away from the Iberian peninsula (sensitivity to position).The results exhibit a clear dependence of the track and shape of the surface cyclone and its associated pattern of low‐tropospheric warm air advection on the characteristics of the PV anomalies. Therefore, spatial details of the mesoscale forecast are shown to be highly sensitive to the precise structure of the upper‐level dynamic forcing. Nevertheless, for the full set of arbitrarily defined initial conditions, one or several low pressure systems develop in the western Mediterranean area and heavy precipitation always occurs. On the other hand, there is an overall tendency of the simulations to favour cyclone development leeward of the Atlas mountain range and local rainfall enhancements over the exposed mountains of eastern Spain. The relative roles of the orography and sea surface latent‐heat flux versus the action of the upper‐level PV centres are examined by means of additional simulations.

Automated database of mesocyclones from the HIRLAM(INM)‐0.5° analyses in the western Mediterranean

AbstractAn automated procedure has been developed to detect and select mesoscale cyclones and describe their tracks by means of the numerical analysis model, the HIRLAM(INM)‐0.5°, operational at the Spanish Institute of Meteorology (INM). This procedure gives us a description of features of these cyclones, such as their frequency of appearance, geographical distribution and horizontal dimensions, and it also enables us to study the movement and the evolution of the cyclone centres. This procedure was developed in order to obtain an automated database of Mediterranean cyclones. To check the performance of this method, the distribution of cyclones was compared with the distribution obtained from a manual catalogue of mesoscale cyclones, and selected cases were studied using both methods. Some results obtained for the western Mediterranean are presented. A large number of cyclones are detected and most of them are located in specific zones, such as the Gulf of Genoa, south of the Pyrenees, south of the Iberian Peninsula and the Alboran Sea. The cyclones follow preferred tracks, which are different depending on the season. Copyright © 2001 Royal Meteorological Society