C. Ramis

The paradoxical increase of Mediterranean extreme daily rainfall in spite of decrease in total values

] Earlier reports indicated some specific isolated regionsexhibiting a paradoxical increase of extreme rainfall in spite ofdecrease in the totals. Here, we conduct a coherent study of thefull-scale of daily rainfall categories over a relatively largesubtropical region- the Mediterranean- in order to assess whetherthis paradoxical behavior is real and its extent. We show that thetorrential rainfall in Italy exceeding 128 mm/d has increasedpercentage-wise by a factor of 4 during 1951–1995 with strongpeaks in El-Nino years. In Spain, extreme categories at both tails ofthe distribution (light: 0-4 mm/d and heavy/torrential: 64 mm/d andup) increased significantly. No significant trends were found inIsrael and Cyprus. The consequent redistribution of the dailyrainfall categories -torrential/heavy against the moderate/lightintensities - is of utmost interest particularly in the semi-aridsub-tropical regions for purposes of water management, soilerosion and flash floods impacts. I

A 30‐year (1964–1993) daily rainfall data base for the Spanish Mediterranean regions: first exploratory study

A dense daily precipitation data base, extending from 1964 to 1993, has been created for the Mediterranean regions of Spain. It is composed of complete and homogeneous series at 410 raingauge stations (347 in the coastal fringe of peninsular Spain, and 63 in the Balearic Islands). The region offers an interesting scenario for mesoclimatological studies on time and spatial rainfall variability: geomorphologically, it is characterized by important coastal relief units and complex distribution of sea and land masses, leading to different exposures to the rain-bearing maritime winds; climatically, the western Mediterranean is subject to strong seasonal variability, since it is a transition zone between the midlatitude low pressure belt and the subtropical highs as a result of its latitude (between 36° and 44° N). In this study, we exploit the data base and present a first pluviometric characterization of the area by means of yearly and seasonal mean products. The results reveal clear and coherent spatial patterns that we interpret, based on typical storm tracks and land, sea, and relief distributions. In addition, a partition of the 30-year period into three decades (1964-1973, 1974-1983, 1984-1993) has been considered in order to assess the possible existence of any trend. A successive drying of the most sensitive areas to the winter Atlantic depressions (western Catalonia, and central and west Andalucia) is observed. In contrast, the second analysed decade is appreciably drier than the other two in the areas more dependent on the Mediterranean disturbances. The occurrence of anomalous autumns being the most responsible. This fact emphasizes the fundamental importance of the autumn season for the pluviometric balance of the considered area, especially in its eastern part where the major amount of precipitation during this season is produced by convective systems.

A classification of the atmospheric circulation patterns producing significant daily rainfall in the Spanish Mediterranean area

This study investigates the synoptic atmospheric circulations associated with 11 typical spatial patterns for significant rainfall days for Mediterranean Spain, which have been identified in an earlier paper. Using cluster analysis on the most relevant T-mode principal components, a classification into 19 fundamental circulations emerged, based only on geopotential fields at 925 and 500 hPa, using ECMWF gridded data for the period 1984–1993. The derived atmospheric patterns comprise a wide variety of flows over the Iberian Peninsula, with a clear distinction between Atlantic and western Mediterranean disturbances. Distinct seasonal distributions are also observed. Despite the comparatively higher resolution of the rainfall patterns, and the uncertainty derived from working with a daily time scale, a clear association emerged between each of the circulation types and a small number of the characteristic rainfall patterns. These associations can be physically interpreted in terms of the position of the 500 hPa trough, disturbances at the 925 hPa level, and the interaction of the surface rain bearing flows with the complex topography of the region. The final part of the analysis concentrated on establishing links between identified circulation patterns and notable (torrential) precipitation events recorded during the 1984–1993 decade. Some of the identified circulations that are important for the occurrence of significant rainfalls, produce few or no torrential rainfall episodes. Most torrential rainfall events in Mediterranean Spain are associated with disturbances located near or over the south of the Iberian Peninsula. Copyright

A diagnostic study of three heavy precipitation episodes in the western Mediterranean region

Abstract A diagnostic evaluation of three project ANOMALIA case studies involving heavy precipitation in the western Mediterranean region has been carried out. The evaluation shows the unique characteristics of each event, as well as some limited similarities. Heavy precipitation events in the western Mediterranean region typically occur downstream of a significant cyclone aloft (often, but not always, exhibiting “cutoff” cyclone characteristics), but important structural and evolutionary differences are found among these cases. At low levels, a long fetch of flow over the Mediterranean Sea frequently interacts with terrain features to produce persistent heavy precipitation. Although most heavy precipitation events occur during the fall season, they can develop at other times. In the first case, the synoptic-scale environment produced low static stability and substantial storm-relative environmental helicity, thereby supporting both heavy rain in the vicinity of Valencia on mainland Spain and on Ibiza in ...

Mesoscale Numerical Study of Two Cases of Long-Lived Quasi-Stationary Convective Systems over Eastern Spain

A set of mesoscale numerical simulations using the Pennsylvania State University‐National Center for Atmospheric Research model is used to investigate two cases of extreme precipitation over eastern Spain. Both cases (3‐4 November 1987 and 20 October 1982) were characterized by quasi-stationary mesoscale convective systems that developed over the Valencia region and lasted more than 30 and 12 h, respectively. Rainfall totals in 24 h exceeded 800 mm on 3‐4 November and 400 mm on 20 October at some localities of that region. The first event occurred within a weak and very stagnant synoptic pattern under a persistent easterly/northeasterly low-level jet stream impinging on the Valencian orography. In contrast, the second case involved a westwardmoving surface low driven by an upper-level jet streak, which evolved along the northern edge of an upperlevel cutoff low over North Africa. In both cases, the mesoscale model forecast spatial details of the precipitation field reasonably accurately, as well as capturing its long duration, but underestimated the storm total precipitation. Model output fields suggest that the development of a surface mesolow by latent heat release, as well as lee cyclogenesis induced by the Atlas Mountains, could have played an important role in both events by providing low-level convergence and enhanced upslope winds. Thus, a factor separation technique is used to explore this issue. For the event of 3‐4 November 1987, latent heat is decisive for explaining the precipitation maximum over central Valencia, and the Atlas orography induces rainfall enhancement over the same zone. For the event of 20 October 1982, the latent heat release is again shown to be important, whereas the Atlas Mountains orography factor appears to inhibit rainfall. This is the first time that it has been documented that the Atlas-induced modulation of the surface pressure field is not a positive factor for heavy precipitations over eastern Spain. This exceptional case may be due to a negative interaction between the Atlas Mountains and the upper-level dynamics and frontal forcing for this event.

The Environments of Significant Convective Events in the Western Mediterranean

Abstract The environmental characteristics associated with 313 significant convective events in the western Mediterranean are investigated using radiosonde ascents made in Mallorca (Spain). The events are separated into five groups, based on the observed event (hail, heavy rain, “dry” storms, storms with heavy rain, and tornadoes). Classic stability indices, as well as values of convective available potential energy and helicity, are considered for each group. These traditional convective indices appear not to provide good guidance for discriminating environments associated with each group of events. In order to classify the environments, each sounding is defined by means of 34 variables that describe the thermal and humidity vertical structure, instability, precipitable water, and helicity. A cluster analysis shows that four different vertical structures appear. Each kind of event shows preference for the environments defined by a cluster. A simple method is presented for sounding classification using th...

A catalogue and a classification of surface cyclones for the Western Mediterranean

The Mediterranean Sea is a region that possesses a high frequency of cyclones. Many of them are lee depressions and so, mesoscale and weak, but others are strong and cover a wide area. In this study some characteristics of surface cyclones in the Western Mediterranean are presented. First a database was built from hand analyses of sea-level pressure from 1992 to 1995. Next, a similar database was obtained from the LAM-INM (Limited Area Model of the Instituto Nacional de Meteorologia of Spain) objective analyses of the 1000 hPa geopotential field. Results show an important increase in the number of cyclones when a mesoscale analysis is carried out. Most of the Western Mediterranean cyclones are mesoscale and weak, and they are not uniformly distributed in space and in time. On the one hand, there are some areas with a high concentration of cyclones. The location of some of them, close to the main mountain ranges, suggests their possible orographic origin. On the other hand, in some areas the cyclones are present during the whole year, but in other areas are seasonally distributed. Finally, a study of typologies of the cyclones was conducted by using the cluster analysis technique. The classification was performed from the intensity of the cyclones and the shape of the sea-level pressure around the centre. Results are similar from both databases and so the existence of seven typologies have been identified. Copyright

Mesoscale Model Simulations of Three Heavy Precipitation Events in the Western Mediterranean Region

A mesoscale numerical model with parameterized moist convection is applied to three cases involving heavy rainfall in the western Mediterranean region. Forecast precipitation fields, although not perfect when compared to the observations of rainfall, appear to have sufficient information to be considered useful forecasting guidance. The results illustrate that a good simulation for this type of event in a region with complex topography is strongly dependent on a good initialization and prediction of the low-level flow and water vapor distribution. For two of the cases that have a marked synoptic-scale contribution, the simulations give reasonably accurate predictions of the precipitation distribution, although the amounts are generally underestimated. The third case exhibits relatively subtle synoptic-scale forcing and is dominated by isolated convective storms (mostly over the sea) that also produced severe thunderstorms (including tornadoes), and the prediction of precipitation is not as promising. Overall, the results are encouraging in terms of potential application of mesoscale models operationally in the western Mediterranean region. Additional experiments beyond the ‘‘control’’ simulations have been performed to isolate the influence of orography and water vapor flux from the Mediterranean Sea on the model simulations. This factor separation indicates that both effects can be important contributors to a successful forecast. Suggestions are offered for future efforts in pursuing the application of mesoscale models to this forecast problem.

Diagnosis and numerical simulation of a torrential precipitation event in Catalonia (Spain)

SummaryA torrential precipitation event occurred in Catalonia (northeastern part of Spain) during 9 and 10 October 1994. More than 400mm were registered in the south of Catalonia. A diagnostic study shows that most of the ingredients to produce heavy rain (large scale upward vertical motion, instability, high moisture content in all the troposphere) were present over the Spanish coast and western Mediterranean. Mesoscale triggering mechanisms have been associated with the orographic forcing, not only through physical lifting of moist air by the coastal mountains, but also by the redistribution of the surface pressure field induced by the Atlas and Pyrenees ranges. A numerical simulation of the event using a meso-β model has been performed. The model forecasts qualitatively well the rainfall distribution but underestimates the maximum rainfalls. The effects of the orography and the evaporation from the sea have been also studied. The simulations have shown that the action of the orography is decisive for the rainfall, pressure and wind distrbutions over the Spanish coast and the western Mediterranean. The isolated action of the evaporation turns to be much less important. However the combined effect of orography and evaporation is the most important factor in the areas where the greatest amount of rainfall occurred.

Sea breeze in Mallorca. A numerical study

SummaryThe Spanish island of Mallorca is located almost in the centre of the western Mediterranean. Very often from April to October, and almost every day during the summer, the sea breeze circulation develops. The shape and the topography of the island determine that the main characteristic feature of the sea breeze is the existence of a convergence line in the centre of the island. In this paper we describe the sea breeze circulation. A very simple non-eulerian numerical model allows us to simulate the evolution of the circulation during 24 hours.