[1] Earlier reports indicated some specific isolated regions exhibiting a paradoxical increase of extreme rainfall in spite of decrease in the totals. Here, we conduct a coherent study of the full-scale of daily rainfall categories over a relatively large subtropical region-the Mediterranean-in order to assess whether this paradoxical behavior is real and its extent. We show that the torrential rainfall in Italy exceeding 128 mm/d has increased percentage-wise by a factor of 4 during 1951 -1995 with strong peaks in El-Nino years. In Spain, extreme categories at both tails of the distribution (light: 0-4 mm/d and heavy/torrential: 64 mm/d and up) increased significantly. No significant trends were found in Israel and Cyprus. The consequent redistribution of the daily rainfall categories -torrential/heavy against the moderate/light intensities -is of utmost interest particularly in the semi-arid sub-tropical regions for purposes of water management, soil erosion and flash floods impacts. INDEX TERMS: 1854

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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 Andalucía) 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. © 1998 Royal Meteorological Society

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 the Balearic Islands, as well as a tornado at Menorca in the Balearic Islands on 7-9 October 1992. The second case involved a slow-moving cyclone that destabilized the stratification and produced several days of heavy precipitation over the period 31 January-6 February 1993. In the third case, in the Italian Piedmont region on 5-6 November 1994, the heavy precipitation included a nonconvective component, with moist but relatively stable air impinging on steep terrain gradients. A set of basic diagnostic tools is applied to the cases, and it is shown that anything but a superficial diagnosis of each case requires flexibility in selecting diagnostic tools. The ways by which heavy precipitation is created can vary substantially from case to case and in different parts of the world; however, there is a common thread. Heavy precipitation is the result of moist, low-level air ascending rapidly, so any diagnosis aimed at forecasting heavy precipitation needs to address the following: vertical motion, static stability, moisture supply, and orographic effects (when appropriate). Forecasting implications of the cases are discussed, with the emphasis on considering these physically relevant processes.

Abstract. Tropical-like storms on the Mediterranean Sea are occasionally observed on satellite images, often with a clear eye surrounded by an axysimmetric cloud structure. These storms sometimes attain hurricane intensity and can severely affect coastal lands. A deep, cut-off, cold-core low is usually observed at mid-upper tropospheric levels in association with the development of these tropical-like systems. In this study we attempt to apply some tools previously used in studies of tropical hurricanes to characterise the environments in which seven known Mediterranean events developed. In particular, an axisymmetric, nonhydrostatic, cloud resolving model is applied to simulate the tropical-like storm genesis and evolution. Results are compared to surface observations when landfall occurred and with satellite microwave derived wind speed measurements over the sea. Finally, sensitivities of the numerical simulations to different factors (e.g. sea surface temperature, vertical humidity profile and size of the initial precursor of the storm) are examined.

ABSTRACT:Medicanes are 'Mediterranean tropical-like cyclones', warm-core cyclones that occasionally put in danger the islands and coastal regions. In spite of large geographical differences between the Mediterranean Sea and the tropical oceans, their genesis mechanisms, based on the thermodynamical disequilibrium between the sea and the atmosphere, are similar.The special characteristics of the medicanes make their detection difficult: only with high resolution meteorological analysis data and dense maritime observations that task would be possible. An alternative method, using satellite data and restricted criteria about the disturbance symmetry, size and lifespan, has been successfully used to detect 12 medicanes from 1982 to 2003.To enhance the medicane prediction capability or even to assess the risk potential in future climates, it is necessary to characterize the special conditions of the synoptic-scale meteorological environments that are needed for their development and maintenance. By comparing these environments against the bulk of Mediterranean cyclonic situations, high values of mid-tropospheric relative humidity, significant diabatic contribution to the surface level equivalent potential temperature, and low values of tropospheric wind shear, are revealed as important parameters involved in medicane genesis, as in tropical cyclones. An empirical genesis index previously derived for the tropical cyclones is also tested in the study, and its behaviour is revealed as a possible discriminative parameter of the precursor environments.In the context of the growing concern about how climate change will affect the number and intensity of hurricanes, a preliminary analysis for medicanes has been done here. By projecting the previous empirical index into three different global climate model (GCM)-simulated climates, spatial distributions of the monthly index values have been evaluated. The monthly mean values and the frequency of extreme values of this index tend to decrease, showing that the number of days with a medicane risk tends to reduce at the end of the 21st century.

SUMMARYA small, quasi-tropical cyclone occurred on 12 September 1996 over the western Mediterranean. Intense convective activity over the region during this period also produced a tornado outbreak in the Balearic Islands and torrential precipitation over eastern mainland Spain.Mesoscale model runs properly simulate the cyclone formation and show convection and heavy precipitation following the cyclone trajectory during its eastward progression. A sensitivity study examining the upper-level dynamic forcing, latent-and sensible-heat uxes from the sea, and orography is conducted. A potential-vorticity (PV) inversion technique is used to reduce the amplitude of the upper-level trough in the model initial conditions. The results show that neither the orography nor the sensible-heat ux from the sea play a signi cant role during this particular cyclone development. Conversely, both the latent-heat ux and the upper-level trough are shown to be crucial for low-level cyclogenesis. Features common to hurricane-like polar lows are found for the cyclone, and an analysis of the precise role of the upper-level structures and the convective development is conducted.A factor-separation technique is used to determine the individual effects of the aforementioned factors, as well as their interaction. At the rst stage of the cyclogenesis, the upper-level PV anomaly enhanced the low-level circulation of the synoptic-scale low and enhanced the latent-heat ux from the sea. During its mature stage, the circulation associated with the small-scale cyclone enhanced the latent-heat ux from the sea, thereby helping to maintain the development of deep convection, and inducing further cyclone deepening by diabatic heating. This scenario has many similarities with the air-sea interaction instability mechanism. Thus, the joint action of the upper-level anomaly, as a spin-up agent, and the latent-heat ux, as a sustainer of convection, emerges as the primary factor for the genesis and evolution of the small quasi-tropical cyclone.