S. Alonso

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.

Mesoscale Dynamics and Vertical Motion in the Alborán Sea

Abstract The inflow of Atlantic water through the strait of Gilbraltar usually exhibits a stationary wavelike pattern in the Alboran sea. We use an objective analysis technique for quantitative scale separation to investigate the interaction of scales in the region. The large scale is clearly dominated by an anticyclonic gyre. Smaller scale analysis shows the existence of several mesoscale cyclonic eddies along the northern boundary of the western Alboran sea anticyclonic gyre. The relationship between the large-scale/mesoscale variability and the induced ageostrophic vertical motion is established using the Q vector formulation. We find that on the macroscale, upward motion occurs upstream of the anticyclonic gyre (upstream of a wave crest) while downward motion takes place downstream (upstream of a trough). We also show that the vertical motion associated with the mesoscale eddies is an order of magnitude higher than the large-scale vertical motion. These patterns of large-scale and mesoscale vertical m...

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 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.

A Statistical Adjustment of Regional Climate Model Outputs to Local Scales: Application to Platja de Palma, Spain

AbstractProjections of climate change effects for the System of Platja de Palma (SPdP) are derived using a novel statistical technique. Socioeconomic activities developed in this settlement are very closely linked to its climate. Any planning for socioeconomic opportunities in the mid- and long term must take into account the possible effects of climate change. To this aim, daily observed series of minimum and maximum temperatures, precipitation, relative humidity, cloud cover, and wind speed have been analyzed. For the climate projections, daily data generated by an ensemble of regional climate models (RCMs) have been used. To properly use RCM data at local scale, a quantile–quantile adjustment has been applied to the simulated regional projections. The method is based on detecting changes in the cumulative distribution functions between the recent past and successive time slices of the simulated climate and applying these, after calibration, to the recent past (observed) series. Results show an overall ...

Tornadoes over complex terrain: an analysis of the 28th August 1999 tornadic event in eastern Spain

Abstract On 28 August 1999, a tornadic storm developed during the afternoon over the Gudar range, near the border between Teruel and Castellon provinces ( Sistema Iberico , eastern Spain). The area has a characteristic complex terrain with peaks up to 2000 m. At least one tornado developed, which attained F3 intensity, producing severe damage in the forest that covers the mountains. The region is well known as a summer convective storm nest and, not surprisingly, a range in the Sistema Iberico is called Sierra del Rayo (lightning range). The meteorological situation on 28 August 1999 shows the presence of a thermal low over the Iberian peninsula, producing warm and moist air advection towards inland Castellon at low levels. Meanwhile, a cold trough crossed the Iberian Peninsula from west to east at upper levels. Deep convection is identified on the Meteosat images during the afternoon, when the upper level trough reached the area where warm and humid Mediterranean air was concentrated. The radar images reveal signals indicating the supercellular character of the tornado-producing storm. Weak echo region, frontal and rear inflow notches are identified on the low-level CAPPIs images. In order to study the roles of the orography and the solar radiation on the ingredients associated with the tornadic storm development, numerical simulations of the event were performed, using the non-hydrostatic MM5 model. Very fine grid resolution, up to 2 km in the horizontal, allows us to determine the role of the complex terrain in favoring environmental conditions associated with the onset of severe convection. In fact, interaction of certain low-level flows with particular topographic features with scales of about 20 to 50 km is found to generate intense small-scale circulations over the storm nesting region. A more detailed analysis of the orographic effect shows that fine scale terrain features (2–5 km) are responsible for the intensification of the convective storms, whereas modification of the low-level flow by the large-scale features (20–50 km) is able to trigger the convective systems. The effect of the solar-induced surface heating is also analyzed and it turns to be crucial not only by intensifying the Iberian thermal low and the easterly warm and moist air advection towards the area but also by promoting mountain breezes.

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 ...

A Method to Reduce the Adverse Impact that Inhomogeneous Station Distributions Have on Spatial Interpolation

Abstract We describe a simple and economic method for reducing the errors that can result from the irregular distribution of data points in linear interpolation schemes that use prescribed, isotropic weighting (IW) functions. The method can be applied to single-step analysis as well as to schemes consisting of more than one step. The starting point of the analysis algorithm is the generation of two datasets (with an IW scheme), one by interpolating the observed field onto the collocated observing sites, the other by interpolating the observed field onto a regular grid. These two datasets are then used independently to estimate two new gridpoint fields as outputs from the same IW analysis scheme. It is assumed that the difference between these two new gridpoint fields is a measure of the error field that results from applying the IW scheme to an inhomogeneous distribution of observing sites, and that this error field is not very different from that associated with the initial gridpoint field analysis. It i...