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TORRENTIAL DAILY RAINFALL PATTERNS IN MEDITERRANEAN SPAIN AND ASSOCIATED METEOROLOGICAL SETTINGS
Romualdo Romero
Clemente Ramis
Meteorology Group. Departament de Física.
Universitat de les Illes Balears.
Palma de Mallorca. Spain

ABSTRACT

Using a 30-years (1964-93) data-base of daily precipitation at 410 sites in Mediterranean Spain, the main spatial patterns controlling "torrential" daily rainfalls in the area are derived. This is done by applying cluster analysis on the most relevant principal directions extracted from a principal component analysis of the between-day (T-mode) correlation matrix. The obtained clusters are quite definite and clearly display the dominant role exerted by the complex orography and its connection with the main rain bearing flows. The western patterns, largely stimulated by Atlantic flows, show similar incidence in winter and autumn. On the contrary, the eastern patterns, which are strongly influenced by the Mediterranean dynamics, dominate in the fall season.

ECMWF gridded data of geopotential at 925 and 500 hPa for the period 1984-93 are used to investigate the synoptic circulations associated with significant rainfall in the same area. Applying cluster analysis on the most relevant T-mode principal components, a classification into 19 fundamental circulations emerges. Some of the identified circulations, which 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.


INTRODUCTION

A dense daily precipitation data-base, extending from 1964 to 1993, was created for the Mediterranean regions of Spain. It is composed of complete and homogeneous series at 410 rain gauge stations (347 in the coastal fringe of peninsular Spain, and 63 in the Balearic Islands, Fig. 1). The raw data consisted of the 3366 available daily precipitation records during some period between 1951 and 1995, provided by the Instituto Nacional de Meteorología of Spain (INM). A first selection was made to consider only stations with a minimum of 1000 data values (almost three years), which yielded a set of 2842 rain gauges. The inventory of these stations showed a great variety of record lengths, with only 5 stations with no missing data. Then, a search was performed to choose the longest sub-period of 1951-95 having the highest number of stations with tolerable completion. The final decision was to keep the 410 stations with 90% of data available during 1964-93 (30 years). An iterative method was followed to check the quality of the data, as well as to fill the missing data by interpolation from the surrounding stations. A detailed description of the method can be found in Romero et al. (1998a).

The region offers an interesting scenario for meso-climatological studies on time and spatial rainfall variability. Geomorphologically, it is characterized by important coastal relief units and complex distribution of sea and land masses (Fig. 1), 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 mid-latitude low-pressure belt and the subtropical highs as a result of its latitude (between 360 and 440 N). Another important climatic characteristic of the region is the torrential aspect of rainfalls, mainly during the autumn. Most of the coastal stations have registered daily rainfalls greater than 200 mm (Font 1983).

The yearly mean precipitation (Fig. 2) shows high amounts about Sierra de Ronda (in the form of an almost circular area with more than 700 mm and peak values in its centre exceeding 1500 mm), appreciable amounts also along Pyrenees, about Sierra de Aracena, Sierra de Cazorla, Sierra de Aitana and the north of Mallorca, contrasting with a general decrease toward inland areas, and very low values in a vast area of the Southeast (south of Valencia, Murcia and eastern Andalucía). In a general sense, the spatial pattern of Fig. 2 reflects a general precipitation gradient along the SE-NW direction. This feature is connected, apart from the orography, to the degree of proximity to the Atlantic waters from where the majority of storms arrive.


Fig. 1. The Spanish Mediterranean area. It includes a smoothed version of its orography, the position of the stations of the daily rainfall data base (410 in total), and the location of places mentioned in the text.


Fig 2. Yearly mean precipitation for 1984-93

As noted previously, Mediterranean Spain is frequently affected by extreme daily rainfalls. Figure 3a shows that extreme rainfalls tend to concentrate in coastal areas, and secondarily, in mountainous interior lands. The maximum values are found in Sierra de Ronda, where a station attains the 21-25 days category. Valencia and Murcia define an extensive area where 100 mm occurrences (in general in the 1-5 or 6-10 ranges) are given by most of the stations. It is notable the high amount of 100 mm days given by the stations located close to Sierra de Aitana. This zone has been classically considered as the most torrential in Spain. Observe how the peninsular arid Southeast participates actively in this case. Extreme rainfalls are also frequent in the island of Mallorca along Sierra de Tramuntana, and in its eastern part where a minor topographic ridge exists. Ibiza also registered 100 mm rainfalls. The seasonal distribution (not shown) demonstrates that most of the 100 mm rainfalls concentrate in the autumn season. Winters occupy the second place, followed by springs. Extreme rainfalls are rare during summer. Events of 200 mm are almost exclusive of autumn, although also tend to occur during winter in Sierra de Ronda.


Fig 3a. Days with extreme rainfalls (> 100 mm) during 1984-93.


Fig 3b. Recurrence intervals of 100 mm events, for 1984-93.

A direct demarcation of the torrential zones of Mediterranean Spain can be done by visualizing the recurrence intervals. For 100 mm daily rainfalls (Fig. 3b), recurrence intervals lesser than 5 years are obtained about Sierra de Aracena, in a wide zone around Gibraltar Strait, in very localized zones of eastern Andalucía close to the coast or at high altitudes, in Murcia and Valencia, in the littoral of northern and southern Catalonia, zones of Pyrenees, and north and east of Mallorca. A deeper analysis of the torrential character of rainfalls is included in Romero et al. (1998a).

In this study the main spatial patterns controlling torrential daily rainfalls in Mediterranean Spain are derived. Furthermore, we investigate the synoptic atmospheric circulations associated with those patterns.


TORRENTIAL DAILY RAINFALL PATTERNS

From the 30-years precipitation data-base, we define as torrential days those days in which at least 2% of the stations registered more than 50 mm, yielding a total of 449 events. Of the 449 selected days, 35.2 % occurred in winter, 14.9 % in spring, 5.1 % in summer and 44.8 % in autumn.

The approach followed to derive the typical precipitation patterns consists in subjecting the T-mode (day-by-day) correlation matrix to principal components analysis (PCA), and carrying out cluster analysis (CA) on the most important extracted components. That is, days participating with similar loadings on the extracted components are clustered together. This approach is aimed to join days with similar precipitation distributions, irrespectively of the precipitation amounts (Sumner et al. 1995).

For deciding the number of PCs to retain, the simple scree test of Cattell (1966) was adopted. We retained 15 PCs, which account for 68.5 % of the total variance. For the cluster analysis, the non-hierarchical k-means method (Anderberg 1973), as implemented in the STATISTICA utility (1994), was used. The Euclidean distance was taken as the similarity index. Hierarchical tree plots generated by Ward's method (Ward 1963) were also considered as reference for deciding how many clusters to create. A solution comprising 8 typical patterns was chosen. These pattern groups are presented in Fig. 4.

The obtained pattern groups are quite definite and clearly display the dominant role exerted by the complex topography and its connection with the main rain bearing flows. Plot of inter-seasonal variability (Fig. 5) reveals a different incidence of torrential rainfall patterns through the year. The western patterns, largely stimulated by Atlantic flows, are similarly important in winter and autumn, whereas the eastern patterns, which are strongly influenced by the Mediterranean dynamics, dominate in autumn. A more detailed analysis can be found in Romero et al. (1998b).


Fig 4. Daily rainfall composites for the 8 pattern groups of torrential rainfall in Mediterranean Spain.
The number of days for 1964-93 included in each pattern group is indicated in parentheses (total, 449)


Fig 5. Seasonal distribution for the 8 pattern groups of torrential daily rainfall


ATMOSPHERIC CIRCULATION PATTERNS

This section attempts to identify the main atmospheric circulation settings associated with the torrential patterns presented in the previous section. There are several studies in the literature which have illustrated and interpreted statistical connections between synoptic atmospheric patterns and certain surface weather and/or environmental parameters. In his book, Yarnal (1993) offers multiple examples of synoptic classifications, ranging from subjective manual classifications to outputs from eigenvector-based techniques. Bonell and Sumner (1992) establish, using S-mode PCA and CA, the main daily precipitation affinity areas for Wales according to surface wind direction. Sumner et al. (1995) associate the distribution of significant rainfalls over the island of Mallorca with recognized dominant surface circulation types.

Climatology-based scenarios of rainfall distribution for defined synoptic types have been conceptually recognized for a long time, and have become apparent through numerous case studies of heavy precipitations in eastern Spain (see Doswell et al. 1998 and references mentioned therein).

The meteorological data used to carry out the synoptic classification are the European Centre for Medium Range Weather Forecast (ECMWF) grid analyses of geopotential height at 925 and 500 hPa. The spatial resolution is 0.750 in both latitude and longitude. Since the ECMWF has only been operative since 1979, the meteorological data-base has been restricted to the last decade of the 30 years of the original rainfall data base (1984-93). During that decade, 1275 days were identified as significant rainfall days (5% of stations registered more than 5 mm) in Mediterranean Spain. Of these, 165 were also classified as torrential (previously defined). The classification of atmospheric circulation thus utilizes 1275 unique circulation patterns, one per significant rainfall day.

Circulations were classified using T-mode PCA and CA, as for the derivation of the torrential patterns in last section. We have considered data only within the rectangular geographical window between 33.750 and 45.750 N, and between 11.250 W and 6.000 E. It thus comprises 408 grid points. Classifications based on larger windows encompassing much larger geographical areas were also tested, but associations with rainfall patterns were very poor, because the classifications were strongly influenced by circulation features of regions remote from Mediterranean Spain.

Application of the scree test suggests the retention of 6 PCs for the geopotential field at 500 hPa (accounting for 96.7% of the total variance), and 8 PCs for the geopotential field at 925 hPa (95.7% of the total variance). Thus, 14 variables were used in the CA, performed again with the k-means method. Solutions involving 4, 7, 11 and 19 clusters were clearly indicated. The former three solutions produced too dilute an association with rainfall distributions, and an appropriate compromise for a still relatively simple collection of patterns, but with enough meaningful associations was found in the 19 cluster solution. These clusters (referred to as atmospheric patterns, APs) are presented in Fig. 6. Table 1 shows how the 1275 significant and the subset of 165 torrential rain days are distributed amongst the derived 19 atmospheric patterns.

Table 1. Distribution of significant days and torrential days for the 19 atmospheric circulation patterns.
Torrential days are also shown as a percentage of the total number of significant days.

In a general context, it is possible to argue that the synoptic types obtained summarize four main more general scenarios. First, a large scale disturbance lies to the west or north-west of the Iberian Peninsula producing humid Atlantic flows which encourage rainfall development in the bulk of western Andalucía (APs 1, 2, 3, 4 and 5). The second occurs with the passage of cold fronts over the Iberian Peninsula linked to higher latitude low pressure systems (APs 7, 8 and 9). This scenario favors the development of rainfalls in north-eastern Spain and in the inland mountainous areas of Andalucía. In the third case, relatively small lows at 500 hPa are found about the southern part of Spain, and the associated low-level flux over the Mediterranean is warm and humid from the east-southeast (APs 6, 10, 11, 12, 13, 14 and 15). This configuration leads to rainfalls over the eastern flank of Spain, including the Balearic Islands. Finally, for the fourth type, upper and low level disturbances are located to the east of the Iberian Peninsula (APs 16, 17, 18 and 19), inducing strong flows with a pronounced northerly component.

As a further refinement of the analysis carried out, the days producing torrential rainfall were extracted, and their association with atmospheric circulation has been investigated. Figure 7 shows, for each AP, how the torrential events distribute among the 8 torrential patterns (Fig. 4). Circulation patterns AP11 and AP16 were not associated with any torrential event. By contrast, AP13 is associated with the development of torrential rains on almost 40 % of occasions (see Table 1). Also noteworthy are AP3, AP6, AP12, AP14 and AP15, with the incidence of torrential rain exceeding 20 %. As can be observed from Fig. 6, these more torrential APs are characterized in the middle troposphere by closed cyclonic circulations or very accentuated short-wave troughs located in the south of the domain. At low levels, they exhibit a significant level of warm advection towards some area of Mediterranean Spain. These conditions have been observed in most studies of severe rainfall events in this area (for example, Doswell et al., 1998; Ramis et al., 1998).

Figure 7, although obtained from only 165 torrential events, is quite informative. The distributions shown are physically consistent with the dynamic processes contained in the corresponding APs. The combination of important topographic enhancement of rainfall for moist Atlantic or Mediterranean airflows, or both, and dynamic factors associated with the upper level disturbance, in fact render the physical interpretation of the relationship between atmospheric and rainfall patterns quite straightforward. Focusing the attention on the most torrential APs, AP3 favours torrential patterns T1 and T2 in western Andalucía, AP6 favours T4 and T6 and in a lesser extent T2 and T5, AP12 projects almost exclusively on T5, AP13 and AP14 favour significantly torrential rainfalls in Murcia, Valencia and the Balearic Islands through T4, T5, T6 and T8, and AP15 produces T4, T6 and T8. See Romero et al. (1998c) for details.

   
   
   
   
   
   
   
   
   

Fig. 6. Composites of the 19 atmospheric circulation patterns. The continuous line represents geopotential field at 925 hPa (contour interval is 10 mgp), and the dashed line that at 500 hPa (contour interval is 20 mgp).


Fig. 7. Percentage frequency of the 8 torrential daily rainfall patterns within
the 19 atmospheric circulation patterns for Mediterranean Spain.


CONCLUSIONS

Application of T-mode PCA and CA on a 30-years data-base of daily precipitation at 410 sites, has permitted to derive a simplified collection of 8 spatial patterns governing torrential daily rainfall events in Mediterranean Spain. A visual inspection of those patterns confirms that the extent of the region and the exposure-sheltering systems induced by the complex topography are sufficiently important as to produce a clear regionalization of torrential rainfalls. Distinct seasonal distributions have been observed: the western Andalucía patterns occur with similar incidence in winter and autumn, whereas the eastern patterns have a clear predilection for the autumn season.

Using a similar methodology, a total of 19 fundamental synoptic patterns has been derived that explain the development of significant daily rainfalls in the Spanish Mediterranean area. A study of the torrentiality of the derived atmospheric patterns reveals that some of them, although important for explaining significant rainfalls, are irrelevant for the occurrence of torrential rainfalls in the region. The situations characterized by disturbances located about the south of the Iberian Peninsula, although not very frequent, exhibit a high propensity towards the development of torrential rainfalls focused on the eastern flank of the Iberian Peninsula and the Balearic Islands.

The results obtained should be most valuable in providing a more detailed forecast of rainfall activity in Mediterranean Spain within the context of synoptic numerical weather prediction, and as a complement for meso-scale resolution models. The assessment of the long-term rainfall variability in the region may also benefit from these results.


ACKNOWLEDGEMENTS

Raw precipitation data and ECMWF meteorological fields were provided by the Instituto Nacional de Meteorologia of Spain. This work has been sponsored by CICYT grant CLI95-1846.


REFERENCES

Anderberg, M. R. 1973: Cluster analysis for applications. Academic Press, 359 pp.

Bonell, M. and Sumner, G. N. 1992: Autumn-winter daily precipitation areas in Wales, 1982/83 to 1986/87. Int. J. Climatol., 12, 77-102.

Cattell, R. B. 1966: The scree test for the number of PCs. Mult. Behav. Res., 1, 245-276.

Doswell III, C. A., Ramis, C., Romero, R. and Alonso, S. 1998: A diagnostic study of three heavy precipitation episodes in the western Mediterranean region. Wea. Forecasting, 13, 102-124.

Font, I. 1983: Climatología de España y Portugal. Instituto Nacional de Meteorología, Apartado 285, 28071, Madrid, 296 pp.

Ramis, C., Romero, R., Homar, V., Alonso, S. and Alarcón, M. 1998: Diagnosis and numerical simulation of a torrential precipitation event in Catalonia (Spain). Meteorol. Atmos. Phys. (in press).

Romero, R., Guijarro, J. A., Ramis, C. and Alonso, S. 1998a: A 30 year (1964-1993) daily rainfall data base for the spanish mediterranean regions: First exploratory study. Int. J. Climatol., 18, 541-560.

Romero, R., Ramis, C. and Guijarro, J. A. 1998b: Daily rainfall patterns in the Spanish Mediterranean area: An objective classification. Int. J. Climatol., (in press).

Romero, R., Sumner, G., Ramis, C. and Genovés, A. 1998c: A classification of the atmospheric circulation patterns producing significant daily rainfall in the Spanish Mediterranean area. Int. J. Climatol., (submitted).

STATISTICA, 1994: STATISTICA for Windows (Volume III): Statistics II. StatSoft Inc., Tulsa OK.

Sumner, G. N., Guijarro, J. A. and Ramis, C. 1995: The impact of surface circulations on the daily rainfall over Mallorca. Int. J. Climatol., 15, 673-696.

Ward, J. H. 1963: Hierarchical grouping to optimize an objective function. J. Amer. Stat. Assoc., 58, 236-244.

Yarnal, B. 1993: Synoptic climatology in environmental analysis, Belhaven Press, London, 195 pp.

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