Classification of daily abundant rainfall patterns and associated large-scale atmospheric circulation types in Southern Portugal

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 28: (2008) Published online 23 July 2007 in Wiley InterScience ( Classification of daily abundant rainfall patterns and associated large-scale atmospheric circulation types in Southern Portugal M. Fragoso,* and P. Tildes Gomes Centro de Estudos Geográficos, Universidade de Lisboa, Lisboa, Portugal ABSTRACT: The aim of this study is to carry out two objective classifications concerning the occurrence of episodes of heavy rainfall in Southern Portugal. The first one seeks to define the main spatial distribution patterns associated with abundant rainfall days in this region. Using a 15-year (from October 1983 to April 1998) precipitation data base corresponding to 98 sites in Southern Portugal, a sample is established by extracting the precipitation days in which the threshold of 40 mm/24 h was exceeded in three or more stations, yielding a total of 199 days. It is found that the Algarve region is the one where episodes of heavy rainfall are most frequent and which exhibits the strongest torrential character. The highest frequency of days with heavy rain occurs in the fall and in the early part of the winter. Through the combined use of principal component analysis (PCA) and k-means clustering, four different rainfall patterns are identified, and each one exhibits specific characteristics. The second classification is obtained using the same computational method (PCA coupled with k-means) and seeks to define the fundamental large-scale atmospheric circulation types associated with abundant rainfall in the region, based on sea level pressure (SLP) data. The data was collected from the NCEP/NCAR Reanalysis dataset, taking SLP as the most discriminating variable. Five circulation types are thus obtained, indicating considerable diversity among the large-scale atmospheric circulation conditions that are conducive to the occurrence of heavy rainstorms in Southern Portugal. The article concludes by discussing the main links between the two classification schemes, in order to ascertain the influence of the large-scale atmospheric circulation upon the spatial distribution patterns of heavy precipitation episodes. Copyright 2007 Royal Meteorological Society KEY WORDS Southern Portugal; principal components analysis; cluster analysis; rainfall patterns; large-scale atmospheric circulation types Received 9 January 2006; Revised 9 April 2007; Accepted 16 April Introduction The southern part of Portugal is usually considered as the region located to the South of the Tagus river and covers an area of approximately km 2 (Figure 1). The region s location and orography contribute to the general features of the climate, which has a typical Mediterranean character, with a well marked dry season centred in the summer months. In general, the average annual rainfall decreases from North to South, as it does as we move inland from the coastal regions. In most of Southern Portugal, the mean value of annual rainfall varies between 500 and 800 mm. The wetter areas correspond to the highest parts of the major mountains, such as the Serra de S. Mamede (which is located in the Alto Alentejo region and exhibits a mean annual level rainfall of over 1000 mm) and the Serra de Monchique (which is located in the Algarve region and has a mean value of over 1200 mm). By contrast, the * Correspondence to: M. Fragoso, Centro de Estudos Geográficos, Faculdade de Letras, Alameda da Universidade, , Lisboa, Portugal. marcelofragoso@mail.telepac.pt average annual rainfall is less than 400 mm in the lower Guadiana river valley. The duration of the dry season is shortest in the northern areas (Ribatejo region) and increases as we move southward, reaching five months in the eastern Algarve lowlands (considering the Bagnouls Gaussen aridity index). The severity of the drought conditions during summer is also more intense in the interior parts of the region and especially in the Alentejo. Ventura (1994, 1996) has demonstrated that the rainfall regime in Southern Portugal, as in many other regions with a Mediterranean climate, is highly irregular and variable. One particularly relevant feature of the rainfall regime in Southern Portugal is the occurrence of episodes of rain of torrential character, especially during the fall season (Brandão and Fragoso, 1999; Fragoso, 2003), which often represents a potential of economic damage. These intense precipitation events may lead to significant damages, by causing flash floods that affect small drainage basins (Ramos and Reis, 2002) and by triggering landslides and bringing about bank erosion, as described by Zêzere et al. (1999) in the case of the region to the North of Lisbon. In the past decade, several episodes of intense Copyright 2007 Royal Meteorological Society

2 538 M. FRAGOSO AND P. TILDES GOMES one concerns the rainfall distribution patterns associated with heavy rain events. This is a very important issue and the results of this analysis could prove useful not only to climatologists but also to other experts, like the environmental planning domain. The primary goal of this study is to improve the knowledge of geographic incidence of heavy rainstorms in Southern Portugal, seeking to provide an objective description of their main spatial patterns. The other main goal of the research is to identify the main large-scale atmospheric circulation types that are favourable to the occurrence of rainstorms. In order to pursue this aim, an objective classification of the most important large-scale atmospheric circulation patterns associated with abundant precipitation events is put forth and discussed. Figure 1. Location and orography of Southern Portugal. The sites mentioned in the text are indicated. rainfall lay at the origin of flash flood events occurring in Southern Portugal. The most catastrophic occurred on 5 November, 1997, when one single storm gave rise to a flash flood that was responsible for 12 casualties in the small area of Ourique, in the Baixo Alentejo region. On the following day, the same storm reached the Spanish city of Badajoz (Cerro dos Reyes area) causing worse disaster, killing 22 people. The climatic relevance of the intense rainfall events in Southern Portugal is also clear from the point of view of its contribution to the total amount of annual precipitation. This contribution is considerable in some years, which shows that as regards these heavy precipitation events, one must take into account, not only their dangers, but also their positive impacts upon human activities such as agricultural production or water resource management. Earlier studies have analysed the relationship between the large-scale atmospheric circulation and the precipitation over the Portuguese territory (Corte-Real, 1995; Corte-Real and Qian, 1998; Corte-Real, 1999; Trigo and DaCamara, 2000; Trigo et al., 2002; Goodess and Jones, 2002; Santos et al., 2005). These studies have pointed out the strong influence of the NAO (North Atlantic Oscillation) upon the variability of precipitation in Portugal, especially as regards the winter rainfall, and also showed the diversity of large-scale APs associated with monthly or daily rainfall. Nevertheless, there is still a lack of research concerning the main rainfall patterns associated with intense rainstorms, as well as their atmospheric environment. In this article, the issue of abundant rainfall in Southern Portugal is analysed from two main perspectives. The first 2. Data bases: Precipitation and atmospheric data 2.1. Precipitation data Two different databases were used in this study. The first one was a daily precipitation database, collected with the support of the National Water Resources Institute (INAG), Portugal. The INAG network comprises raingauges that perform daily observations at 9 a.m., so their series are organized according to 24-h time units (from 9a.m.ofday 1 to 9 a.m. of the subsequent day). It must be underlined that the temporal resolution of these precipitation data cannot be refined. In creating this data set, we have sought to draw a balance between spatial resolution (higher number of rain-gauge stations) and the longest possible time period. Only in the early 1980s did the spatial coverage of the rain-gauge network in Southern Portugal become relatively satisfactory, despite some gaps in the Alentejo mountainous areas. Ninety eight stations were selected (resulting in a mean spatial resolution of around 3 stations/1000 km 2 ), providing continuous and reliable data over a common period of 15 years (between 1 October 1983 and 30 April 1998). The selection of the 98 sites resulted from the previous quality control of the precipitation records, which allowed for the rejection of all the stations that were characterized by problems such as heterogeneity of data or serious gaps and errors in the time series. A decision was made to consider only those rain-gauge stations for which at least 95% of the data relative to the period between 1983 and 1998 was available. Therefore, the initial precipitation data set considered for the purpose of this study is a matrix, comprising data from 98 sites and 5326 days, which constitutes enough data to allow for the identification of the main rainfall patterns. A second procedure consisted in the extraction of a subset of data from the initial matrix, in order to retain only those rainy days that exhibited torrential character. It was therefore necessary to choose some criteria that would lead to the establishment of a sui threshold, in order to allow for a clear distinction between what constitutes a day of abundant (or heavy) rainfall in the Southern Portugal region and what does not. The

3 RAINFALL PATTERNS IN SOUTHERN PORTUGAL 539 most common threshold applied for this purpose in areas with a subtropical climate is an amount of 50 mm/day (Chagnon, 1994; Romero et al., 1999; Peñarrocha et al., 2002) The selection of the optimal threshold should not be the result of an arbitrary decision; rather, it should be the result of the statistical analysis of the daily precipitation series, which can be done by plotting the frequency distribution curves obtained using different class limits. We,thus, found that the value of 40 mm/24 h was the most adequate as a precipitation threshold to be used in selecting the rainy days associated with extreme events. Moreover, this option produced a final subset of data that comprised a significant number of days, leading to results with statistical significance. Hence, a decision was made to keep from the initial precipitation data set only those days in which at least three stations registered more than 40 mm, yielding a total of 199 days. This subset constitutes a suitable sample, comprising numerous events of a strong torrential character: in 44 rainy days (22.1% of the sample), the amount of precipitation exceeded 100 mm in at least one station of the network. Autumn (October, November and December) is clearly the season with the highest frequency of occurrence of abundant rainfall events in Southern Portugal: 60.3% of the 199 days of the sample. Winter accounts for 22.6% of the total number of rainy days and spring for 12.6%. The number of such events occurring in summer is very limited, with just nine heavy rainfall days (4.5%) in the sample corresponding to that part of the year Atmospheric data Daily averages of several variables were used, including SLP, precipitable water (PW) and geopotential heights fields (500 hpa, Z500). This data was obtained from the NCEP/NCAR Reanalysis dataset (Kalnay et al., 1996), made available by the Climate Diagnostics Centre (CDC) in their website. It is a gridded dataset with a spatial resolution of 2.5 that includes data for the entire globe from 1948 until the present. For the purpose of this research, we extracted the data that corresponded to a geographical window defined by 25 N 55 N and 40 W 30 E, in whose centre Southern Portugal is located, yielding a total of 377 points (variables). Considering that this study was specifically concerned with heavy rainfall events, a decision was made to use the same 199-day sample that was used in classifying the precipitation distribution patterns. In the joint analysis of the two data sets, it was necessary to bear in mind that the daily rainfall data referred to 24-hour time units that comprised the period between 9 a.m. in a given day and 9 a.m. in the following day, whereas the atmospheric data referred to the period between 0 and 24 h. After analysing the bias associated with this problem, we have concluded that the best solution consisted in selecting, from the reanalysis data, the daily time units with the biggest overlapping period when compared to the rainfall data. With this option, a better agreement was found between the large-scale atmospheric circulation and the occurrence of rainstorms over Southern Portugal. 3. Methodology As was mentioned in Section 1, the aim of the general approach followed in this study was to identify the main rainfall patterns associated with torrential events in Southern Portugal as well as the corresponding largescale atmospheric circulation patterns, by constructing two objective classifications. The methodology used is discussed by Sumner et al. (1995) and Sumner (1996). Earlier studies that applied the same statistical approach (PCA coupled with k-means) produced good results for a number of different areas (Kidson, 1994; Sumner, 1996). More recently, Sumner et al. (1995), Romero et al. (1999a, 1999b) and Peñarrochaet al. (2002) applied the same technique to Mediterranean Spain and inspired our interest in performing a similar study with regard to Southern Portugal. The PCA coupled with k-means clustering is a suitable statistical approach in order to filter and classify daily climate data. The procedures that were followed in this research consisted of two main steps. In order to render these procedures clear, we shall first describe the details of the methodology used in establishing a classification of daily (abundant) rainfall patterns (the first step). The issues concerning the classification of the corresponding large-scale atmospheric circulation patterns (the second step) shall be subsequently described, as well as the strategy used to establish some links between the two cluster schemes. Thus, the identification of the main atmospheric circulation types was based on the rainfall patterns that were previously identified, which corresponds to the adoption of a bottom-up approach which was deemed more appropriate than the alternative process (known as the top-down approach), according to the results presented in the aforementioned papers Daily abundant rainfall patterns The first procedure consisted of transforming the initial data set (98 stations/variables and 199 days/cases) into a between-day correlation matrix. As was demonstrated by Sumner et al. (1995), Sumner (1996), Romero et al. (1999a) and Peñarrocha et al. (2002), the use of a between-day (t-mode) correlation matrix has the advantage of providing a primary distinction within the data, highlighting those days which presented more similarities in their spatial patterns. PCA analysis was then applied to this correlation matrix, in order to extract the main modes of variation of the data and to reduce its complexity to a few principal components (PCs). This procedure requires that several decisions and judgements be made, namely as regards the type of computation (rotated or non-rotated methods) and the number of extracted components to retain. After a number of attempts, the most satisfactory results were found by using the non-rotated method. Alternative methods

4 540 M. FRAGOSO AND P. TILDES GOMES Figure 2. LEV for the first 20 principal components extracted from the PCA applied to heavy rainfall data. such as VARIMAX rotation (Richman, 1986) produced much more complex results, substantially reducing the explained variance in the first six PCs without providing any advantage in terms of its interpretation. Therefore, the first 13 PCs were extracted from the non-rotated solution, following the observation of the slope breaks in the logarithmic eigenvalue (LEV) (Figure 2). A clear break is visible at the 13th PC, where the accumulated percentage of explained variance reaches 73% of the total. Cluster analysis (CA) using the (non-hierarchical) k-means method was then performed on a matrix with 199 case loadings (the abundant rainfall days) and 13 variables (the PCs extracted from the PCA). The aim of this procedure was to group days with similar loadings into clusters, which amounts to deriving the most representative daily rainfall patterns. In performing this clustering procedure, the index similarity adopted was the euclidean distance. The use of the k-means method also requires that a previous decision be made with regard to how many clusters should be created, which means that the optimal number can only be found after testing several solutions in a number of different attempts. It was concluded that the best results were obtained by using four clusters and the patterns generated by alternative solutions (five or six clusters) were in fact, not significantly different Large-scale atmospheric circulation patterns A subset of the NCEP/NCAR Reanalysis dataset was used to create a matrix containing as variables, the 199 days previously used in classifying the rainfall patterns and 377 points in space, as cases. The same method used in the classification of the rainfall patterns was applied in order to identify the main circulation types associated with the abundant rainfall days (exceeding 40 mm), based only on SLP reanalysis data. PCA was thus performed on this matrix, and five non-rotated PCs retained, corresponding to an accumulated explained variance of 76.5%. CA using the (nonhierarchical) k-means method was then used to identify the correlation between the 199 days and the 5 extracted PCs. The subsequent procedure consisted in performing a comparison between the classifications obtained using four, five and eight clusters in order to identify the optimal solution. An option was made in favour of the classification using five clusters, which produced different large-scale circulation types and distinct features of the respective SLP patterns. Finally, composite charts of the five SLP patterns were constructed, mapping composite charts of other NCEP reanalysis atmospheric variables for each cluster. This article will show figures containing composite charts of the SLP fields, Z500 and PW. The described methodology corresponds to the sequence of procedures that have produced more interesting and understandable results. An alternative methodology was essayed and it consisted of compiling direct composite maps (E.g. from SLP and 500 hpa geopotential fields) based on the clusters of the rainfall patterns previously obtained. However, the large-scale circulation types identified in these direct composite maps were not easily distinguishable as the circulation types resulted from the objective classification, and thus it was decided to exclude them from this approach. 4. Results and discussion 4.1. Rainfall patterns In this section, the main features of the two classifications are discussed, in order to verify the distinction between the clusters and the physical significance of the rainfall and SLP patterns identified, as well as their global and seasonal frequency. The daily abundant rainfall patterns (DARP) are shown in Figure 3, in which the mean amount of daily precipitation (mm) registered in each of the four clusters has been plotted. The contour lines in these rainfall composites were drawn using the krigging method. If we compare the four maps presented, we find that the distinction between the DARPs is quite evident and that all of them represent a different kind of spatial distribution of the precipitation. In each DARP, there are distinct areas that are especially affected by abundant rainfall, and this conclusion can also be drawn from the observation of Table I. It is quite evident that, in a general overview, the Algarve is the region most affected by abundant rainfall events (at least using the 24-h time scale). Two of the four rainfall patterns (DARP 3 and DARP 4) are associated with heavy rainstorms affecting the Algarve region, and it is interesting to note that those rainfall patterns are also the most frequent ones (Table I). Another relevant fact that is clear from the analysis of Figure 3 is the higher propensity for the occurrence of heavy precipitation events over the Algarve Mountains. In the stations located in these mountains, the threshold of 40 mm/24 h was exceeded in 73.2% of the 199 heavy rainfall days and, in the same areas, the daily events with a notable torrential character (exceeding 100 mm/24 h) occurred 24 times in

5 RAINFALL PATTERNS IN SOUTHERN PORTUGAL 541 Figure 3. Composites of the daily abundant rainfall patterns (DARP) obtained in the classification. (Note: each pattern has its own precipitation scale). Table I. General characteristics of the four clusters obtained for the heavy rainfall patterns classification (locations in Figure 1). Heavy rainfall pattern (DARP) Number of Days included Areas most affected (maximum precipitation) DARP1 40 The whole region with maxima over the mountains DARP2 42 Northern area (Ribatejo region) and a secondary maximum over the Algarve mountains DARP3 62 Eastern Algarve with a maximum over the southern flank of the Caldeirão mountain DARP4 55 Western Algarve with a maximum over the Monchique mountain 199 the 15 years of the study period, which can be considered a considerable frequency. DARP 1 exhibits moderate rainfall in the whole region, with a general increase in the case of most of the higher uplands, such as S. Mamede, Monfurado, Cercal, Monchique and Caldeirão. The maximum precipitation occurs in the Algarve Mountains, but the main feature of this particular pattern is the wide distribution of the rainfall areas, which comprise the entire region. The majority of the days associated with this pattern occurred in the fall (45% of cases in the trimester between October and December) and winter (30% of the events took place in January, February or March) seasons. DARP 2 represents heavy rainfall over the northern part of the region, namely in Ribatejo and Alto Alentejo. In this pattern, the precipitation decreases substantially from North to South, except in the case of the Algarve, where a secondary maximum is registered over the mountains. Abundant rainfall occurs mostly in the northern areas of the region, suggesting that this pattern could be related to rainstorms crossing Northern and Central Portugal. The highest percentage of days associated with

6 542 M. FRAGOSO AND P. TILDES GOMES this pattern corresponds to the fall season (59.5% of the cases). DARP 3 is characterized by very abundant and intense rainfall occurring in eastern Algarve. The maximum precipitation is located in the southern and eastern flanks of the Caldeirão Mountain, i.e. the upper parts of the slopes that are most exposed to warm and wet southerly fluxes. The abundant rainfall concentrated over the eastern part of the Algarve becomes more moderate to the west and decreases rapidly to the North. In the study period, this was the most frequent of the four DARP, and it is mostly associated with rainstorms occurring in the fall (66% of these events occurred in October, November or December). DARP 4 represents abundant rainfall in western Algarve, with a maximum in the highest areas of the Monchique Mountain. As in the case of the previous pattern, heavy rains are also spatially concentrated in this cluster, and the considerable daily amounts suggest the clear torrential character of the rainfall events. This pattern exhibits a gradual decrease in precipitation from west to east and a much sharper decrease from south to north. As in the other pattern groups, the days included in this cluster consisted mostly of autumn days (63.6% of the total number of cases) Main atmospheric circulation types As explained in Section 3, the classification of the main large-scale atmospheric patterns (APs) associated with abundant rainfall days yielded five different clusters. Composite charts of the large-scale atmospheric circulation schemes for each type are shown in Figures 4 and 5. Bearing in mind the description made in Section 3, it is worth stressing once again that this classification was based on the SLP fields. As can be concluded from the visual inspection of Figures 4 and 5, all these APs are clearly distinguishable. It is interesting to verify that such a diversity of large-scale circulation characteristics could bring about similar consequences, i.e. the occurrence of heavy rainstorms in Southern Portugal. The common characteristics of all the patterns can be summarized as consisting of two main features: the activity of dynamic low pressure systems, contributing to the development of deep convection; and the influence of low levels fluxes with a southerly component, providing the advection of warm and moist air. Nevertheless, their specific characteristics are also very important, and may account for the diversity of the rainfall patterns generated, as described in Section 4.1. Table II indicates some aspects and general features concerning the distinction between the five APs. The AP 1 type is associated with a blocked circulation pattern at 500 hpa level (with divergence of the westerlies and is characterized by the presence of a stationary depression centred to the west of the Algarve. This blocked circulation is not visible in Figure 4 because the averaging of the AP 1 composite map resulted in a Figure 4. Composite charts of large-scale atmospheric patterns (AP 1, 2 and 3) associated with abundant rainfall days (SLP data is represented by continuous lines and 500 hpa geopotential fields by dashed lines). The grey scale represents the mean amount of PW. considerable smoothing of the main pattern identified in individual maps. A warm and wet southerly flow at the low levels over Southern Portugal provides substantial moisture to the depression, where the thermodynamic instability is probably increased by the presence aloft of cold air. This is a pattern typical of the fall season, as 62% of the days included in this group occurred between October and December. AP 2 is the most frequent pattern and corresponds to a quite different large-scale circulation scheme. A large and deep depression extends its influence over the whole of western Europe, characterized by a fast westerly flow at the level of the medium and high troposphere. This is the only AP obtained that corresponds to high zonal index conditions. In spite of its distant location, the influence of the low pressure centre, which is located to the west of the British Islands, reaches as far South as Morocco. The

7 RAINFALL PATTERNS IN SOUTHERN PORTUGAL 543 Figure 5. Composite charts of large-scale atmospheric patterns (AP 4 and 5) associated with abundant rainfall days (SLP data is represented by continuous lines and 500 hpa geopotential fields by dashed lines). The grey scale represents the mean amount of PW. cold season (December February trimester) accounts for most (52%) of the days included in this pattern. AP 3 is characterized by the influence of a trough depression; the location of this trough results in the influence of a south-westerly current over the region. The axis of the trough extends along the western coast of the Iberian peninsula and elongates towards the Morocco seaboard. Under these circulation conditions, the Southern Portugal region is subject to the influence of the eastern part of a low-pressure system, where the upward motion of the air fluxes is more active. This pattern could mostly be observed in autumn (44.5% of the cases occurred in the October December trimester), but also in winter (34% of the total number of included days). Among the five patterns, AP 4 is the one characterized by the influence of a depression located at the lowest latitudes, near the 35 N parallel. Despite the low intensity of the surface cyclone, as well as its relatively small size, aloft the depression system is linked with a deep and oblique trough, extending along the western coasts of Portugal and Morocco. Taking the geographic location of the low pressure system into account, it is quite evident that the tropical air most likely contributes significantly to the air mass structure of the depression system. As in the case of several other patterns, AP 4 occurs mostly in the autumn (which accounts for 63.3% of the total number of days included in this type). Finally, the AP 5 pattern exhibits some similarities with regard to AP 1. In both of these two patterns, the surface low is centred to west of the region and it is linked to a cut-off-low at the 500 hpa geopotential. However, in the case of AP 5, the low pressure system is larger and slightly more intense than in the first type described. Besides, the characteristics of the blocked circulation at the upper levels are different, indicating a cut-off cyclone. These conditions bring about the influence of a southwesterly flow at the low levels of the atmosphere, upon which a cold core is superposed, contributing to a context of high instability over the Southern Portugal region. Table III was constructed with the aim of ascertaining the relationship between the two classifications, i.e. in order to assess how the APs can explain which rainfall pattern is most likely observed. The table depicts the correspondence between the relative frequency of each type of DARP and the various APs. By analysing the contents of Table III, it becomes evident that there is only one obvious linkage between the two cluster schemes. The occurrence of precipitation days when abundant rainfall affects the eastern Algarve (DARP3) is quite strongly associated with the influence of lows located over the Cadiz Gulf (AP 3). The cyclogenesis over this subtropical oceanic basin, as well as the more southern location of the low pressure systems, seems to play an important role in ensuring that the eastern Algarve is crossed by a southeasterly flow (warm and wet, especially during the fall season) in the lower troposphere. On the other hand, all the other DARP could be generated by several different APs, which suggests Table II. General features of the atmospheric patterns (AP s), based on SLP data. (In this table, the Southern Portugal territory is simply designated as the region ). Atmospheric pattern Location of the low pressure system Surface size of the low pressure system Surface flux direction over the region Upper-level (500 hpa) circulation features over the region AP 1 Southwest of the region Medium S cold pool influence AP 2 Northwest of the Iberian very large SW vigorous zonal flow Peninsula AP 3 Over the Portuguese coast Medium S talweg (eastern flank trough influence) AP 4 South of the region (overalgarve//cádiz Atlantic Bay) Medium SE deep trough extending from the British Islands to the Atlantic coast of Marocco AP 5 West of the region Large S cold pool (cut-off) influence

8 544 M. FRAGOSO AND P. TILDES GOMES Table III. Distribution of the 199 heavy rainfall days for each type of the atmospheric patterns classification (in bold, percentages greater than 30%; percentages greater than 25% are underlined). Atmospheric pattern Abundant rainfall days %of total DARP 1 DARP 2 DARP 3 DARP 4 AP AP AP AP AP that the large-scale surface circulation probably does not have a distinct influence upon the definition of the rainfall patterns. Naturally, we must regard these results with caution, as the spatial resolution of the atmospheric data does not allow for any information with regard to the influence of smaller (sub-synoptic or meso-scale) depressions. 5. Conclusion Using precipitation data from a 15-year period ( ), four DARPs were identified in Southern Portugal. Two of them (named DARP 3 and DARP 4 in the previous section) are clearly associated with a higher amount of rainfall, comprising rainfall days with a torrential character. Both patterns particularly affect the Algarve region, exhibiting a strong rainfall maximum in the mountainous areas ( Serra do Caldeirão and Serra de Monchique ). The torrential rainfall days included in these two patterns occurred mostly in the fall season (66% and 63% of the cases, respectively). The other rainfall patterns obtained represent days with more moderate precipitation with a wide distribution of rainfall throughout Southern Portugal (DARP 1), or affecting the Ribatejo region (DARP 2). In order to ascertain the large-scale atmospheric circulation associated with each of the DARPs, an objective classification based on SLP data was established and five APs were identified. The occurrence of precipitation days in which abundant rainfall affects the eastern Algarve (DARP3) are strongly associated with the influence of lows located over the Cadiz Gulf (AP 3). However, the results obtained do not suggest any other clear linkages between the atmospheric and rainfall patterns. 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