A STUDY ON THE INTRA-ANNUAL VARIATION AND THE SPATIAL DISTRIBUTION OF PRECIPITATION AMOUNT AND DURATION OVER GREECE ON A 10 DAY BASIS

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 23: (2003) Published online in Wiley InterScience ( DOI: /joc.874 A STUDY ON THE INTRA-ANNUAL VARIATION AND THE SPATIAL DISTRIBUTION OF PRECIPITATION AMOUNT AND DURATION OVER GREECE ON A 10 DAY BASIS A. BARTZOKAS,* C. J. LOLIS and D. A. METAXAS Laboratory of Meteorology, Department of Physics, University of Ioannina, Ioannina, Greece Received 12 July 2001 Revised 6 November 2002 Accepted 6 November 2002 ABSTRACT The intra-annual variation of precipitation amount and duration and their spatial distribution during the year are studied on a 10 day basis for the Greek region, using S-mode and T-mode factor analysis. (i) For the intra-annual variation of precipitation amount, two modes were revealed: the first shows one broad maximum during the conventional winter in stations affected by the sea; the second presents two maxima, the first during late autumn early winter and the second during late spring, corresponding to the northern mainland stations. (ii) For the spatial distribution of precipitation, three main patterns were revealed: the first one is the winter pattern, with the maximum over the west windward area; the second is the summer pattern, with a maximum over the north inland region; and the third is the autumn pattern, with the maximum over northwestern Greece. (iii) For precipitation duration, two types of intra-annual variation were revealed. The first one is similar to the first of the analysis for precipitation amount; the second presents two maxima, the first during the beginning of December and the second during the middle of February, corresponding to the areas of northwestern and northeastern Greece. (iv) For the spatial distribution of precipitation duration, three main patterns were revealed: the first is the summer pattern, which is similar to the second of the analysis for precipitation amount; the second is the winter pattern, with the spatial maximum located over the eastern mainland and western Crete; finally, the third one is the autumn pattern, with the maximum in northwestern Greece. During the third 10 day period of October and the second 10 day period of February, precipitation seems to present singularities, possibly due to fluctuations in atmospheric circulation. The above intra-annual variations and spatial distribution patterns are connected to the seasonal variations of the depression trajectories, the atmospheric instability, the influence of sea-surface temperature as a cyclogenesis factor, and the windward or leeward character of the various areas (orographic effect). Copyright 2003 Royal Meteorological Society. KEY WORDS: Greece; factor analysis; precipitation amount; precipitation duration; 10-day periods 1. INTRODUCTION The intra-annual variation of precipitation and its spatial distribution over Greece have already been studied extensively. Most previous studies have focused on precipitation amount, based on the easily accessed monthly data (e.g. Balafoutis, 1977; Markou-Iakovaki, 1979; Kotini-Zabaka, 1983; Bloutsos, 1993; Fotiadi et al., 1999), whereas studies on precipitation duration are rare. Although the above studies present satisfactorily the main characteristics of precipitation variation and distribution, fluctuations connected with time scales smaller than 1 month are obscured and have not been detected. The revelation and the explanation, if possible, of these fluctuations are very important for Greece, a country with a very complex topography, and the present study aims at this, using 10-day period (decad) data as the main tool. The paper consists of four sections. First, the intra-annual variation of precipitation amount over Greece is studied, defining areas with * Correspondence to: A. Bartzokas, Laboratory of Meteorology, Department of Physics, University of Ioannina, Ioannina, Greece; abartzok@cc.uoi.gr Copyright 2003 Royal Meteorological Society

2 208 A. BARTZOKAS, C. J. LOLIS AND D. A. METAXAS characteristic intra-annual variation of precipitation. Second, the seasons of precipitation, i.e. the sub-periods of the year presenting specific spatial distributions of precipitation, are revealed. Third, the intra-annual variation of the duration of precipitation is examined, defining areas with a characteristic annual variation of this parameter. Fourth, the seasons of precipitation duration, i.e. the sub-periods of the year presenting characteristic spatial distributions of precipitation duration, are revealed. Thus, a complete illustration of the spatio-temporal precipitation regime (amount and duration) in the country will be given, as an important tool towards better water management and optimization of various water needs and uses. 2. DATA AND METHODS The database consists of 10-day (decad) values (three values per month) of precipitation amount (millimetres) and duration (minutes) for 38 Greek stations, evenly distributed (Figure 1), for the 37 year period , kindly provided by the Hellenic National Meteorological Service. The decad precipitation values were Figure 1. The 38 stations used

3 RAINFALL VARIATION OVER GREECE 209 averaged over the 37 years and the mean intra-annual variations of precipitation amount and duration, consisting of 36 decads, were constructed for each station. In order to study the characteristics described in the first section, the multivariate statistical method of factor analysis (FA) was used (SPSS statistical package). This method, with its various modes and options, has been extensively used and described in the past by many researchers, as well as by us (Jolliffe, 1986; Manly, 1986; Richman, 1986, Bartzokas et al., 1994); therefore, this is not presented here. We just underline that FA is considered as one of the most appropriate methods for reduction of the dimensionality of a large data set and thus for the revelation of the main modes of its variation. Since the method is based upon correlation coefficient matrices, the data values must be variables that are normally distributed. Therefore, we had to exclude from the analysis the values of July and August, because in many stations of the southern part of the country, and in most south Aegean islands, precipitation is almost zero, even as a 37 year average. Hence, our final data sets consist of 30 (instead of 36) mean decad values of precipitation amount and duration, starting with 1 10 September (first decad) and finishing with June (30th decad). Test analyses with all (36) decads supported this decision, as the dominant zero values of July and August obscured some very important spatio-temporal features in the rest of the year. For the study of the intra-annual variations of precipitation amount and duration, FA (S-mode) was applied on the data matrices consisting of 30 lines (decads) and 38 columns (stations). Thus, stations presenting common mean intra-annual variation of precipitation (separately for amount and duration) were objectively detected and grouped, revealing the main subdivisions of the country, so far as it concerns these two parameters. For the spatial distribution of precipitation amount and duration, the matrices of the initial data sets were transposed and FA (T-mode) was applied on the new matrices of 38 lines (stations) and 30 columns (decads). In this way, decads exhibiting a common spatial distribution of precipitation (separately for amount and duration) were grouped objectively, defining the seasons of the year from a precipitation point of view. The method was applied experimentally many times, retaining, as significant, different number of factors every time. For the optimum number of factors, the physical interpretation of the results, the percentage of the total variance explained, and the statistical significance of the factor loadings were taken into account. The latter could be easily investigated, since by using correlation instead of covariance matrices, viz. by standardizing the data of each column, the loadings are, in fact, the correlation coefficients between the original data time series and the resultant factor scores time series. Finally, we note that, at the stage of rotation, Varimax was found to be most suitable, giving the best results. 3. RESULTS 3.1. Intra-annual variation of precipitation amount A series of test analyses with various numbers of factors retained showed that the optimum number of factors is two. Although the third eigenvalue is greater than unity (29.9, 3.5, 1.3, etc.), results with three factors were rejected, inasmuch as the third factor was found to be very weak; i.e. the number of stations depending primarily on this was very small (two or three) and the maximum loadings did not exceed The two factors retained explain 88% of the total variance of the intra-annual variation of precipitation at the 38 available stations. Factor 1 accounts for 59% of the total variance and comprises stations influenced by the sea. The most typical stations of this factor are those located in the islands of the central and southern Aegean, where loadings exceed 0.90 (Figure 2(a)). This means that time series of standardized factor scores (zero mean, unit variance) in Figure 2(b) represent accurately the intra-annual variation of precipitation in this area (above 81% of the variance). Areas of the northern Aegean and of the Ionian sea with loadings around 0.80 (64% of variance) can also be considered as clearly classified in this factor. The intra-annual variation of precipitation in these areas is simple, shown by one broad maximum from mid December to early February and one minimum, obviously in summer. Further comments about this minimum cannot be made, since July and August have been omitted. However, it is well known that, during these months, precipitation in the central and southern Aegean is a very rare event.

4 210 A. BARTZOKAS, C. J. LOLIS AND D. A. METAXAS (a) 3 PRECIPITATION AMOUNT - FACTOR 1 (59%) 2 Factor 1 scores S O N D J F M A M J (b) Figure 2. Intra-annual variation of precipitation amount. Factor 1: (a) factor loadings isopleths (values lower than 0.4 are not statistically significant at the 95% confidence level and the corresponding isopleths are not drawn) and (b) factor scores time series (standardized) Factor 2 accounts for 29% of total variance and it mainly comprises stations in the continental areas of northern Greece (Figure 3(a)). In this area, the intra-annual variation of precipitation amount presents two maxima and two minima (Figure 3(b)). The primary maximum is also broad, but it appears about 2 months earlier than in the maritime stations, in the period from late October to early December. The primary minimum seems to appear in summer, as in factor 1. The secondary maximum is shown around late May, and the secondary minimum appears around early February. Apart from the stations affected mostly by the sea and those of the inland and mountainous areas, there are a number of stations influenced by both factors. These stations cannot be classified clearly into either of the two factors, as they exhibit mixed characteristics. Most of these stations are located along or near the coasts of the mainland and they might belong to a third transitional group. However, the intra-annual variation of

5 RAINFALL VARIATION OVER GREECE 211 (a) 3 PRECIPITATION AMOUNT - FACTOR 2 (29%) 2 Factor 2 scores S O N D J F M A M J (b) Figure 3. As in Figure 2, but for factor 2 precipitation in these stations is not statistically independent from those of the other two groups; therefore, FA did not consider it as a separate mode of variation. At these stations, in spite of the rotation of the axes, the loadings of the two factors are almost equal (e.g. Ioannina 0.70 and 0.63, Lamia 0.68 and 0.60, Kavala 0.65 and 0.65, etc.). The annual variation of precipitation for Ioannina (northwestern Greece) is presented in Figure 4, where a transitional behaviour is seen; i.e. the broad winter maximum appears later than that of factor 2 (about days), but there is a resemblance with factor 2 in the late May maximum. We also note the prominent maximum value of mid February (a t-test showed that it differs significantly from the early and late February ones), which is in agreement with a similar maximum in the intra-annual variation of relative geostrophic vorticity at 500 hpa, located slightly northwards (Metaxas et al., 1994). This vorticity maximum is apparently connected to a temporary enhancement of the zonal circulation over the central Balkans causing abundant rainfall over windward northwestern Greece during the middle of February.

6 212 A. BARTZOKAS, C. J. LOLIS AND D. A. METAXAS Precipitation amount (mm) IOANNINA S O N D J F M A M J Figure 4. Mean intra-annual variation (September June) of precipitation amount in Ioannina ( ) The above two basic intra-annual variation modes of precipitation can be explained if the main tracks of depressions in the Mediterranean (Alpert et al., 1990a,b; Trigo et al., 1999) are considered, along with the land sea differential heating. Thus, in the southern maritime stations, the main maximum appears 1 2 months later than in the northern inland stations, in accordance with the southward displacement of the depression tracks from autumn to winter. The secondary maximum is observed in inland stations only, due to the intense heating of land and atmospheric instability (Fotiadi et al., 1999). This late-may maximum coincides with that detected by Metaxas (1972) for thunderstorm frequency over the same areas Spatial distribution of precipitation amount Test analyses showed that the optimum number of factors is three (eigenvalues: 17.2, 7.6, 2.2, etc.), accounting for 90% of total variance, which is an impressively high percentage considering the difficulties presented when handling precipitation data. A similar analysis was carried out by Metaxas et al. (1999) based on monthly data, which led to two factors, obscuring the third mode of variation of the present analysis. Factor 1 accounts for 39% of the total spatial variance of precipitation, comprising mainly the winter decads. In Table I it is seen that this winter period (as far as it concerns the horizontal distribution of precipitation) starts after the middle of November and finishes around the end of March, with most typical decads being those of January, when all loadings exceed The spatial distribution of precipitation in winter is presented by the standardized factor scores (zero spatial mean) in Figure 5. During this period of the year the maxima of precipitation are located on the western windward slopes of the Pindus mountain range, in western Crete, as well as in the eastern Aegean islands and probably along the western Turkish coast. On the other hand, precipitation minima appear in eastern continental Greece. Factor 2 explains 29% of the total variance and refers to the period after the middle of April. In Table I it is seen that the core of this period is after the middle of May, when all loadings exceed This is the period of the continental thunderstorms maximum and it can be termed summer. In fact it is early summer, but it can be assumed that it extends to the omitted months of July and August. This consideration is enhanced by the relatively high loadings of the first two decads of September, when a substantial influence of the summer period is apparent (loadings above 0.60). In Figure 6 it is seen that, during summer, the maximum precipitation amounts are recorded in the continental areas and mainly in the Pindus mountain range, and the minimum is over the southern Aegean and Ionian seas; this is in accordance with the results of the study by Lolis et al. (1999) for July and August with monthly data. Factor 3 (22% of total variance) comprises mainly the period from the middle of September to the end of October, but beyond this there is an evident influence from the beginning of September and up to the end of November (Table I). This period can be characterized as autumn for the spatial distribution of precipitation, with the maximum located over the northern Ionian Sea and northwestern Greece, west of the Pindus mountain

7 RAINFALL VARIATION OVER GREECE 213 Table I. Factor loadings for the analysis on the spatial distribution of precipitation amount (values lower than 0.32 are not statistically significant at the 95% confidence level and have been omitted. Values greater than 0.70 indicate that at least 49% of variance is explained by one factor only and appear in bold) Decad Factor 1 Factor 2 Factor September September September October October October November November November December December December January January January February February February March March March April April April May May May June June June 0.95 range (Figure 7). We note that, on Corfu (Kerkyra) island, the value of the factor scores (in fact precipitation) exceeds the spatial average by almost three standard deviations. Finally, the minimum is shown in the eastern Aegean islands, where values lower than one standard deviation from the spatial average are seen. The above results reveal that (concerning precipitation only, i.e. without taking into account any other meteorological parameter) Greece is characterized by three seasons only, namely winter, summer and autumn. The summer is in fact early summer, as, in summer, most of the Mediterranean climate areas are completely dry due to the predominance of a subtropical anticyclone over the Mediterranean and because sea-surface temperature is, on average, lower than the air temperature above it (mainly in July), implying static stability. On the contrary, north of the Mediterranean, where the influence of these two factors does not exist, a secondary precipitation maximum appears in July. A fourth season, corresponding to the conventional spring, was not found. A couple of transitional decads in late March and early April (Table I) cannot be characterized as spring, since such short transitional periods also appear between summer and autumn and autumn and winter. The main features of winter (most rainfall is recorded on the windward slopes of western Greece) must be attributed to the effect of depressions, which, during this period, mostly move in trajectories from west to east, and to the orographic effect. In summer, the precipitation distribution

8 214 A. BARTZOKAS, C. J. LOLIS AND D. A. METAXAS Figure 5. Spatial distribution of precipitation amount. Factor 1 scores (standardized) (highest amounts are recorded far from the coasts and in the mountains) is primarily due to crossing of upper-air troughs, associated with land heating and breezes (Metaxas, 1978; Balafoutis, 1991; Fotiadi et al., 1999), affecting the atmospheric stability. Finally, in autumn (highest amounts in northwestern Greece), the precipitation distribution is associated with the gradual displacement of depression trajectories to lower (than in summer) latitudes; the warm sea must also play an important role, along with the topography of western Greece. Thus, in this season, precipitation is abundant in northwestern Greece and not in the eastern Aegean. The results presented above can be considered as an extension/supplement of the work of Metaxas et al. (1999), who used monthly values and revealed two seasons only: winter and summer Intra-annual variation of precipitation duration Similar to the above analysis for the precipitation amount, the analysis for the precipitation duration leads to two factors that explain 95% of the total variance. Factor 1 (48% of total variance) refers mainly to the stations of the central and southeastern Aegean and southern Ionian Seas, where factor loadings exceed 0.80 (Figure 8(a)), much like the analysis for precipitation amount (Figure 2(a)). These stations exhibit a similar intra-annual variation of precipitation duration, which is presented by the factor scores time series (Figure 8(b)). It can be seen that, over the Greek seas, the duration of precipitation is at a maximum during January (about two standard deviations above the annual mean) and at a minimum in summer, as was found for the precipitation amount (Figure 2(b)). Secondary weak minima also appear in mid November, mid February and mid May, and secondary maxima appear in late October and early March, but they are not all of the same magnitude. Factor 2 is of about equal strength (47% of total variance) and mainly comprises the stations of the western mainland and the northeastern part of the country (Figure 9(a)), a classification that bears some similarity to that of factor 2 of the amount analysis (Figure 3(a)). In these areas the duration is longest during the

9 RAINFALL VARIATION OVER GREECE 215 Figure 6. As in Figure 5, but for factor 2 Figure 7. As in Figure 5, but for factor 3

10 216 A. BARTZOKAS, C. J. LOLIS AND D. A. METAXAS (a) 3 PRECIPITATION DURATION - FACTOR 1 (48%) 2 Factor 1 scores S O N D J F M A M J (b) Figure 8. Intra-annual variation of precipitation duration. Factor 1: (a) factor loadings isopleths and (b) factor scores time series (standardized) period mid November early December (Figure 9(b)) and then it falls to a minimum in late January early February, followed by another remarkable peak in mid February. Minimum values are also seen in summer, as was expected. In this analysis, the problem of stations not clearly classified in any of the two retained factors appears stronger, since at all stations the loadings were found to be statistically significant at the 0.05 level for both factors (above 0.40), thus complicating the classification. This is an indication that the analysis could be performed with one factor only, with the series of its scores representing the unique intra-annual variation of precipitation duration at all stations. This is also indirectly indicated by the large difference between the first and the second eigenvalues: 35.4 and 1.0 respectively. Such an analysis was experimentally carried out and yielded an almost sinusoidal intra-annual variation with a broad maximum during the conventional winter, when, as is the case

11 RAINFALL VARIATION OVER GREECE 217 (a) 3 PRECIPITATION DURATION - FACTOR 2 (47%) 2 Factor 2 scores S O N D J F M A M J (b) Figure 9. As in Figure 8, but for factor 2 for the Mediterranean climate, precipitation lasts longer than in summer. However, since the purpose of this work is to reveal the fine structures of the intra-annual variation of precipitation duration, then the analysis with two factors was accepted, despite the slight disadvantage of large transitional zones and small core regions. The above results in the analysis with two factors can be explained by taking into account the trajectories of the depressions during the cold period of the year (Alpert et al., 1990a,b; Trigo et al., 1999). In autumn and in early December (which for Greece is not purely a winter month), when depressions travel eastwards at relatively high latitudes, precipitation lasts longer in the windward northwestern parts of the country. The air masses, having left most of their moisture in the west, result in smaller amounts of rainfall and of shorter intervals on the leeward eastern continental Greece and also light and short-lived rainfall in the southern Aegean. These depressions are reinforced over the northern Aegean, causing longer rainfall over

12 218 A. BARTZOKAS, C. J. LOLIS AND D. A. METAXAS northeastern Greece, as they continue travelling towards Turkey, Bulgaria and the Black Sea. The second peak, in mid February, is in accordance with that of precipitation amount for the town of Ioannina (Figure 4), which belongs to the core area of factor 2. A similar peak is also apparent in the intra-annual variation of precipitation amount in northern continental Greece (Figure 3(a)), though much weaker, due to the different core area of factor 2. After late December, when depressions cross Greece at lower latitudes and because of the normally large temperature difference between the sea and the air above it (sea is much warmer and the air masses are therefore unstable), precipitation duration is longer over the central and southern Aegean and southern Ionian Sea than over the northern Aegean and continental Greece. This is in agreement with common meteorological experience that, in high winter months (January February), thunderstorms occur much more frequently over the sea and coasts than inland Spatial distribution of precipitation duration The analysis of the spatial distribution of precipitation duration revealed, as in the corresponding analysis of precipitation amount, three main modes of distribution (eigenvalues: 23.6, 2.8, 1.1, etc.), explaining 92% of the total variance. The three seasons defined are described in the following section. Table II. Factor loadings for the analysis on the spatial distribution of precipitation duration (values lower than 0.32 are not statistically significant at the 95% confidence level and have been omitted. Values greater than 0.70 indicate that at least 49% of variance is explained by one factor only and appear in bold) Decades Factor 1 Factor 2 Factor September September September October October October November November November December December December January January January February February February March March March April April April May May May June June June 0.94

13 RAINFALL VARIATION OVER GREECE 219 Factor 1 explains 38% of the total variance and refers mainly to the summer decads. This season is from late April till late September (loadings above 0.70), whereas the core period (loadings above 0.90) is from late May till probably the end of August (Table II). The spatial distribution in summer is presented by the factor scores in Figure 10. It is seen that, during this period of the year, the longest duration of precipitation is observed in northern continental areas of the country, whereas the shortest duration is in the southern Aegean and Ionian Seas, where precipitation is a very rare event. Factor 2 (29% of total variance) prevails from the beginning of January until mid March, with the exception of the mid February decad. Also, the period comprises the last decad of October (Table II). During this season, which can be called winter, the precipitation events with longest duration take place in the eastern part of continental Greece and in western Crete, whereas the minimum duration of precipitation is recorded in northeastern and western Greece and eastern Crete (Figure 11). Factor 3 is the weakest, accounting for 25% of the total variance. Its core period is not as clear as in previous cases and the maximum loadings appear in two non-neighbouring decads, the end of November and the middle of December, which must be considered as the most representative parts of this season. The third important interval of this season is that of mid February, which has been excluded from the winter classification. During this season, the maxima of precipitation duration are recorded in northwestern Greece (in Ioannina the score exceeds the spatial average by three standard deviations) and the minima over the Aegean and the eastern part of continental Greece. This season can be called autumn or late autumn. In summer, the duration pattern (Figure 10) is similar to the pattern of the amount (Figure 3(a)), as both display high values over northern continental Greece due to intense land heating, along with atmospheric instability, which is not the case in the dry maritime and coastal areas of southern Greece (Mediterranean climate). In winter (Figure 11), the pattern is unexpected, as depressions usually travel from west to east. However, in high-winter months (January and February), which mainly belong to winter (in the precipitation duration analysis), apart from the prevailing zonal circulation, meridional circulation may also prevail for Figure 10. Spatial distribution of precipitation duration. Factor 1 scores (standardized)

14 220 A. BARTZOKAS, C. J. LOLIS AND D. A. METAXAS Figure 11. As in Figure 10, but for factor 2 long periods. Under meridional circulation, precipitation is frequent over the eastern parts of continental Greece, while western Greece may be completely dry (zero duration) due to the Pindus mountain range orientation (northwest southeast). On the contrary, under zonal circulation conditions, precipitation is more frequent/longer on the windward western part, but some smaller amounts also fall over the eastern part, because of the maritime origin of the air masses. Thus, the total precipitation duration in winter is longer over eastern continental Greece, although the amounts are much less there. In autumn (Figure 12), the maximum precipitation duration over northwestern Greece must be attributed to the first autumn depressions, travelling eastwards at latitudes lower than in summer, associated with potential instability due to passage over the relatively warm sea. According to the above results, meridional flow is not a frequent phenomenon in late November and December ( autumn for precipitation duration), which is why there are not high values along eastern continental Greece, as is the case in January and February (factor 2). This is possibly due to the fact that, in late winter (January March), when land cooling is intense, cold air invasions connected either to the extension of the Siberian anticyclone or to a blocking anticyclone located over central Europe are frequent over the Balkans. These synoptic conditions are usually persistent and they are characterized by a north northeasterly flow over Greece. This argument is in agreement with the results of Lejenäs and Økland (1983), who found that 500 hpa blocking frequency over central Europe increases significantly from December to January, remaining at high values until March. Blocking conditions over central Europe favour the domination of meridional flow over Greece, causing longer precipitation events over the eastern rather than the western Greek mainland. Finally, we note that the mid February decad, which does not belong to winter, should not be considered as an erroneous result, since it has also been observed in Ioannina precipitation (Figure 4), and in the precipitation duration of northwestern Greece (Figure 9(b)). The same argument is valid for the third decad of October, which is classified in winter. According to Metaxas et al. (1994), relative geostrophic vorticity over the

15 RAINFALL VARIATION OVER GREECE 221 Figure 12. As in Figure 10, but for factor 3 northern Balkans shows a minimum during the third decad of October, indicating enhanced anticyclonic activity there and consequently more frequent north northeasterly flow over Greece, a winter feature. We also note the secondary maxima appearing in the intra-annual variation of precipitation amount (factor 1; Figure 2) and duration (factor 1; Figure 8) during the same decad. A detailed explanation of the atmospheric circulation singularities during these decads needs a special study. The three seasons defined for duration of precipitation have been termed winter, summer and autumn, as was the case for precipitation amount. Although the beginning and the end of each season is not the same in both analyses, viz. seasons with the same name are not identical, the lack of spring, both in duration and in amount, is an indication that the results might be considered equivalent. 4. CONCLUSIONS The intra-annual variation and the spatial distribution of precipitation amount and duration over the Greek area were studied, applying S-mode and T-mode FA on mean decad data. According to the results, the following conclusions can be made. 1. Over the southern marine and coastal areas, the intra-annual variation of precipitation amount and duration is simple, with one broad maximum from mid December to early February, typical of the Mediterranean climate. 2. Over the northern mainland areas, the intra-annual variation of precipitation amount presents two maxima: the first from late October to early December and the second around late May. The first is attributed to depression activity and the second to the contribution of atmospheric instability.

16 222 A. BARTZOKAS, C. J. LOLIS AND D. A. METAXAS 3. Over the areas of northwestern Greece, the intra-annual variation of precipitation duration presents two maxima: the primary one during the beginning of December and the secondary one during the middle of February. 4. From mid November to late March, the spatial distribution of precipitation amount presents maxima over western Greece, western Crete and the eastern Aegean, whereas from the beginning of January until mid March the distribution of precipitation duration presents maxima over eastern mainland areas. 5. From mid April to early or mid September, precipitation amount and duration are respectively higher and longer over the northern mainland, due to low-level convergence by the sea breezes from the western and eastern coasts and due to occasional upper-air trough passages north of Greece, associated with cold air masses and instability. During this season, precipitation is a rare event over southern marine areas. 6. From mid September to the end of October for precipitation amount and around the end of November and the middle of December for precipitation duration, spatial maxima are shown over northwestern Greece, due to the first cold-season depressions moving over the central Balkans towards the Black Sea. 7. Two interesting singularities concerning precipitation amount and duration appeared in mid February and late October. The atmospheric circulation characteristics that provoke these singularities should be studied in future work. REFERENCES Alpert P, Neeman BU, Shay-El Y. 1990a. Climatological analysis of Mediterranean cyclones using ECMWF data. Tellus A 42: Alpert P, Neeman BU, Shay-El Y. 1990b. Inter-monthly variability of cyclone tracks in the Mediterranean. Journal of Climate 3: Balafoutis Ch Contribution to the study of the climate of Macedonia and western Thrace. PhD thesis, University of Thessaloniki (in Greek). Balafoutis Ch Principal component analysis of Albanian rainfall. Journal of Meteorology 16(157): Bartzokas A, Metaxas DA, Ganas IS Spatial and temporal sea-surface temperature covariances in the Mediterranean. International Journal of Climatology 14: Bloutsos AA Harmonic analysis of the annual variation of precipitation over Greece. In Water Deficiency and Floods. Symposium Organized by the Geotechnical Chamber of Greece, Thessaloniki, March 1992; (in Greek). Fotiadi AK, Metaxas DA, Bartzokas A A statistical study of precipitation in NW Greece. International Journal of Climatology 19: Jolliffe IT Principal Component Analysis. Springer-Verlag: New York. Kotini-Zabaka S Contribution to the study of the climate of Greece. Normal weather per month. Publication of the Research Centre of Climatology of the Academy of Athens, Publication no. 8 (in Greek). Lejenäs H, Økland H Characteristics of Northern Hemisphere blocking as determined from a long time series of observational data. Tellus A 35: Lolis CJ, Bartzokas A, Metaxas DA Spatial covariability of the climatic parameters in the Greek area. International Journal of Climatology 19: Manly BFJ Multivariate Statistical Methods: A Primer. Chapman & Hall: London. Markou-Iakovaki P Precipitation in Crete island. PhD thesis, University of Athens (in Greek). Metaxas DA Space and time distribution of thunderstorm frequency in Greece. Publication of Laboratory of Meteorology, Ioannina University, no. 4 (in Greek). Metaxas DA Evidence on the importance of diabatic heating as divergence factor in the Mediterranean. Archiv fuer Meteorologie, Geophysik und Bioklimatologie Serie A 27: Metaxas DA, Bartzokas A, Lianou MK The variability of the relative geostrophic vorticity in Europe, the Atlantic and N. Africa during the year. In Proceedings of the International Symposium/Workshop on Climatic Variability and Impact to Agriculture, (Dalezios NR (ed.), University of Thessaly, Volos, 21 April; Metaxas DA, Philandras CM, Nastos PT, Repapis CC Variability of precipitation pattern in Greece during the year. Fresenius Environmental Bulletin 8: 1 6. Richman MB Rotation of principal components. Journal of Climatology 6: Trigo IF, Davies TD, Bigg GR Objective climatology of cyclones in the Mediterranean region. Journal of Climate 12: