SPATIAL DISTRIBUTION OF SOME DYNAMIC PARAMETERS DURING THE EVOLUTION OF SELECTED DEPRESSIONS OVER THE AREA OF CYPRUS

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 24: (04) Published online in Wiley InterScience ( DOI: /joc.1105 SPATIAL DISTRIBUTION OF SOME DYNAMIC PARAMETERS DURING THE EVOLUTION OF SELECTED DEPRESSIONS OVER THE AREA OF CYPRUS KLEANTHIS NICOLAIDES a, * SILAS MICHAELIDES a and THEODORE KARACOSTAS b a Meteorological Service, Nicosia, Cyprus b Department of Meteorology and Climatology, Aristotelian University, Thessaloniki, Greece Received 29 April 03 Revised June 04 Accepted June 04 ABSTRACT During the cold season, the cyclonic systems that reach the eastern Mediterranean from the surrounding geographical region and those that are formed over the area are largely associated with unsettled weather conditions over the island of Cyprus. For a better understanding of the evolution of these synoptic-scale systems, some of the dynamic characteristics of selected groups of baroclinic depressions that affected the area during the winter season were studied. More specifically, the spatial isobaric distribution of the fields of relative vorticity, divergence, vertical motion and a static stability parameter were considered in relation to the evolution of each grouping of such depressions. The study focuses on 1 depressions in the colder months of November to March, from the beginning of January 1988 till the end of December Five groups of depressions were established and studied separately: those originating from north, east, south and west and those that form in situ. The time of maximum deepening of each depression was determined from the surface synoptic analyses and the spatial distributions of the above parameters were established for this time, 1 day before and 1 day after. The data used are the standard-level isobaric analyses compiled by the National Centers for Environmental Prediction National Center for Atmospheric Research. The results presented in this study refer only to three tropospheric isobaric levels, namely 8, 0 and 0 hpa. Copyright 04 Royal Meteorological Society. KEY WORDS: Cyprus; east Mediterranean; cyclonic disturbances; depressions; dynamical characteristics 1. INTRODUCTION The cyclogenetic potential of the Mediterranean basin, especially during the cold season, has long been recognized and is well documented (Gleeson, 1954; Meteorological Office, 1962; Reiter, 1975). Also, numerous diagnostic studies have shed light on the complex processes underlying the formation and evolution of such systems (e.g. Kallos and Metaxas, 1980; Karacostas and Flocas, 1983; Michaelides, 1987; Prezerakos and Michaelides, 1989; Maheras et al., 00). Unsettled weather over the area of Cyprus is largely determined, at least during the winter period, by the evolution of baroclinic depressions that either reach the area travelling from the surrounding wider area, or develop over the area of the eastern Mediterranean (El-Fandy, 1946). Baroclinic depressions are more frequent during the cold period, when a low-index circulation is established over the Northern Hemisphere. The cold period can safely be assumed to span from late autumn to late spring (El-Fandy, 1946; Boast and McGinnigle, 1971; Metaxas, 1976; Kallos and Metaxas, 1980). Michaelides et al. (04) have shown that the area of the eastern Mediterranean is affected by baroclinic depressions mainly from the west, and more frequently during February and December. It has also been shown that depressions affecting the area with pressure of their innermost closed isobar of less than 998 hpa have their origin from the west and the north. Depressions having their origin from the north and innermost closed isobar pressure value of * Correspondence to: Kleanthis Nicolaides, Meteorological Service, CY-1418 Nicosia, Cyprus; kleanthi@spidernet.com.cy Copyright 04 Royal Meteorological Society

2 18 K. NICOLAIDES, S. MICHAELIDES AND T. KARACOSTAS less than 998 hpa are more frequent than those from the west, and the mean time of their presence in the area is less than the mean time of the presence of depressions from all other directions. (Michaelides et al., 04). Generally, depressions reaching the area from all directions, or depressions developing in situ, stay over the area for less than 48 h (Nicolaides et al., 1998). Of all the depressions affecting the area, those that are rejuvenated, due to favourable dynamic conditions, are quite few; generally, however, they amount to more than the depressions developing locally as new depressions (Gleeson, 1954; Meteorological Office, 1962; Michael, 1986; Nicolaides et al., 1998). Depressions affecting the area of Cyprus are associated with different types of weather. Climatological studies have shown that showery weather is associated mainly with depressions affecting the area of Cyprus from the west, i.e. the main body of the Mediterranean Sea. Cold and mainly dry weather is predominant with depressions affecting the area of Cyprus from the north, and weather connected with dust events is largely associated with depressions affecting the area from the east and south. This variability of the weather over the area of Cyprus is the main reason for examining the dynamic characteristics of the associated depressions, during the cold months, with respect to their geographical origin. The study of the synoptic-scale dynamic characteristics of the cold-season depressions can provide a better insight into the mechanisms underlying the evolution of such systems and help to explain the development of the associated weather of the area. The aim of the present research is the study of the evolution of the spatial distribution of the fields of relative vorticity, divergence, vertical motion and a static stability parameter of groups of depressions affecting the area of Cyprus during the cold season. In this respect, all the depressions in a 9 year period have been grouped on the basis of their origin. 2. DATABASE AND METHODOLOGY This present study covers the period from January 1988 to December All the depressions that affected the area of Cyprus during the five cold-season months, namely November March, have been considered. The depressions selected for the present study were originally identified on surface synoptic charts analysed for every 4 hpa and archived by the Meteorological Service of Cyprus. In order to identify the depressions, a reference area bounded by the meridians 25 E and E and the parallel circles N and N has been selected. This area, shown in Figure 1, has a relatively small number of surface meteorological stations, compared with the broad continental European region. The reference area comprises the eastern Mediterranean basin, parts of Greece, Turkey, Syria, Lebanon, Israel and northern Egypt. A total of 1 depressions have been considered in the present study. For a depression to be considered, it must fulfil the following criterion: on the corresponding surface analysis, the cyclonic system must have at least one closed isobar within the area of study, as defined above, having pressure equal to or less than 1016 hpa. After its identification on a surface analysis for the first time, a cyclonic system fulfilling the above criterion was counted as a depression affecting the area of Cyprus and an inventory for its evolution was established and maintained. This inventory consists of a detailed report, comprising, among other information, the date and the direction from which each depression appeared in the study area, the subsequent dates that it stayed over the area and, as follow up, of the value of its innermost closed isobar. It was also noted whether a depression was pre-existing in the surrounding area and penetrated the study area later, or whether the depression had developed in situ. These inventories were subsequently used in separating all of the 1 depressions considered into five classes, each delineating the origin of the depressions over the area of Cyprus. The four classes corresponding to the four directions of entrance are shown in Figure 1, and refer to depressions entering the area from the north, east, south and west. A fifth class refers to depressions that were formed locally for the first time. Table I cross-tabulates the number of depressions with respect to the month in which they first make their appearance in the area and their origin. The results shown in Table I clearly indicate that the vast majority of depressions originate from the west. Indeed, of the 1 depressions affecting the area, almost half originate from Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

3 EVOLUTION OF DEPRESSIONS OVER CYPRUS 1831 North West South East Figure 1. The area used for the calculations of the dynamic parameters. The smaller area is the one considered for the identification of depressions and their directional classification with respect to depression origin Table I. The number of depressions, , cross-tabulated with respect to the month in which they first make their appearance in the area and their origin Origin November December January February March Total From north From east From south From west In the area Total the west and half of the depressions from this direction occur in December and February. The number of depressions from other directions is several orders of magnitude smaller than those from the west. It is worth noting that the observed number of depressions formed locally in this study is in very good agreement with the number calculated by others (see Reiter (1975)), amounting to about two per year. The procedure for selection and classification was adopted in a statistical analysis of depressions affecting the area of Cyprus, presented in a companion paper (Michaelides et al., 04). The inventory of each depression was also used to determine the time at which the depression was at its deepest over the area, and this time was termed D0. The times corresponding to 24 h before and 24 h after D0, the time of maximum deepening, were termed M1 and P1 respectively. For each one of the above times, namely M1, D0 and P1, the relative vorticity ζ, the horizontal divergence of the wind vector δ, the vertical motions ω and the parameter of static instability σ at the standard isobaric surfaces were calculated. These fields were subsequently used to calculate the average distributions corresponding to each of the five groups. Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

4 1832 K. NICOLAIDES, S. MICHAELIDES AND T. KARACOSTAS The equations used for the calculations are δ = 1 u r cos φ λ + 1 v r φ v tan φ r ζ = 1 v r cos φ λ 1 u r φ + utan φ r ω p = δ σ = gt c p pg R T p where r is the radius of the Earth, λ is the longitude, φ is the latitude, u and v are respectively the zonal and meridional components of the horizontal wind vector, g is the vertical component of the vector of the acceleration of gravity, T is the temperature, c p is the specific heat of dry air under constant pressure, p is the atmospheric pressure and R is the universal gas constant. The parameter σ was used in energetic studies (e.g. Michaelides, 1987). The database used for the calculations comprises the fields of wind and temperature with reference to the 10 standard isobaric surfaces of 1000, 8, 700, 0, 0, 0, 2, 0, 1 and 100 hpa archived by the National Centers for Environmental Prediction (NCEP) in the USA, at 0000UTC and on a horizontal grid length of The area of the calculations is bounded by the meridians W and E and the parallel circles N and 65 N. This area is larger than the area used for the identification and classification of depressions, thus providing a more general view of the dynamical processes associated with each class of depressions (see Figure 1). 3. RESULTS The calculations have produced results in the form of average isobaric distributions of the parameters mentioned above, for the depressions originating from north, east, south and west and also for the depressions developing over the area. Although calculations were made for all isobaric levels, the discussion in this article is limited to three tropospheric levels: one lower tropospheric (8 hpa), one mid-tropospheric (0 hpa) and one upper tropospheric (0 hpa). The discussion below proceeds first with the field of the mean static stability index, at the lower tropospheric isobaric level of 8 hpa; then the field of mean divergence at the 8, 0 and 0 hpa levels is presented in association with the field of static stability; and that of the mean relative vorticity at the 0 hpa level. In the discussion, emphasis will be given to the area of interest, i.e. the wider area of the eastern Mediterranean. Results concerning the distribution of the omega field and its structure will not be presented or discussed, although all necessary calculations were carried out and graphs were prepared, since these fields follow the same pattern to the fields of horizontal divergence Depressions originating from the north The results for the depressions entering the area from the north are presented in Figure 2. The first field to present is that of the 8 hpa static stability index. Generally, the distributions show strong values of static stability over the European continental area, with the strongest values over Russia, while maritime areas and southern continental areas are characterized by weaker static stability values. The greater positive values (i.e. more stable air) are due to the presence of the Siberian anticyclone (a source of the polar continental airmass) over Russia, a predominant synoptic system during the cold months. The weaker values (i.e. more unstable air), mainly over the area of the Mediterranean, are a result of instability induced by sensible heat transfer from the warmer Mediterranean to the colder air above. Low static stability index values are also found over Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

5 EVOLUTION OF DEPRESSIONS OVER CYPRUS a) Static Stability 8 hpa M Static Stability 8 hpa D Static Stability 8 hpa P b) Divergence 8 hpa M Divergence 8 hpa D Divergence 8 hpa P c) Divergence 0 hpa M Divergence 0 hpa D Divergence 0 hpa P d) Divergence 0 hpa M Divergence 0 hpa D Divergence 0 hpa P e) Rel. Vorticity 0 hpa M Rel. Vorticity 0 hpa D Rel. Vorticity 0 hpa P1 Figure 2. (a) 8 hpa static stability index distribution (isopleths are every 0.1 K 2 m 1 ), (b) 8 hpa divergence distribution, (c) 0 hpa divergence distribution, (d) 0 hpa divergence distribution (for all divergence distributions isopleths are every s 1 ) and (e) 0 hpa distribution of relative vorticity (isopleths are every s 1 ), for depressions entering the area from the north and for times M1, D0 and P1 Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

6 1834 K. NICOLAIDES, S. MICHAELIDES AND T. KARACOSTAS the western Sahara desert, implying that the destabilization of the lower troposphere air could be caused by the relatively warmer land surface. The steep gradient mainly over the southern European coastline and around the Black Sea is a manifestation of the mean position of the polar front during the cold season. Specifically, at time M1, the weakest static stability values are over the area of the central Mediterranean and the area of Saudi Arabia, while the strongest values are found over the areas of central Europe and east of Turkey. The field of divergence is generally characterized by a weak gradient at the 0 hpa isobaric level, marking the level of non-divergence, and stronger gradients at the 8 and 0 hpa isobaric levels. At the lower tropospheric level of 8 hpa, the areas of the Aegean and Ionian Seas, the Balkans, Turkey, the Caucasus, the Near East and Saudi Arabia are characterized by negative horizontal divergence. Low divergence values are found over eastern Turkey, while over western Saudi Arabia the divergence has its lowest value. Aloft, at the 0 hpa isobaric level, the field of divergence of the horizontal wind vector is characterized by positive values over the area of the Balkans, Turkey and the Black Sea, the Near East and Saudi Arabia. This coupled low-level convergence and higher level divergence signifies the induction of vertical motions over this area. This implies that, although the static stability distribution at time M1 displays marked instability over the wider area of the central Mediterranean, the potential area for development is that of the Balkans and Turkey. At time M1, the field of relative vorticity is not so encouraging for cyclonic development over the area of the east Mediterranean: the mid-tropospheric level of 0 hpa shows strong positive values over the Ionian Sea. At time D0, the static stability field at the 8 hpa level shows that the area of central Europe becomes more stable, while the lower troposphere over the area of the eastern Mediterranean becomes more unstable. At this time, the increased instability of the lower troposphere over the eastern Mediterranean is ascribed to the destabilization of the northerly colder airflow over the warmer Mediterranean waters. At the same time, the field of divergence at 8 hpa continues supporting the cyclonic development, mainly over the area of Turkey and also over the area of the eastern Mediterranean. Furthermore, lower tropospheric development is favoured over the area of the Near East and western Saudi Arabia. The strong lower tropospheric convergence over Turkey is coupled with strong divergence aloft, suggesting that the area of Turkey is favourable for cyclonic development. Similar development is also favoured over the area of the Near East and Saudi Arabia. The relative vorticity distribution, at the mid-tropospheric level of 0 hpa, shows that the strongest values are over the Aegean, to the west of the areas of potential upward motions and cyclone development, while negative values are found over central and western Europe. At time P1, the static stability field shows that the lower tropospheric unstable air covering the area of the eastern Mediterranean extends towards the area of the Near East and Saudi Arabia. The low-level tropospheric instability over mainly the eastern Mediterranean is coupled with a low-level divergent field, while the low-level tropospheric convergence noted over the Near East and Saudi Arabia is coupled with low-level tropospheric unstable air. Superimposed on the area of low-level tropospheric convergence is an area of upper tropospheric divergence (at the 0 hpa level). This indicates that, at time P1, the potential area of cyclone development is the area of the Near East. At time P1, the 0 hpa relative vorticity field weakens, with the maximum displaced further eastwards, over the area of the eastern Mediterranean. The distribution of the negative relative vorticity field found over western Europe suggesting anticyclonic shear, and that of positive values suggesting cyclonic shear over eastern Europe, Turkey and the Near East, is indicative of the mean position of the jet stream aloft. Thus, in the case of depressions entering the area from a northerly direction, the jet stream has its starting position over northeastern Scandinavia, heading towards the Alps and then eastwards over northern Egypt. At the time of cyclone formation at mean sea level (i.e. time D0), the jet stream strengthens over the Balkans, heading towards northern Egypt and then turning towards the eastern Mediterranean. At time P1, the jet stream turns northwest over the eastern Mediterranean and west over the Near East, where strong cyclonic shear is noted Depressions originating from the east In the 9 year period studied here, the number of depressions entering the area from an easterly direction is only 23 (see Table I). The results for this group of depressions are presented in Figure 3. At time M1, the static stability index at the lower tropospheric level of 8 hpa generally shows the same distribution Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

7 EVOLUTION OF DEPRESSIONS OVER CYPRUS 1835 a) Statics stability 8 hpa M Statics stability 8 hpa D Statics stability 8 hpa P1 b) Divergence 8 hpa M Divergence 8 hpa D Divergence 8 hpa P1 c) Divergence 0 hpa M Divergence 0 hpa D Divergence 0 hpa P1 d) Divergence 0 hpa M Divergence 0 hpa D Divergence 0 hpa P1 e) Rel. Vorticity 0 hpa M Rel. Vorticity 0 hpa D0 Figure 3. As Figure 2, but for depressions entering the area from the east Rel. Vorticity 0 hpa P1 Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

8 1836 K. NICOLAIDES, S. MICHAELIDES AND T. KARACOSTAS as that for depressions entering from the north. Strong static stability values (i.e. stable lower tropospheric air) characterize the European continental area, while the maritime and the North Africa continental area is characterized by weaker static stability values (i.e. unstable lower tropospheric air), reflecting the importance of the low-level sensible and latent heat transfers on the stability of the lowest layers. The zone of the strong gradient of the static stability index over the southern European coastline and around the Black Sea coincides roughly with the mean position of the polar front. Specifically, at time M1, the lowest static stability values (i.e. more unstable lower tropospheric air) are located over the area of the eastern Mediterranean. The area of the eastern Mediterranean is found to be in an area of low static stability index (namely, unstable lower tropospheric air); this area is also found in an area of lower tropospheric negative divergence (namely convergence) that extends from further eastwards. The area of Turkey and the Balkans is covered by a strong positive divergence field. This combination results in cyclonic development at time M1, over the area of the eastern Mediterranean, at least over the lower tropospheric level, while the strong divergence field over Turkey and the Balkans results in anticyclonic development. The upper tropospheric level is found to work in such a way that does not fully support the lower tropospheric convergence over the eastern Mediterranean. So the area of the central Mediterranean, the Balkans, central Europe and the western part of the eastern Mediterranean are covered by a broad convergence area aloft, while the areas of the eastern part of the eastern Mediterranean, the Near East and Saudi Arabia are predominantly covered by a divergent wind field. This means that the actual cyclonic development area is confined only to the eastern part of the eastern Mediterranean and further eastwards, rather than the western part. The field of relative vorticity exhibits a weak gradient with a stronger gradient located over the Balkans, France and southern Egypt, coinciding with the position of the jet stream aloft, since central Europe is marked by negative relative vorticity values (anticyclonic shear) and the area of the Black Sea, Turkey, eastern Mediterranean and Egypt is marked by positive relative vorticity values (cyclonic shear). At time D0, the distribution of the field of static stability shows an increase of the instability of the lower tropospheric air over the southern part of the eastern Mediterranean, compared with time M1. This is the result of the warmer easterly to southerly surface airflow over the area. At time D0, the divergence zone at the lower level of 8 hpa over northern Turkey becomes stronger, suppressing cyclonic development, while cyclonic development is favoured with the support of strong convergence over the Near East and Saudi Arabia and to a lesser degree over the eastern Mediterranean. Over the Near East, the low-level convergence (i.e. 8 hpa) and the upper level divergence (i.e. 0 hpa), coupled with lower tropospheric instability, support the mechanism of cyclone development over the area of the eastern Mediterranean and further eastwards. At time D0, the relative vorticity field displays its strongest gradient over the area of Turkey and the Balkans, with positive values over the area of the Mediterranean and negative values over continental Europe. The distribution clearly shows that positive relative vorticity is over the area where cyclonic development takes place. At time P1, the area of the eastern Mediterranean is characterized by increased instability at the lower 8 hpa isobaric level. The distribution of divergence is found to have a generally weak gradient over the isobaric level of 0 hpa, while a stronger gradient is noted over the isobaric levels of 8 hpa and 0 hpa. At time P1, the divergence field shows that development over the area of the eastern Mediterranean is much weaker than at D0, while strong divergence is found over northern Egypt and Turkey. Cyclonic development occurs, at least at the lower tropospheric levels, over the Near East and further eastwards, while the strong divergence field covering the area of Turkey and Egypt favours anticyclonic development. Aloft, at the upper tropospheric level of 0 hpa, stronger convergence is found over the eastern Mediterranean and further westwards, while divergence is found over the area of the Near East and further eastwards. This upper level divergence distribution supports cyclone development over the area of the Near East and further eastwards, which is also supported by the strong positive values of the relative vorticity field over the area. Studying the time evolution of the relative vorticity field, the presence of a relatively weak jet stream aloft is inferred over eastern Europe heading towards the Balkans, bending over the area of southern Egypt and heading towards the area of Saudi Arabia, at time M1. At time D0, the jet stream intensifies over the area of eastern Europe and the Black Sea, leading to a strengthening of the field of positive relative vorticity over the area of the eastern Mediterranean. At P1, the jet stream appears to be relatively weak over Turkey, while the Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

9 EVOLUTION OF DEPRESSIONS OVER CYPRUS 1837 area of the positive relative vorticity is displaced eastwards, over the area of the Near East. At the same time, the area of the Near East is found to be in an area of strong lower convergence and upper strong divergence, suggesting further cyclonic development Depressions originating from the south The least frequent depressions are those originating from the south, amounting to just 14 in the 9 year period considered here (Table I). The results for this group of depressions are presented in Figure 4. The distribution of the static stability index at the isobaric surface of 8 hpa shows that the area of continental Europe is covered by a very stable lower atmosphere. Large static stability values are found over central Europe, the northern Balkans, central Turkey and to the north of the Caucasus range. However, the lower tropospheric values of static stability index are greater than the respective values found over the same area, regarding the depressions entering the area from either north or east, thus resulting in a steeper gradient further to the south. At the 8 hpa level, the field of divergence shows that the central and eastern Mediterranean are regions of convergence, with the larger values found over the Adriatic Sea, while the areas of central Turkey and the Black Sea are sheared by strong low-level divergence. Aloft, at the mid-tropospheric level of 0 hpa, the field of divergence is negative (suggesting weak mid-tropospheric convergence) over a great part of the eastern Mediterranean. Furthermore, at the upper tropospheric level of 0 hpa, the same area is also found under the influence of a zone of convergence. Bearing in mind the above vertical distribution of the divergence field over the area of the eastern Mediterranean, the potential for cyclonic development is uncertain. Cyclonic development is favoured over the area of Saudi Arabia and the Balkans. The field of relative vorticity is positive, but weak, over the area of the Mediterranean and North Africa, while it is stronger over the area of the Near East. At time D0, the static stability index becomes weaker, signifying that the lower tropospheric air is more unstable due to the warm southerly surface airflow. The field of divergence at lower levels strongly supports cyclonic development over the area of the eastern Mediterranean, since strong negative divergence values (i.e. convergence) are found over this area. The mid-tropospheric level of 0 hpa shows that the eastern Mediterranean is found in an area of weak divergence. Cyclonic development is supported by the upper tropospheric level of 0 hpa, since the area of the eastern Mediterranean is characterized by strong divergence values. At time D0, the field of relative vorticity is found with positive values (namely cyclonic shear), greater than those at time M1, over the area of the eastern Mediterranean. Thus, by comparing the divergence fields at the lower, mid and upper tropospheric levels and that of relative vorticity at the midtropospheric level at time M1 with the same fields at time D0, it is concluded that cyclonic development over the eastern Mediterranean is mostly favoured at time D0 rather than M1. At time P1, the field of static stability index shows insignificant changes compared with that of time D0. The divergence field at the lower tropospheric level of 8 hpa shows a strong gradient over the wider area of the eastern Mediterranean, Egypt and Turkey. Thus, over the area of Egypt a strong divergence field is formed, while over the area of Turkey the divergence field found at time D0 becomes even stronger (compared with that of time D0). The area of the Near East, Saudi Arabia and eastern Mediterranean is found in a convergence zone. Specifically, the eastern Mediterranean is found in an area of weaker convergence than D0, while stronger lower tropospheric convergence is found over the Near East and further eastwards. This signifies that the new area for ascending motion is the area of the Near East and further eastwards. Hence, the weakening of convergence at the lower tropospheric level of 8 hpa over the area of the eastern Mediterranean and the strong lower divergence over Turkey and Egypt signify the cessation of further cyclonic development over the wider area of the eastern Mediterranean. This is also supported by the upper tropospheric level of 0 hpa, since the area of the eastern Mediterranean is found under an area of upper convergence. The area of the Near East is an area of upper divergence and it is coupled with lower convergence, as stated earlier, signifying the new cyclonic development area. This cyclonic development is also supported by the mid-tropospheric distribution of positive relative vorticity towards the area of the Near East. Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

10 1838 K. NICOLAIDES, S. MICHAELIDES AND T. KARACOSTAS - a) Static stability 8 hpa M Static stability 8 hpa D Static stability 8 hpa P1 b) Divergence 8 hpa M1 Divergence 8 hpa D0 Divergence 8 hpa P1 c) Divergence 0 hpa M Divergence 0 hpa D Divergence 0 hpa P1 d) Divergence 0 hpa M Divergence 0 hpa D Divergence 0 hpa P1 e) Rel. Vorticity 0 hpa M1 Rel. Vorticity 0 hpa D0 Rel. Vorticity 0 hpa P1 Figure 4. As Figure 2, but for depressions entering the area from the south Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

11 3.4. Depressions originating from the west EVOLUTION OF DEPRESSIONS OVER CYPRUS 1839 In the 9 year period, 73 depressions originating from the west were counted, thus representing almost half of the total number of the cyclonic systems studied here (Table I). The isobaric distributions of the dynamic parameters corresponding to the depressions originating from the west are presented in Figure 5. At time M1, on the distribution of the static stability index, at the lower level of 8 hpa, the contrast between maritime and continental areas is marked; generally speaking, in the wintertime, maritime air is expected to be unstable and continental air to be stable. Hence, the field of static stability shows that the broad Mediterranean region is the area with the most unstable lower tropospheric air, in view of the fact that this lowest layer is being destabilized by heating from below from the generally warmer Mediterranean Sea. Stable air is found over the large continental area of Eurasia, characterizing the polar continental airmass, having its source originating from the predominant synoptic system of the area during the cold season, namely the Siberian anticyclone. At time M1, the area of the central Mediterranean is the area with the lowest value of static stability index; hence, it is the area with the most unstable lower level tropospheric air. This lower tropospheric instability is found at time D0 over the area of the Cretan Sea and the eastern Mediterranean, and at time P1 it is found over the area of the eastern Mediterranean. Regarding the field of divergence of the horizontal wind vector at M1 and at the lower tropospheric level of 8 hpa, the central and eastern Mediterranean are areas of convergence (i.e. negative divergence). Hence, strong convergence is found over the Ionian Sea, while stronger convergence is found over the area of southwest Turkey. Aloft, over the upper tropospheric level of 0 hpa, the areas of the Balkans, Turkey, the Black Sea, the eastern Mediterranean and the eastern part of the central Mediterranean form a wide divergence area, with the greatest values found over southwest Turkey. The vertical profile of the field of tropospheric divergence is such that the arrangement of lower level convergence and upper level divergence over the areas of the Balkans, Turkey, the Black Sea, the eastern Mediterranean and the eastern part of the central Mediterranean suggests cyclonic development. Stronger cyclonic development is favoured over the area of southwestern Turkey, since the stronger lower convergence is coupled with the stronger upper divergence over this area. The distribution of relative vorticity is found to have a gradient over Italy: negative values (anticyclonic shear) over western Europe and the British Isles and positive values (cyclonic shear) over eastern Europe, the central and eastern Mediterranean and North Africa, with the greatest positive relative vorticity values being found over the central Mediterranean. At time D0, cyclonic development is supported over the area to the east of the central Mediterranean and the eastern Mediterranean. The area of the Ionian Sea is still found in an area of lower tropospheric convergence, but cyclonic development is limited because of lack of support (i.e. lack of upper divergence). Further cyclonic development is located over the area of Turkey, the eastern Mediterranean, the Near East and Saudi Arabia. The lower tropospheric convergence is coupled with the upper tropospheric divergence, resulting in ascending motion in the troposphere and cyclonic development. The field of relative vorticity operates towards enhancing the cyclonic development over the above area, since positive vorticity values (cyclonic shear) are found over the eastern Mediterranean and the Aegean. At time P1, the lower tropospheric field of divergence shows a weakening convergence zone over the area of the eastern Mediterranean, while the stronger zone is over the Near East, the Black Sea and the Caucasus. The area of the central and western Mediterranean is an area of lower divergence, thus favouring descending motions in the lower troposphere. At time P1, cyclonic development is favoured over the area of the Near East, Saudi Arabia, the Black Sea and the Caucasus, but stronger cyclonic development is favoured mainly over the Near East and further eastwards and the area of Saudi Arabia. Regarding the mid-tropospheric relative vorticity field, positive values are found over the area of the eastern Mediterranean, west of the strong cyclonic development area. The structure of the relative vorticity field shows a weak gradient at time M1 over the area of southern Italy, which is strengthening at time D0. At time D0, the strong positive relative vorticity values are found over the eastern Mediterranean, suggesting that the cyclonic shear associated with the jet steam aloft either becomes stronger over central Mediterranean or the cyclonic curvature increases over the area. At time P1, the relative vorticity field weakens, with its maximum displaced further east, over Cyprus. Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

12 18 K. NICOLAIDES, S. MICHAELIDES AND T. KARACOSTAS a) Static stability 8 hpa M Static stability 8 hpa D Static stability 8 hpa P b) Divergence 8 hpa M Divergence 8 hpa D Divergence 8 hpa P c) Divergence 0 hpa M Divergence 0 hpa D Divergence 0 hpa P d) Divergence 0 hpa M1 Divergence 0 hpa D Divergence 0 hpa P e) Rel. Vorticity 0 hpa M1 Rel. Vorticity 0 hpa D Rel. Vorticity 0 hpa P1 Figure 5. As Figure 2, but for depressions entering the area from the west Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

13 3.5. Depressions developing over the area EVOLUTION OF DEPRESSIONS OVER CYPRUS 1841 Although only a few of the depressions affecting the area are actually initiated over the area of Cyprus, these systems are of particular importance to local meteorologists (El-Fandy, 1946; Kallos and Metaxas, 1980). The findings regarding the average distributions of these depressions are presented in Figure 6. At time M1, the static stability index distribution is found with its lowest values (namely, unstable lower tropospheric air) over the area of the central and eastern Mediterranean. Moreover, at time D0, and later at time P1, the static stability index distribution is found with its lowest values over the southern part of the eastern Mediterranean, noting the increased instability of lower level tropospheric air. The European continental area is at all times an area with positive mean static stability values, noting the stable lower level tropospheric air, mostly over the area of central Europe and that of Russia. At time M1, the distribution of the horizontal divergence at the lower tropospheric level of 8 hpa shows a zone of convergence over the area of Turkey, the eastern Mediterranean, the Near East and Saudi Arabia. In contrast to this lower level convergence, the same geographical areas are found to be divergent aloft. The field of relative vorticity at the mid-tropospheric level of 0 hpa is weak, but positive. Under these conditions, the areas of Turkey, the eastern Mediterranean, the Near East and Saudi Arabia are areas with a potential for ascending motion and, thus, for cyclonic development. At time D0, the static stability index is found with its lowest value (namely, stronger lower tropospheric instability) over the southern part of the eastern Mediterranean. At the same time, the distribution of divergence shows the formation of a strong convergence maximum over southeastern Turkey, extending towards the area of the eastern Mediterranean and the Near East, suggesting an intense cyclonic development, mainly over southeastern Turkey and the Near East. At time D0, the Black Sea is an area of lower tropospheric divergence, denoting anticyclonic development. Cyclonic development over the area of southeastern Turkey, the eastern Mediterranean and the Near East is supported by the distribution of divergence at the upper tropospheric level of 0 hpa, where these areas are characterized by strong divergence values. Also, at time D0, the relative vorticity field becomes stronger than at time M1, resulting in the entire map s absolute maximum in vorticity being located between Crete and Cyprus. Such conditions are favourable for cyclonic development over the area of southeastern Turkey, the eastern Mediterranean and the Near East. At time P1, the lower tropospheric field of divergence suggests that the main cyclonic development zone is displaced slightly eastwards, covering the area of eastern Turkey, the Near East and Saudi Arabia, while the area of the eastern Mediterranean is found in an area of weakening convergence, indicating that support for cyclonic development ceases gradually. This weakening cyclonic development over the area of the eastern Mediterranean is also supported by the distribution of divergence over the upper tropospheric level of 0 hpa. Given that the eastern Mediterranean is essentially an area of convergence, there is thus a tendency for cyclolytic processes. At the same time P1, and at the upper tropospheric level of 0 hpa, the areas of the Near East and Saudi Arabia are found in an area of divergence, which results in ascending motions, cloud formation and cyclonic development. At time D0, at the mid-tropospheric level of 0 hpa, the positive field of relative vorticity over the area of the eastern Mediterranean becomes stronger, hence favouring cyclonic development over this area. At time P1, the distribution of relative vorticity is found with its maximum value over the easternmost part of the eastern Mediterranean; thus, the area of cyclonic development moves further to the east. 4. CONCLUDING REMARKS The synoptic climatology of depressions in the Mediterranean area has been widely studied (Alpert et al., 1990; Trigo et al., 1999; Campins et al., 00). Recently, the general characteristics of the climatology of various dynamic parameters over the Mediterranean region have been investigated (Flocas et al., 01; Michaelides et al., 02). The present study is considered as a contribution to the documentation of the synoptic climatology of some dynamic parameters associated with the development of synoptic-scale cyclonic systems affecting the eastern Mediterranean. The incentive behind the present analysis is to produce average spatial distributions of some dynamical parameters pertinent to the development of baroclinic disturbances. These can be used as Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

14 1842 K. NICOLAIDES, S. MICHAELIDES AND T. KARACOSTAS a) Static stability 8 hpa Static stability 8 hpa D Static stability 8 hpa P1 M1 b) Divergence 8 hpa M1 Divergence 8 hpa D0 Divergence 8 hpa P1 c) Divergence 0 hpa M1 Divergence 0 hpa D0 Divergence 0 hpa P1 d) Divergence 0 hpa M Divergence 0 hpa D Divergence 0 hpa P1 e) Rel Vorticity 0 hpa M1 Rel Vorticity 0 hpa D0 Rel Vorticity 0 hpa P1 Figure 6. As Figure 2, but for depressions forming over the area Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

15 EVOLUTION OF DEPRESSIONS OVER CYPRUS 1843 guidance to meteorologists in attending to the development of such synoptic-scale features of various origins in the area. Depressions over the area of the eastern Mediterranean are mainly a cold-season event. The classification adopted was dictated by the phenomena associated with each class of depressions: depressions entering the area from the north are associated with cold airmass intrusions, low temperatures and snow showers; depressions entering from the east and south are mainly associated with dust events (Michaelides et al., 1999); and depressions entering from the west are associated with rainy weather. From the discussion of the results above, it is clear that the underlying topography is, to a high degree, responsible for the distribution of the lower tropospheric level dynamic parameters under study, as is the direction from which a depression is entering the area, which could be useful in weather forecasting. The upper tropospheric level mainly handles the dynamic mechanisms of the systems involving their evolution and development. The mid-tropospheric level of 0 hpa acts as the level of non-divergence, and it is clearly found with very weak gradient. The main remark concerning the lower tropospheric mean static stability value is its strong association with topography and latitude. The lower tropospheric air is more stable over continental areas than maritime areas, where the lower tropospheric air is more unstable. The influence of the warmer Mediterranean Sea and that of the colder continental area modify the stability of lower tropospheric air. Unstable air over the Mediterranean signifies the potential of the maritime air to support cyclone development in the lower troposphere. Furthermore, weaker instability (compared with the instability over the Mediterranean) is found over the Atlantic, which, however, varies with latitude (since the Atlantic Ocean is warmer to the south than to the north). The static stability index also varies over the continental areas, since northern continental areas comprise more stable lower tropospheric air than the southern continental areas. The strong static stability values, found over Russia, are associated with the cold-season s predominant synoptic system, namely the Siberian anticyclone. There are cases where lower tropospheric instability is coupled with lower level divergence, a combination that is not very supportive with regard to further cyclone development. Such a combination is found in the case of depressions entering from the north, at all times M1, D0 and P1. The distribution of the divergence of the horizontal wind vector at the lower level of 8 hpa appears to be influenced by the orography, mainly over the area of the Alps and Turkey. Generally, it shows that the developing areas at lower levels are connected not only to the unstable lower tropospheric air, but also where orography and the upper dynamics favour development. The Turkish plateau is the area where development occurs at time M1 when depressions approach from the north, whereas the development of depressions from the east occurs over the area of the Near East and the eastern Mediterranean. Depressions entering the area from the south are initiated over the eastern Mediterranean, whereas depressions expected from the west have a long development path, starting from the area of southern Italy. Depressions developing in the area are found to be associated with a lower level convergence zone extending from eastern Europe, the Balkans, the Black Sea, Turkey and the Near East. Upper tropospheric dynamic processes support or suppress further development. Relative vorticity is found to be positive in all areas where cyclonic development occurs. Greater positive values were found at time D0, in all cases. Path, strength and curvature, as the main characteristics of the jet stream, are found to be very important for each group of depressions, since each group is driven by a particular jet stream set of characteristics. REFERENCES Alpert P, Neuman BU, Shay-El Y Climatological analysis of Mediterranean cyclones using ECMWF data. Tellus 42-A: Boast R, McGinnigle JB Extreme weather conditions over Cyprus during April Meteorological Magazine 101: Campins J, Genoves A, Jansa A, Guijarro JA, Ramis C. 00. A catalogue and a classification of surface cyclones for the western Mediterranean. International Journal of Climatology : El-Fandy MG Barometric lows of Cyprus. Quarterly Journal of the Royal Meteorological Society 72: Flocas HA, Maheras P, Karacostas TS, Patrikas I, Anagnostopoulou C. 01. A -year climatological study of relative vorticity distribution over the Mediterranean. International Journal of Climatology 21: Gleeson TA Cyclogenesis in the Mediterranean region. Archives of Meteorology, Geophysics and Bioclimatology, Series A (6): Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)

16 1844 K. NICOLAIDES, S. MICHAELIDES AND T. KARACOSTAS Kallos G, Metaxas DA Synoptic processes for the formation of Cyprus lows. Rivista Meteorologia Aeronautica XL(2 3): Karacostas TS, Flocas AA The development of the bomb over the Mediterranean area. La Meteorologie 33: Maheras P, Flocas HA, Anagnostopoulou C, Patrikas I. 00. On the vertical structure of composite surface cyclones in the Mediterranean region. Theoretical and Applied Climatology 71: Metaxas DA Intense cold intrusions in the Aegean during winter. Bulletin of the Hellenic Meteorological Society 1: Meteorological Office Weather in the Mediterranean, Vol. 1, 2nd edition. HMSO: London. Michael P Study of the weather systems affecting the area of eastern Mediterranean during the winter period. MSc thesis, University of Athens, Greece. Michaelides SC Limited area energetics of Genoa cyclogenesis. Monthly Weather Review 115: Michaelides S, Evripidou P, Kallos G Monitoring and predicting Saharan Desert dust events in the eastern Mediterranean. Weather 54: Michaelides S, Nicolaides K, Karacostas T. 02. Isobaric distributions of dynamic field over the wider European region. In 6th Panhellenic Conference on Meteorology, Climatology and Atmospheric Physics, Ioannina, Greece, September; (in Greek). Michaelides S, Nicolaides K, Karacostas T. 04. Statistical characteristics of the cold season depressions over the area of Cyprus. Meteorologicaky časopis 7: Nicolaides K, Michaelides S, Karacostas T Statistical analysis of the winter baroclinic depressions over the area of Cyprus. In 4th Panhellenic Conference on Meteorology, Climatology and Atmospheric Physics, Athens, Greece, September; 1 5 (in Greek). Prezerakos NG, Michaelides SC A composite diagnosis in sigma coordinates of the atmospheric energy balance during intense cyclone activity. Quarterly Journal of the Royal Meteorological Society 115: Reiter ER Handbook for Forecasters in the Mediterranean. Naval Postgraduate School: Monterey, CA, USA. Trigo IF, Davies TD, Bigg GR Objective climatology of cyclones in the Mediterranean region. Journal of Climate 12: Copyright 04 Royal Meteorological Society Int. J. Climatol. 24: (04)