Scholarly record

IDENTIFICATION OF CIRCULATION PATTERNS PRODUCING FLASH FLOODS OVER SOUTH-WEST OF IRAN

Najafpour Bahram

Abstract

In this study, to identify circulation patterns producing flash flood in the South-West of Iran, 51 large floods in the Qantareh Station have been selected which have been among the data from the daily flood in a period of 30 years of Mond basin. The Mond basin is one of the sub-basin of Persian Gulf Basin and is located in south west of Iran. Then, the hydrologic features of selected floods ha ve been calculated. Afterwards, hydrologic features of these floods using mean and st andard deviation have been standardized. Then performing a cluster analysis via Ward integration method the data of six main groups of flood have been derived. Finally one day was determined as a sample day of each group to perform synoptic analysis of precipitation days causing the flood of sample day via the environment-to-circulation procedure. To analyze precipitation days synoptically, maps of sea level pressure(SLP), geopotential height, temperature, specific moisture of 500 hPa level and Isohyets maps of daily basin have been used. The upper atmosphere data have been provided from NCEP\NCAR database. In this paper, only the synoptic analysis of precipitation days of sample flood of flood cluster number 4 is provided. The results showed that precipita tion cluster providing sample flood of flood cluster number 4 were due to synoptic conditio ns of locating a cut-off low in the East Mediterranean and South of Black Sea. Key words: circulation patterns, geopotential height, cluster analysis, flash flood, Iran 1. I1TRODUCTIO1 Climate studies are one of today's necessities. Emergen ce of climatic phenomena such as drought, devastating floods, and climatic changes in recent years has doubled this need. Today, climatologists with a variety of methods such as modeling of atmospheric conditions [31], [6],[15] using indices of tel econnection [23], [29], [17], [19], [18], [12] using data from remote sensing [10], [13] st atistical analyses [5], [16] and synoptic studies [27], [1],[26], [2] investigate the c limatic research. There is a long literature review on methods of climatic synoptic analysis. But the big jump in climate synoptic studies in recent decades is provided by usi ng computer for the synoptic researches and accessing databases such as the NCEP\NCAR. Nowadays this database is widely used [14], [32], [11]. These conditions have made huge and long-term data analysis possible using mathematical and sta tistical techniques [20], [35] , [22]. So today, synoptic climatologists are able to do detailed studies such as climate forecast and other climatic phenomena [33]. While in the past, synoptic studies were mostly done by manual methods; synoptic researchers use more qua ntitative methods to analyze their studies today. Each of these methods can be done in two ways: circulation-to-environment and environment-to-circulation. In the environment- to-circulation method first environmental phenomena such as floods, drought, pollution, dust days, etc at ground level are studied; then, the upper atmos phere synoptic condition is studied [34]. This 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org International Multidisciplinary Scientific GeoConference SGEM 2011 874 method is commonplace in synoptic climatology [37]. For example, Betts et al [4] used synoptic environment-to-circulation method in a study on the coastal East Atlantic storms. Dayan and Lamb [8] also used th e same method to study global and synoptic- scale weather patterns controlling wet atmo spheric deposition over central Europe. Delden [9] also used environment-to-cir culation method for synoptic analysis of thunderstorms in west Europe. 2. Materials and methods In this study, by selecting the method of synoptic environment-to-circulation analysis, circulation patterns of flash floods of Mond basin (Fig. 1) have been identified. The Mond basin is located in south west of Iran. for this purpose, first a study is done over a statistical period of 30 years of floods in Qantar eh Station that is located in the outlet of the Mond basin (Fig. 2), 51 clusters of severe and pervasive flood in this station which are also seen in the other hydrologic gauging station of the basin have been selected. The selected floods have been measured in daily scale. A flood cluster is a flood that has been continued through a day or more. The selection criterion of these floods was that their mean was one standard deviation more than the annual average floods in the Station [26]. Then, by statistical analysis of selected floods, their base time, peak time, volume of flood peak, the average of floods, total volume of floods and runoff height were calculated. Then, these results us ing mean and standard deviation were standardized [30], [25]. After that, by doing a cluster analysis of integration method on the standard data, the six flood clusters were obtained (Table 1). Next, by choosing a day as the sample day for each cluster of six floods, the flood features of sample day were calculated (Table 2). The sample day is a day that has the highest correlation with their group members (Table 3). Finally Clusters of precipitation causing sample floods were identified (Table 4). A precipitation clus ter is a rainfall day that has been falling through a day or more. To perform synoptic analysis, an area with dimensions of zero to 60 degrees north and 10 to 70 degrees east was selected. For synoptic analysis, maps of geopotential height, temperature and specific moisture of 500hPa level and sea level pressure (SLP) are used two days before to two days after the preci pitation days causing flood of sample days of each cluster flood. These maps have been provided from NCEP\NCAR database. At the end, Isohyet map of precipitation days causing the flood of sample days have been drawn. Here for the sake of space, just synoptic analysis of precipitation cluster causing flood of sample cluster No 4 has been performed. Fig.1. locations of the Mond Basin on the Iran's map Fig. 2 . The location of gauging stations 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org Hydrology Water Resources 875 3. Discussion and results 3-1. Cluster analysis One of the common clustering methods is Ward ’s cluster analysis [28]. This method is based on minimum variance within groups and maximum variance between groups [24]. In order to categorize the selected floods of Qantareh station, a cluster analysis was done by Ward’s agglomeration method on the standardized data of 51 selected flood features of Qantareh station. Then the sel ected floods of Qantareh station were divided in to six major clusters. At the end, main features of each flood cluster such as mean, base time, time to peak, and height have been calculated. Table 1 shows the features of flood clusters in Qantareh station. In this table in the seventh column one day is determined as a sample day for each cluster flood. This day is used for the synoptic analysis of the flood clusters. Table 2 shows the features of the floods of the sample days and table. 3 show the correlation of the floods of sample day of flood clusters in Qantareh station. Table.1. The features of flood clusters in Qantareh Station Sample flood number Flood height (mm) Flood mean (m3/s) Peak flood (m3/s) Time to peak (Day) Base time (Day) Cluster number 6 4.48 257.01 885.12 2.48 6.82 1 9 8.36 211.79 1005.9 10.5 16.5 2 3 26.41 782.37 2604.7 6.25 14.25 3 44 13.58 636.86 2214.7 3.37 9.37 4 28 2.14 112.08 359.15 3.65 12.8 5 14 7.96 369.74 1104 4.10 9.34 6 Table. 2. The features of the floods of the sample day Cluster number sample flood numbe r Start Date End Date Base time (Day) Time to peak (Day) Peak flood (m3/s) Flood mean (m3/s) Flood heigh t (mm) 1 6 1972.02-19 1972-02-26 8 3 1198.72 356.62 6.8 2 9 1976-03-14 1976-03-30 17 10 1455.52 284.18 11.4 3 3 1971-01-11 1971-01-25 15 8 2588.6 754.78 26.8 4 44 1995-12-11 1995-12-18 8 2 2096.03 627.82 11.9 5 28 1990-02-03 1990-02-10 8 5 333.58 112.1 2.1 6 14 1980-01-22 1980-01-30 8 4 1174.34 457.67 8.7 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org International Multidisciplinary Scientific GeoConference SGEM 2011 876 Table. 3. The correlation of the floods of sample day Cluster number sample flood number sample flood correlation Correlation within groups Group members 1 6 0.9975 0.9950 13 2 9 0.9993 0.9993 2 3 3 0.9997 0.9994 4 4 44 0.9965 0.9932 8 5 28 0.9972 0.9945 17 6 14 0.9951 0.9910 9 3-2. Precipitation Cluster causing sample floods After performing cluster analysis and computing the features of six flood clusters in Qantareh Station and also determining the sample day, precipitation clusters causing the flood of sample day have been identified. Fo r this purpose by referring to the calendar of precipitation clusters of the basin, precipitation cluster that was occurred synchronous with the flood of the sample days or the days before was selected as precipitation cluster causing the flood of the sample days. Next, the main features of each cluster such as precipitation start time, end da te, base time, etc. have been calculated. Table 4 shows features of precipitation cluster causing flood of the sample days. Here for the sake of space only the synoptic analysis of precipitation days causing sample floods of the flood cluster number 4 has been performed. Table. 4. Features of precipitation clusters causing sample floods 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org Hydrology Water Resources 877 3-3. Synoptic analysis Based on the calculated correlation (Table 3) flood No 44 from a total of 51 chosen floods is selected as the sample flood of flood cluster number 4. Flood cluster number 4 has eight members and its sample flood has a correlation of 0.9965 percent with other group members. The correlation within the group of flood cluster number 4 is 0.9913 percent. The flood started on December 11 th in 1995 and after 8 days, it was terminated on December 18th1995. The Sample flood took 2 days to reach the peak and the amount of flood in peak time was 2096.03 cubic meters per second (Fig. 3). The Flood mean in base time was 627.82 cubic meters per second and its direct runoff was 11.9 mm. The sample flood of the flood cluster number 4 has produced 433.95 million cubic meters of flood water during the base time. Precipitation cluster causing sample flood of the flood cluster number 4 started on December 10 th in 1995 and continued for 3 days. The average daily precipitation of this precipitation cluster was 24.1 mm and its total amount was72.3 mm. This precipitation cluster on average cove red 83.89 percent of Mond basin and has produced volume of water equal to 2638.95 million cubic meters and the average precipitation of its center was 66.97 mm. Precipitation center is a place with the highest daily precipitation on a daily isohyet map [21] . If the volume of sample flood is divided over the water volume of this cluster, the runoff coefficient will be equal to 0.1646. Figure 4 shows precipitation-flood chart of the sample flood of the flood cluster number 4. Fig. 3. Precipitation-flood chart of sample flood of the flood cluster number 4 Figure 4 shows the map of 500hPa geopotential height of precipitation cluster of sample flood of flood cluster number 4. On November 8 th 1995 the map that refers to two days before the start of precipitation cluster in th e basin, a cut-off low with the contour line of 5550 meters in the south of the Black Sea at latitude 40 degrees north is located. The expansion of this cut-off-low to the sout hern latitudes has caused zonal moving of westerlies in the south west of Iran. The map of 500 hPa temperature level on this day also shows the existence of cut-off-low in this day (Fig. 5). The geopotential height map of the day December 9 th 1995 is almost like the map of the day before. In the geopotential height map of December 10th 1995 precipitation day, the cut-off-low on the 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org International Multidisciplinary Scientific GeoConference SGEM 2011 878 map of the previous day has been advancing to the lower latitudes. This factor caused the formation of a weak trough on the Red Sea. The contour lines of 5600 meters have crossed over the middle atmosphere of the basin. Shallowness of this trough has caused the basin to have little or no precipitation in this day. The spatial average of precipitation for this day is 0.32 mm and the amount of precipitation of the precipitation center is also 5.55 mm. The spatial average of precipitation is the pixel mean precipitation in a daily isohyet map which has been obtained by using Kiriging Interpolation Method. The geopotential height map of the precipitation day on December 11 th1995 shows the movement of the cut-off-low of the previous day map to east than the previous day and its dominance on the east Mediterranean and south of Black Sea. In this map another cut-off-low with the contour line of 5390 meters is located in the south-east Scandinavian Peninsula and north of the Bl ack Sea. The cut-off-low has deepened and shifted the trough of the previous day to the East. Increased precipitation in the basin has been remarkable in this day so that in this day the spatial average of precipitation has increased to 21.91 mm and the amount of precipitation of the precipitation center has increased to 96.56 mm. On the geopoten tial height map of the precipitation day December 12th1995 two cut-off-lows of the previous day map integrated and formed a unique cut-off-low with the center height of 5350m and settled in the central Europe. A cut-off-low tongue of these contour lines with 5550 meters has been expanded along the North West-South East to North-West of Ira n and West of the Caspian Sea. The center height of this cut-off-low has been reduced to 50 meters than the previous day. These conditions have increased the temperature contrast in this region and accordingly atmospheric instability on this day. The e xpansion of the menti oned cut-off-low tongue to the North-West of Iran has caused a trough along the North West - South East so that the axis of this trough is located on the North West of Persian Gulf. On this day, the basin has undergone a special precipitation day so that the spatial mean of precipitation of this day on Mond basin is 50.07 mm and the amount of precipitation of the precipitation center is 98.77 mm. On the geopotential height map of December 13 th 1995 which is a day after the precipitation days in Mond basin, the cut-off-low of previous day is located in northern Italy a nd Westerlies in a zonal flow dominates the area of Iran. Figure 8 shows isohyet maps of precipitation cluster causing the sample flood of flood cluster number 4. 0 1 02 03 04 05 06 07 0 10 20 30 40 50 60 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 (a) 1995-12-08 (b) 1995-12-09 (c) 1995-12-10 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org Hydrology Water Resources 879 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 0 1 02 03 04 05 06 07 0 10 20 30 40 50 60 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 (d) 1995-12-11 (e) 1995-12-12 (f) 1995-12-13 Fig.4. Geopotential height 500hPa level maps of precipitation days of sample flood of flood cluster number 4 Figure 5 shows the temperature map of 500hPa level in precipitation days of sample flood of flood cluster number 4. The obvious phenomenon of the temperature maps of precipitation days causing sample flood of the flood cluster number 4 is the following of cut-off-lows of this level from the low level centers of the geopotential height maps. Formation and movement of these cut-off-lows caused the formation of a trough temperature on the direction of the wester lies. The cut-off-lows in these maps corresponds with those of the geopotential height maps (Fig. 4) of this flood cluster. On these maps, most of these cut-off-lows with isotherms less than 246 degrees of Kelvin are visible. On the temperature map of 500hPa level of the precipitation days, pressed isotherms were observed in the range of 20-30 degrees north of the northern Red Sea which increases the existence of a front over this area. 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 (a) 1995-12-08 (b) 1995-12-09 (c) 1995-12-10 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 (d) 1995-12-11 (e) 1995-12-12 (f) 1995-12-13 Fig. 5. The temperature maps of 500hPa level in precipitation days of sample flood of flood cluster number 4 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org International Multidisciplinary Scientific GeoConference SGEM 2011 880 Figure 6 shows specific humidity map of the precipitation cluster causing the sample flood of flood cluster number 4. In all maps of specific humidity of 500hPa level of precipitation days of sample flood of flood cluster number 4, three important points were observed. Firstly, the moisture of systems causing Mond basin precipitations is provided from two different sources originated from the east of Atlantic Ocean and Africa. The second point is that the moisture of these two sources is increased from two different directions and enters the south- west of Iran and the Mond basin. The first direction is originated in the East Atlan tic, crosses Mediterranean Sea, Black Sea and then reaches west and north-west of Iran. Th e second direction is originated from East Africa, crosses Red Sea, Persian Gulf and the Oman Sea and then reaches south-west of Iran. This direction is mentioned in the resear ch of the other scholars too [3], [7], [36]. The third point is that the sp ecific humidity on the reviewed map has been concentrated from 10 to 40 degrees north and then it diminishes. 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 0 1 02 03 04 05 06 07 0 10 20 30 40 50 60 0 1 02 03 04 05 06 07 0 10 20 30 40 50 60 (a) 1995-12-08 (b) 1995-12-09 (c) 1995-12-10 0 1 02 03 04 05 06 07 0 10 20 30 40 50 60 0 1 02 03 04 05 06 07 0 10 20 30 40 50 60 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 (d) 1995-12-11 (e) 1995-12-12 (f) 1995-12-13 Fig. 6. Specific humidity maps of 500hPa level in precipitation days of sample flood of flood cluster number 4 Figure 7 shows the sea level pressure (SLP) map of precipitation cluster causing sample flood of flood cluster number 4. On the map of December 8 th 1995 three pressure centers are observed. A low pressure center with a central pressure at 1004 hPa is seen in the north-east of The Kharazm Lake. The tongue of this low pressure center has spread along the northeast - southeast regions to eastern areas of Iran. Another low pressure tongue with the isobar of 1008 hPa is located in the south-west of the Red Sea. A high pressure center with central pressure of 1040 hPa has also settled in the north of Black Sea. On the sea level pressure map, of December 9 th 1995 relatively severe pressure differences have been viewed in and around Iran. So that, along the southwest- northeast of the map, first the amount of pressure decreases and then increases in the central areas to the northeast of Iran. Then the pressure has decreased again because of the presence of a low pressure center over the northeast of the Kharazm Lake. These 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org Hydrology Water Resources 881 conditions are indicative of temperature and humidity conflicts between the air masses of this region. The map of sea level of the first day of the precipitation cluster (December 10 th 1995) shows the settlement of the high pressure center of the map of the previous day over the north of Caspian Sea. The tongue of this high pressure center has spread along the north-south to the Oman Sea. On the other hand, the tongue of the low pressure center of the north-east Africa has been settled in the south west of Iran. Due to these conditions, Iran’s southwest areas and the Mond basin are the crash place of two air masses – cold air from the northeast and warm air from the southwest. On the map of December 11th 1995 the low pressure tongue of the sea level map of the previous day of northeast Africa, has been settled on the Ar abian Peninsula as an independent low pressure center with the pressure isobar of 1010hPa. Considering anti-clockwise movement of low pressure centers in the northern hemisphere, the dominance of this low pressure center on the Arabian Penins ula has caused the mois ture transfer of Persian Gulf to the regions of south-west Iran. On the other hand, the expanding and strengthening of the tongue of the high pressure center of the east of the Kharazm Lake on this day has also caused instability in this area. On the Sea level pressure map of December 12th 1995 which is the last precipitation day of the precipitation cluster, the movement of the low pressure center dominated on the Arabian Peninsula center in the map of the previous day toward the south and east, has caused the spread of its tongue on the south and south west of Iran and injected the moisture of Oman Sea and the Persian Gulf into the Mond basin. So, on this day the spatial mean of the precipitation of basin has increased to 50.07 mm. Figure 9 shows isohyet maps of precipitation cluster of the sample flood of the flood cluster number 4. 0 1 02 03 04 05 06 07 0 10 20 30 40 50 60 H L L 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 H L L 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 H L a) 1995-12-08 (b) 1995-12-09 (c) 1995-12-10 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 H L 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 H L 0 1 02 03 04 05 06 07 0 10 20 30 40 50 6 0 L L H (d) 1995-12-11 (e) 1995-12-12 (f) 1995-12-13 Figure. 7. Sea level pressure maps of precipitation days of sample flood of flood cluster number 4. 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org International Multidisciplinary Scientific GeoConference SGEM 2011 882 (a) Isohyet map of precipitation day 1995-12-10 (b) Isohyet map of precipitation day 1995-12-11 C) Isohyet map of precipitation day 1995-12-12 D) Isohyet map of total precipitation days causing sample flood of flood cluster number 4 Figure. 8. Isohyet maps of precipitation days of sample flood of flood cluster number 4 4. CO1CLUSIO1 As it was observed in the 500hPa geopotential height maps of the precipitation cluster causing sample flood of flood cluster number 4 has been the result of the combination of synoptic conditions. The most important part of these synoptic conditions is the settlement of a cut-off-low in the east Med iterranean and south of the Black Sea. These conditions have caused the formation of a trough on the Red Sea and the shift of its axis from the Red Sea to the Persian Gulf. The settlement of the eastern domain of this trough over the south west of Iran has caused the precipitation cluster causing sample flood of flood cluster number 4 in the Mond basin. 500hPa temperature maps of this precipitation cluster also confirm the presence of a cut-off-low in the mentioned level. In the specific humidity maps of 500 hPa leve l the moisture sources of the precipitation cluster of the sample flood come from two sources of east Africa and east Atlantic. 11th International Multidisciplinary Scientific GeoConference SGEM2011 www.sgem.org Hydrology Water Resources 883 These two sources are also strengthened along their direction, in away that the east Africa source has been strengthened with the Red Sea and the Persian Gulf and the east Atlantic source has been reinforced with the Mediterranean Sea. On the maps of sea level pressure the precipitation cluster causing the sample flood started over the Mond basin when a low pressure tongue from sout hwest of the Red Sea and a high pressure tongue from the northeast entered Iran. Strengthening and dominance of the mentioned low pressure tongue on the Arabian Peninsul a and its movement toward the south and east has also caused moisture transfer of the Persian Gulf and Oman Sea to areas of southwest of Iran and accordingly a huge precipitation and a flash flood in the Mond basin. 5. ACK1OWLEDGME1TS I would really appreciate the guidance of Seyed Abolfazl Masoodian, Associate Professor, Department of Geography, Isfahan University, Iran; I also thank Iran's Ministry of Energy because of sharing floo ds data, and the NCEP/NCAR for using the upper atmosphere data. 6. REFERE1CES [1] Alijani B. Variations of 500 hPa flow patterns over Iran and surrounding areas and their relationship with the climate of Iran. Theor Appl Climatol, 72, pp 41-54, 2002. [2] Bardossy A. & Filitz F. Identification of flood produci ng atmospheric circulation patterns, Journal of Hydrology, 313, pp 48-57, 2005. [3] Barth H.J. & Steinkohl F. 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