Low-level Jet(LLJ) and western Iran heavy rain: case study Dec 1991

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1 2011 2nd International Conference on Environmental Science and Technology IPCBEE vol.6 (2011) (2011) IACSIT Press, Singapore Low-level Jet(LLJ) and western Iran heavy rain: case study Dec 1991 Elham Mobarak Hassan Faculty of Agriculture & natural resources, Islamic Azad university,ahvaz branch, Iran mobarak_e@yahoo.com Amir Hossein Meshkatee Faculty of Basic Science, Science and Research, Islamic Azad university,tehran,iran, ameshkatee@yahoo.com Abstract The western Iran is affected by Mediterranean cyclones in the cold season. In this study the role of low-level jet(llj) in forming and developing heavy rain, exceeding more than 30 mm, and relationship between them are investigated in Dec 1991 by using Mediterranean cyclone location, humidity flux convergence, equivalent potential temperature and maximum wind situation. The Initial data was obtained by NCAR reanalysis. Six hours prior to rain event, the cyclone center is located over Turkey associated with extending pressure trough to Iraq and western Iran thus pressure gradient is increased in these regions, showing by synoptic analysis. The maximum wind speed more than 20 ms,in 850 hpa, is located south to southwest of cyclone along North Africa coast. Prior to occurrence of rain, southerly wind ms has formed over Saudi Arabia resulted by meridional wind component ( v ) enhanced to 14 ms in exit of low-level jet. The warm advection demonstrates transports low-latitude (Saudi Arabia, Sudan) warm air toward Iraq and Iran which is associated with forming and increasing southerly wind speed it can provide favorable conditions for the convective rain. Moreover, southerly wind can transport humid air from Red sea and Eden Gulf to high latitude, consequently low- level humid is increased in south Forming in exit of LLJ, the southerly wind is coincided with equivalent potential temperature advection and humidity flux convergence. As a result, southerly wind over Saudi Arabia provide some favorable condition for produce convective heavy rain by humidity and instability enhancement in The maximum humidity flux convergence increased to 10 grkg s in 700 hpa level and also mixing ratio to 8 grkg in 850 hpa in west of Iran six hours before rain events. Keywordst; Low-level jet ; souterly wind; equivalent potential temperature; humidity flux convergence I. INTRODUCTION This paper presents a LLJ, thermodynamic and synoptic analyses of meteorological conditions associated with heavy rains in the west border of Iran on 9 Dec Mediterranean weather systems have affected border of western Iran in cold season with rain, snow and falling temperature phenomena. Some of the Mediterranean system are accompanied total 24-h rainfall exceeding 30 mm. The precipitation is a meteorological phenomena that highly affects weather of some parts of regions. The several processes which sufficient moisture and upward motion are two essential conditions are necessary for precipitation forming [1]. Numerous investigators have studied heavy rain events and severe convective storms in world [2] [3]. Some earlier studies was demonstrated that heavy rain were well correlated highly with a ageostrophic low-level jet [4][5]. The low-level jets is defined as a region of relatively strong winds in the lower parts of the atmosphere usually between 925 to 850 hpa layer that are a common phenomena in the boundary layer [6]. This is named as a jet stream found typically below 850 hpa. The cyclogenesis is associated with enhancement of the 850 hpa wind field, as a result LLJ is formed. Furthermore, it often refers to a southerly wind maximum also occurs South/Southwest of a low-level cyclone. Previous Analyses indicate that an intense LLJ with wind speeds in exceed of ms in the hpa layer, was positioned at the west of warm sector cyclone or the east of a cold front extended to the South the cyclone center [7]. LLJ is an important factor in moisture transport and acts to increase transport to Northward warm and moisture air toward heavy rain or convection region [8] [6] [9] [10] [11]. LLJ can be increase to doubling the magnitude of the moisture transport into the region of heavy snow [9][2]. The numerical simulation has been performed to study LLJ effect in heavy rain [12]. The simulation with a weaker initial jet results to 1) reduction of low-level flow 2) decrease low-level thermal and moisture advection associated with the low-level flow[12]. Since some of LLJ did not cause any heavy rainfall other favorable conditions obviously must also exist for heavy rain events to occur, Therefore It can be considered a nearly necessary condition (not sufficient) in producing heavy rain events [13].The purpose of this study is to determine the role of low-level jet in transporting humid toward western and northwestern Iran and the development of heavy rain. A case study is discussed On 9 December 1991 produce rain more than 30 mm during 24 hours more than seven stations which is chosen here as the heavy rain. II. METODOLOGY AND DATA The data used in this study was obtained from the NCEP/CAR archives of operational initialized analyses available every 6 hours for the period Dec The data grid resolution is and referred to the mean sea level and isobaric levels of 1000, 925, 850, 700, 500, 400, V1-163

2 300, 250, 200, 150, 100 hpa. The domain considered was from 10 to 50 N (North) and from 10 to 60 E (East). The synoptic structure will be examined by using mean sea level pressure. The wind field was plotted as isotach and equivalent potential temperature for the 850hpa level used to analysis the LLJ. The humid flux convergence and mixing ratio demonstrates for humidity low-level troposphere. Although the total rain occurred at 06(UTC) 09 Dec,the conditions were considered on 00/08(UTC) Dec 30-h before and 00/09 Dec six hours before heavy rain took place in Meridional cross section are used for recognizing vertical wind and humid factors in 24-h during ending to 00/09 Dec. III. SYNOPTIC AND TERMODYNAMIC ANALYSIS The occurrence total rain more than 30 mm during 24-h on 9 Dec 1991 is illustrated in Fig. 1 in which data from a network of rain gages is used to determine the distribution of rainfall along border of The total rain over nine stations is demonstrated by table.1 with geographic location. The maximum rain is 98 mm in Sardashat station located in N and 45.5 E. Prior to 00(UTC) 08 Dec 1991, the cyclone was formed over central Mediterranean Sea on 05 Dec crossed the Mediterranean toward Northeast (not shown), eventually low pressure with 1000 hpa contour is located over Crete island and Southwest of Turkey as moved toward Northeast during 5 to 8 Dec (Fig.2a). Sea level pressure accompanied by 850 hpa level wind vector at 00 (UTC) 08Dec are presented in Fig.2a. Maximum wind speed more than 21 ms is situated near 25E in southern cyclone and 32.5N along coastline of Northern Africa (Fig. 2-b) that it can be recognized as lowlevel jet (LLJ) definition, in 850 hpa. Cyclonic curvature wind is located in south of low pressure associated with southerly/southwesterly wind in east of cyclone covering east of Mediterranean sea and Turkey. The wind,in 850 hpa, blow from North Libya and Egypt toward south Turkey crossing Mediterranean sea,therefore it can carry warm and moisture air. It is seen southerly wind can transport air mass over Mediterranean sea toward south Turkey. The Maximum meridional wind component ( v ) with 14 ms value contributed to southerly wind development, crossing Turkey and east of Mediterranean Sea, which is situated in maximum wind speed (LLJ) exit,in 850 hpa. Increasing meridional component over Turkey produced southerly wind and cyclonic curvature in east of maximum wind speed (LLJ exit). The other southerly winds with 8 ms value that is formed by Maximum meridional wind component ( v ) with 8 ms value, covering Saudi Arabia toward Persian Gulf and south of Iraq, is demonstrated by Fig.2b. The southerly wind, which was largely responsible for the intensification of the meridional wind component ( v ) is seen near Northern Saudi Arabia Fig.2b. The impact of the increased southerly wind in the lower troposphere and at near the earth s surface in the region where the LLJ is developed is also illustrated by the surface equivalent potential temperature at 850 hpa level. The analysis of equivalent potential temperature at 00/08 Dec indicates warm air center is located Northeast of African near Sudan (Fig.2c). The equivalent potential temperature depicts extended warm air advection tongue from Sudan into Northern Saudi Arabia to western Iran, passing The 320 k isotherm southwestern Iran near 30N then continuing to Turkey and east of Mediterranean sea; consequently, it transports warm air into cyclone s east, at 00(UTC)08Dec. The northerly wind is accompanied with cold advection from western Russia region flows south toward Italy and North of Libya. Northern cold air and southern warm air are faced in east of Mediterranean Sea while warm advection ahead and cold advection rear of cyclone are took place. The southerly wind formed in southern Saudi Arabia can transport warm air this area to northward. The convective rain more than 2 cm,at 00/08 Dec, associated with warm air tongue is covered the most parts west of Turkey and black sea located North east of cyclone (Fig.2c). One of the index for humidity lower troposphere is mixing ratio whose maximum value is near Sudan and south of Red sea North,extending tongue Northeast direction toward east of Mediterranean so that 4 gr kg contour pass Turkey and eastern Mediterranean sea and second extending tongue to Persian gulf and south east of Iran with 6 gr kg (Fig.2d) at 00/08 Dec. Maximum humidity flux convergence with gr kg s value, in 700 hpa, is located over Turkey coinciding with maximum heavy rain as warm air advection flows toward this region,at 00/08 Dec (Fig.2d). The humidity flux convergence is demonstrated vertical transport humidity up to 700 hpa from surface. The humid air transport in vertical plan from surface up to 700 hpa associated with southerly wind speed 10 ms value in 25 to 30 N (fig.2d) and vertical upward wind (not shown). As a result, at 00 (UTC) 08 Dec, it seems warm and humid air over Mediterranean sea and North of Egypt is carried to cyclone, it can provide sufficient humidity and instability for convective rain by forming and increasing southerly wind located exit of maximum wind speed (LLJ). By 00(UTC)09 Dec, six hours before heavy rain, cyclones is weakened about 5 hpa increasing to 1005 hpa in the central pressure. As the cyclone is moved northeastward over Turkey, pressure trough is formed toward Iraq and north Saudi Arabia (Fig.3a). In the other side, the Siberian pressure ridge is developed east of Iran as pressure gradient is increased in western Iran associated with wind speed intensification. The analyses also reveal the low-level jet location in south-southeast of the cyclone over north coast of Africa extending to north of Saudi Arabia during 30 hours before heavy rain ending in western Iran to 00/09Dec. V1-164

3 The cyclonic curvature associated with cyclone is decreased while the wind direction is westerly /south westerly/southerly in LLJ exit extended to northern Saudi Arabia. in 850 hpa (Fig.3a and b). Intensifying Southerly wind to 14 ms in LLJ exit which is associated with pressure trough over Saudi Arabia along western of Iran is caused by increasing meridional wind component from 8 to 14 ms during 24 hours by 00/09 Dec (Fig.3b). The equivalent potential temperature is demonstrated enhanced warm advection toward southwest of Iran as contour 320 k shifted to 35N and 325 k closed to contour observed over Saudi Arabia by 00/09 Dec(Fig.3c). In the other side cold advection flow to east of cyclone toward Mediterranean sea and Egypt, therefore gradient temperature is intensified while pressure trough is closed to northern Saudi Arabia and south Increasing warm air advection is resulted by increasing southerly wind. Convection rain more than 4 cm is covered west border of Iran accompanied with warm air advection (Fig.3c). It can be seen, convection rain is increased from 2 to 4 cm during 24 hours as 00/09 Dec as v -component and southerly wind intensified. Comparing Fig.2b and 3b demonstrate intensified southerly wind that transport warm air from Red sea toward western Iran, it can create instability and upward motion for carrying vertical moisture. Mixing ratio is increased to 8 gr kg over southwest of Iran accompanied enhancing convective rain, during 24 hour ending by 00/09 Dec (Fig.3d). Comparison fig.2d and 3d, it is seen moist sufficient transport from Red sea and Aden sea to western Iran accompanied with southerly wind,in 850 hpa, therefore both of area are moist source for convective rain of This processes is one of necessary conditional for heavy rain and provides abundant humidity, in 850 hpa. By 00/09 Dec, humidity flux convergence pattern is shifted eastward reached to western Iran as the increased value to 8 10 gr kg s (Fig.3d) located east of pressure trough line coincided with maximum convective rain and increased mixing ratio (Fig.3d and c). The western Iran rainfall maximum with 4 cm value area is well coincided with surface moisture convergence pattern. Increasing humidity flux convergence is due to intensifying southerly wind from 8 to 14 ms (Fig.3b and d). The moisture and temperature advection create convective environment to the development of heavy rain. The southerly wind in LLJ exit was an important factor in increasing the magnitude of the moisture transport into the region of heavy rain. Western Iran experience a significant increase wind speed and meridional wind component during 30 hours before heavy rain IV. CONCLUSION The Mediterranean cyclone situation accompanied with low-level jet, humidity flux convergence and potential temperature was analyzed on 08 and 09 Dec 1991 in this study. Before the occurrence rain, Synoptic analysis revealed low pressure with central pressure 1010 hpa was located east of Mediterranean and south of Turkey while pressure trough associated with low pressure was extended toward Iraq and Iran that has leaded to increase pressure gradient in Although in some cases LLJ cross cyclone toward center with southerly wind[7], In this study maximum low-level wind(llj) more than 20 ms is situated along south of cyclone and north Africa coast. By 00/09 Dec, southerly wind ms coincide with trough pressure has formed along Saudi Arabia as a result meridional wind component enhancement up to 14 ms in exit of low-level jet. The Saudi Arabia and Sudan warm air advection associated with southerly wind northward in the lower troposphere appear to help trigger the rain fall. These findings are consistent with earlier studied e.g. [8][6][10][112]. It appears, LLJ contributes to form southerly wind and heavy rain on 09 Dec and to provide more favorable environment for the heavy rain by enhancing warm air advection and moisture transports in the lower troposphere from Red sea, south of Arabia and Aden gulf to The southerly wind transports humid air from Red sea and Aden sea to northward that as wind southerly speed was increased, humidity enhanced near south of Iraq and west of Iran, therefore source of humidity for heavy rain is Red sea and Aden sea surface in 850 level. The humidity flux convergence demonstrates enhancing air humidity in vertical direction from surface to 700 hpa. The six hours before rain, maximum convergence flux increases to 8 10 gr kg s in 700 hpa level and mixing ratio to 8 gr kg, in 850 hpa in By 00/09 Dec, forming and increasing southerly wind speed across Saudi Arabia can be created and intensified warm and moist air to northward as the shown by previous studied by [8][9]]10][11]. REFERENCES [1] M.R. Sinclair, A Diagnostic Study of the Extratropical Precipitation Resulting from Tropical Cyclone Bola, Mon. Wea. Rev,Vol. 121,P ,1993. [2] R.A. Maddox, and C.Doswell, An Examination of jetstream Configurations,500 mb Vorticity Advection and Low-level Thermal Advaction Patterns During Extended Periods of Intense Convection, Mon. Wea. Rev,Vol. 110,P , [3] K.Ma, and L.Bosart,, A Synoptic Overview of a Heavy Rain Event in Southern China,Wea & For,Vol. 2,89-112, [4] T. Akiyama, Ageostrophic Low-Level Jet Stream in the Baiu Season associated with Heavy Rainfalls Over the Sea Area, J. Meteor.Soc. Jap,Vol. 51, P ,1973. [5] K. Ninomiya, and T.Akiyama, Band Structure of Mesoscale echo clusters associated with Low-Level Jet Stream, J. Meteor. Soc.Japa,Vol. 52, P , V1-165

4 [6] K.A. Browning,and W.Pardoe, Structure of Low-Level Streams ahead of Mid-Latitude Cold Fronts, Quart.J.R.Met.Soc,Vol. 99,P , [7] G.M.Lackmann., Cols-FrontalPotential Vorticty Maxim,the Low- Level Jet,and Moisture Transport in Extratropical Cyclones, Mon. Wea. Rev,Vol. 130, P.59-74, [8] L.W.Uccellini,P.Kocin.,R.A.Petersen.,C.H.Wash., & K.F.Bril. The Presidents Day Cyclone of Februray 1979:Synoptic Overview and Analysis of the Subtropical Jet Streak Influencing the Pre- Cyclogenetic Period, Mon. Wea. Rev,Vol. 112,P.31-55,1984. [9] L.W.Uccellini,and L.D.Johanson., The Coupling og Upper and Lower Tropospheric Jet Streaks and Implications for the Development of Severe Convective Storms,Mon. Wea. Rev,Vol. 107, P , [10] Y-L.Chen, X.A.Chen.,S.Chen. and Kuo.Y-H., A Numerical Low- Level Jet during TAMEX IOP 5,Mon. Wea. Rev,Vol. 125, P , [11] M.J. Mitchell, R.W.Arritt., and K.Labas., A Climatology of the Warm Season Great Plains Low-Level Jet Using Wind Profiler Observations.Wea.& For,Vol. 10, P ,1995. [12] K.F.Brill,L.W.Uccellini,R.P.Burkhart.,T.T.Warner.&R.A.Anthes, Nu merical Simulations of a Transverse Indirecte Corculation and Low- Level Jet in the Exit Region of an Upper-Level Jet, Jou. Atmos.Sci, Vol. 42, P , [13] G.T-J.Chen.,C-C.Wang.,&D.T-W.Lin., Characteristics of Low-Level Jets over Northern Taiwan in Mei-Yu Season and Their Relationship to Heavy Events,Mon.Wea.Rev,Vol. 133., P.20-42, Figure 1. Total rain (mm) on 09 Dec1991 in western Iran TABLE I. TOTAL RAIN(MM) IN 09 DEC 1991, IN RAIN GAGES STATION Station Longitude Latitude Total rain 09(DEC) Khoy Tabriz Oroomieh Mahabad Piranshahr Takab Saghez Sardashat Khoramabad V1-166

in 850 hpa and (thick line) convective rain( cm );d)(hatched) humidity flux convergence( 10 gr kg s ) in 700 hpa and mixing ratio in 850 hpa ( gr kg ) and wind vector in 850 hpa; 00(UTC) 08 Dec 1991.

5 Figure 2. a)mean sea level pressure(hpa) and wind vector in 850 hpa ; b)(thin line )wind speed and (thick line) meridional wind speed in 850 level( ms ); 8 c)(thin line) equivalent potential temperature( k ) in 850 hpa and (thick line) convective rain( cm );d)(hatched) humidity flux convergence( 10 gr kg s ) in 700 hpa and mixing ratio in 850 hpa ( gr kg ) and wind vector in 850 hpa; 00(UTC) 08 Dec Figure 3. a)mean sea level pressure(hpa) and wind vector in 850 hpa ; b)(thin line )wind speed and (thick line) meridional wind speed in 850 level( ms ); 8 c)(thin line) equivalent potential temperature( k ) in 850 hpa and (thick line) convective rain( cm );d)(hatched) humidity flux convergence( 10 gr kg s ) in 700 hpa and mixing ratio in 850 hpa ( gr kg ) and wind vector in 850 hpa; 00(UTC) 09 Dec V1-167

This is a refereed journal and all articles are professionally screened and reviewed

Advances in Environmental Biology, 5(10): 3425-3433, 2011 ISSN 1995-0756 3425 This is a refereed journal and all articles are professionally screened and reviewed ORIGINAL ARTICLE The Divergence Field