Download Low-level Jet(LLJ) and western Iran heavy rain: case study Dec... Elham Mobarak Hassan Amir Hossein Meshkatee

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
Amir Hossein Meshkatee
Faculty of Agriculture & natural resources,
Islamic Azad university ,Ahvaz branch, Iran
E-mail : [email protected]
Faculty of Basic Science, Science and Research,
Islamic Azad university,Tehran,Iran,
E-mail : [email protected]
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 −1 ,in 850 hPa, is located south to southwest of cyclone along
North Africa coast. Prior to occurrence of rain, southerly wind
12-16 ms −1 has formed over Saudi Arabia resulted by
meridional wind component ( v ) enhanced to 14 ms −1 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 southwestern Iran. 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 western Iran. The
maximum humidity flux convergence increased to
−8 × 10 −8 grkg −1 s −1 in 700 hPa level and also mixing ratio to
8 grkg −1 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 1991.
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 30- 35 ms in the 800-850 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 08-09 Dec 1991. The
data grid resolution is 2.5×2.5 and referred to the mean sea
level and isobaric levels of 1000, 925, 850, 700, 500, 400,
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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
western Iran.
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 western Iran.
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 36.15 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 −1 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 −1 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 −1 value that is
formed by Maximum meridional wind component ( v ) with
8 ms −1 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 −1 contour pass
Turkey and eastern Mediterranean sea and second extending
tongue to Persian gulf and south east of Iran with
6 gr kg −1 (Fig.2d) at 00/08 Dec.
Maximum humidity flux convergence with -5
− 5 × 10 −8 gr kg −1s −1 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 −1
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.
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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 −1 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 −1
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 southwestern Iran. 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 −1 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 western Iran. 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 −8 gr kg −1s −1 (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 −1 (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.
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
western Iran.
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 −1 is situated along south of
cyclone and north Africa coast.
By 00/09 Dec, southerly wind 12-14 ms −1 coincide with
trough pressure has formed along Saudi Arabia as a result
meridional wind component enhancement up to 14 ms −1 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
western Iran.
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 −8 gr kg −1s −1 in 700 hPa level and
mixing ratio to 8 gr kg −1 , in 850 hpa in western Iran.
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].
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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.
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40
39
38
37
36
35
34
33
32
31
30
44 45 46 47 48 49 50 51 52 53 54 55
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
44.96
46.28
45.08
45.71
45.13
47.11
46.26
45.5
48.28
38.55
38.08
37.53
36.76
36.66
36.38
36.25
36.15
33.43
28
24
60
44
74
34
68
98
34
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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 −1 );
c)(thin line) equivalent potential temperature( k ) in 850 hPa and (thick line) convective rain( cm );d)(hatched) humidity flux convergence( 10 −8 × gr kg −1s −1 )
in 700 hPa and mixing ratio in 850 hPa ( gr kg −1 ) and wind vector in 850 hPa; 00(UTC) 08 Dec 1991.
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 −1 );
c)(thin line) equivalent potential temperature( k ) in 850 hPa and (thick line) convective rain( cm );d)(hatched) humidity flux convergence( 10 −8 × gr kg −1s −1 )
in 700 hPa and mixing ratio in 850 hPa ( gr kg −1 ) and wind vector in 850 hPa; 00(UTC) 09 Dec 1991.
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