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Role of Low Level Jetstream in Intense Monsoon Rainfall episodes over the
West Coast of India
Dr. P.V. JosephProfessor Emeritus
Department of Atmospheric SciencesCochin University of Science and Technology
Fine Arts Avenue, Kochi-682 016e-mail: porathur@vsnl.com
Topics presented• Low Level Jetstream during SW monsoon
• Factors associated with Heavy Monsoon
Rainfall along the west coast of India
a. Vorticity of Low Level Jet Stream
b.Vertically Integrated Moisture
c. Convective cloud formation
• A hypothesis for intense monsoon rainfall
episodes
Horizontal and Vertical Profiles of Wind in LLJ on 9 July 1961 (Joseph and Raman,1966)
Rainfall (OLR) and wind at 1.5km (850hPa) at MOK(Joseph and Sijikumar,Journal of Climate, 2004)
Rainfall (OLR) and wind at 1.5 km in Active Monsoon(Joseph and Sijikumar,Journal of Climate, 2004)
Rainfall (OLR) and wind at 1.5 km in Break Monsoon(Joseph and Sijikumar,Journal of Climate, 2004)
A recent study by Francis and Gadgil for the planning of ARMEX has shown the following, analysing dataof 1951-1987
a. Heavy Monsoon Rainfall (HMR) of more than 15cm
per day occur along west coast during monsoon
causing considerable damage
b. Such HMR events have maximum frequency near
19N and between 14N and 16N latitudes
c. Most favorable time for HMR events is between 15
June and 15 August
d. 62% of these events are associated with systems
organised on the synoptic and larger scales (TCZ)
.Mumbai19.0N,72.9E.Honavar
14.2N,74.4E
Cases of rainfall at Honavar of 20cms per day or more during monsoon seasons of 1975 - 1989
43.2207 June 1989
46.6313 June 1988
22.8705 August 1986
30.3816 June 1986
21.0026 June 1985
25.9203 August 1982
21.2723 June 1982
20.6415 July 1979
29.4419 July 1977
21.4217 July 1977
25.6616 June 1977
22.1208 June 1977
Rainfall (cms) during previous 24 hours
Observation at 0830 hrs of
Composite 850hPa wind for 12 cases of monsoon rain at Honavar (1975-1989) of 20cms per day and more.
Red lines are isotachs of wind speed in m/s at 1m/s interval
.
Composite of OLR for 12 cases of monsoon rain at Honavar (1975-1989) of 20cms per day and more.
Honavar
Cases of rainfall at Colaba, Mumbai of 20cms per day or more during monsoon seasons of 1975 - 1990
25.9015 August 1990
42.1216 June 1990
30.9325 June 1985
34.5517 June 1985
54.4302 July 1984
24.1623 September 1981
20.6201 August 1979
41.7231 July 1975
Rainfall (cms) during previous 24 hours
Observation at 0830 hrs of
Cases of rainfall at Santacruz, Mumbai of 20cms per day or more during monsoon seasons of 1975 - 1990
21.5213 September 1984
22.3617 June 1985
24.0102 July 1984
25.3417 July 1983
20.8521 July 1982
27.5619 July 1982
31.8223 September 1981
26.4705 August 1976
22.3409 July 1975
Rainfall (cms) during previous 24 hours
Observation at 0830 hrs of
Frequency of Heavy Monsoon Rain at Colaba and Santacruz 1950-1990
NIL≥ 5002≥ 50
NIL≥ 4004≥ 40
06≥ 3006≥ 30
28≥ 2019≥ 20
72≥ 1552≥ 15
Santacruz
in cms
Colaba
in cms
Composite 850hPa wind for 8 cases of monsoon rain at Colaba,Mumbai of (1975-1990) 20cms per day and more.
Red lines are isotachs of wind speed in m/s at 1m/s interval
.
Composite of OLR for 8 cases of monsoon rain at Colaba,Mumbai (1975-1990) of 20cms per day and more.
Mumbai
.
Composite 850hPa wind for 9 cases of monsoon rain at Santacruz,Mumbai of (1975-1990) 20cms per day and more.
Composite of OLR for 9 cases of monsoon rain at Santacruz,Mumbai (1975-1990) of 20cms per day and more.
Mumbai
Case Studies for Heavy Rainfall at Honavar, Mumbai & S. Gujarat
Heavy rain recorded at 03 GMT (0830 IST)
Honavar 07 June ‘89 …43.2cmColaba 02 July ‘84 ….54.4cmSantacruz 27 July ’05 …94.4cmColaba 01 Aug ’79 …20.6cmS. Gujarat 27 June ’02…Pardi 61cm
Valsad 54 cm
850 hPa wind and LLJ axis on 06 June 1989Case of Honavar rainfall – 43.22cm
.
Zonal wind shear (m/s per 500km) at 850hPa - Honavarfrom 29May to 12 June 1989. Heavy rain on 6 June.
-(δu/δy)
Vorticity (in box around Honavar) at 850hPa from 25May to 15 June 1989. Heavy rain on 6 June.
(δv/δx - δu/δy)
Vertically Integrated Moisture (kg/m2) - Honavarfrom 29May to 12 June 1989. Heavy rain on 6 June.
Vertically Integrated Moisture (kg/m2) on 2 June 1989(isolines more than 40kg/m2 at 2kg/m2 intervals)
.
Vertically Integrated Moisture (kg/m2) on 4 June 1989(isolines more than 40kg/m2 at 2kg/m2 intervals)
.
Vertically Integrated Moisture (kg/m2) on 6 June 1989(isolines more than 40kg/m2 at 2kg/m2 intervals)
.
.Colaba
.Santacruz
24 hour Rainfall recorded at 0830IST
on 02 July 1984Colaba - 54cmSantacruz - 24cm
24 hour Rainfall recorded at 0830IST
on 27 July 2005Colaba - 7cmSantacruz - 94cm
850 hPa wind and LLJ axis on 01 July 1984. Case of Colaba rainfall – 54.43cm
.
850 hPa wind and LLJ axison 29 June 1984
.
Zonal wind shear (m/s per 500km) at 850hPa - Mumbaifrom 20June to 10July 1984. Heavy rain on 1 July.
-(δu/δy)
Vorticity (in box around Mumbai) at 850hPa from 20June to 10 July 1984. Heavy rain on 1 July.
(δv/δx - δu/δy)
Vertically Integrated Moisture (kg/m2) - Mumbai from 20June to 10 July 1984. Heavy rain on 1 July.
Vertically Integrated Moisture (kg/m2) on 1 July 1984(isolines more than 40kg/m2 at 2kg/m2 intervals)
.
850 hPa wind and LLJ axis on 26 July 2005. Case of Santacruz rainfall – 94.4cm
.
850 hPa wind and LLJ axis on 24 July 2005.
.
Zonal wind shear (m/s per 500km) at 850hPa - Mumbai,from 18July to 03August 2005. Heavy rain on 26 July.
-(δu/δy)
Vorticity (in box around Mumbai) at 850hPa from 15July to 05 August 2005. Heavy rain on 26 July.
(δv/δx - δu/δy)
Vertically Integrated Moisture (kg/m2) - Mumbai from 18July to 03 August 2005. Heavy rain on 26 July.
.
850 hPa wind and LLJ axis on 31 July 1979. Case of Colaba rainfall– 20.62cm
Zonal wind shear (m/s per 500km) at 850hPa - Mumbai, from 20July to 10August 1979. Heavy rain on 31 July
-(δu/δy)
Vorticity (in box around Mumbai) at 850hPa from 20July to 10 August 1979. Heavy rain on 31 July.
(δv/δx - δu/δy)
Vertically Integrated Moisture (kg/m2) - Mumbai from 20July to 10 August 1979. Heavy rain on 31 July.
.Colaba
.Santacruz
Nowcasting and warning for very heavy rain
• Step 1- Monitor growth of850hPa zonal wind shear andvertically integrated moisturein a 500km x 500km boxaround Mumbai.
• Step 2 – When aboveparameters reach criticalvalues begin radar watch forthunderstorm development and operate a mesonet for hourlyrainfall and surface wind. Atypical grid for mesonet ofabout 50 stations is marked on the map of Mumbai.
850 hPa wind and LLJ axis on 26 June 2002. Case of S.Gujarat rainfall – 61cm and 54cm
Zonal wind shear (m/s per 500km) at 850hPa – S.Gujarat,from 18June to 03July 2002. Heavy rain on 26 June.
-(δu/δy)
Vertically Integrated Moisture (kg/m2) – S. Gujarat from 18June to 3 July 2002. Heavy rain on 26 June.
A Hypothesis for the Heavy Rainfall alongWest Coast occurring in Mesoscale Systems• There is intense cyclonic vorticity north of the LLJ axis(850hPa)• The wind field below 850hPa is much weaker • When cyclonic shear vorticity in the frictional boundarylayer associated with the LLJ at a coastal stationincreases, cumulus / cumulonimbus convectionincreases
• Increased convection leads to pumping of moisture into the troposphere • Thus as the shear vorticity at a station increases theVertically Integrated Moisture in the atmosphere alsoincreases and the rainfall potential increases
Hypothesis contd…..
• With shear and moisture increased & withfavourable CAPE, large thunderstorms form withstrong vertical upward velocities inside • Around each thunderstorm there will be anenvironment with vertical downward motion which suppresses thunderstorm formation there. Eachthunderstorm is a mesoscale system• The downward motion field aroundthunderstorms (in an area about 9 times the size of the thunderstorm area) will transport the intensecyclonic vorticity of the LLJ at 850hPa to lowerlevels in the boundary layer
Hypothesis contd…..
• Increased cyclonic vorticity at levels like950 and 900 hPa with greater frictionaleffects will lead to increased vertical motion in the atmosphere leading to more intenseconvective storms • The increased cyclonic vorticity at levelsbelow 850hPa where the existing wind fieldis weak can lead to formation of mesoscalecyclonic vortices as suggested in literatureand for which ARMEX was conducted
Ground Level
850hPa Level(Large Vorticity)
Mechanism of Vorticity transfer to 900 and 950hPa from 850hPa
Vertical mass flux (estimated) inside cloud (Mc) and in theenvironment outside cloud (M) in cloud clusters over Marshallislands (Yanai et al 1973). M is the large scale mass flux. Note the large vertically downward flux outside cloud at low levels .
∼-
From book Storm and Cloud Dynamics by Cotton and Anthes p 215.
At CUSAT Sijikumar in his PhD thesis looked at this problem. He performed MM5 simulations for heavy rainfall events at Honavar (latitude 14.2N) with initial conditions 48 hours before the reported heavy rainfall events.
He used :Cumulus Parameterization : Betts MillerPBL scheme : Mellor Yamada EtaMoisture scheme : Simple ice (Dudhia)
The two-way nested domains (three) used in the experiment are shown below. Grid sizes 10km, 30km and 90km.
For the case of heavy rainfall of 43cm at Honavar at 03z of 07 June 1989 withfull orography of the Western Ghats andinitial data of 00z 05 June 1989, amesoscale vortex formed at 950hPa by18z of 05 June away from coast. Itmoved towards Honavar on the secondday (06 June 1989).
850 hPa wind and LLJ axis on 05 June 1989. Case of Honavar rainfall of 6June – 43.22cm
.Honavar
Simulated 950hPa wind after (a)18 hrs to (f)48 hrs at 6 hour intervals(with Western Ghats orography in inner most domain)
Simulated 950hPa wind after (a)18 hrs to (f)48 hrs at 6 hour interval(with no Western Ghats orography in inner most domain)
It is found that the mesoscale vortex is only slightly weaker in the no-orography caseshowing that the most important factor isthe shear vorticity of the LLJ at 850hPa.The experiment was repeated for a fewmore cases at Honavar. Similar mesoscalevortices were obtained. But similar experiments performed forMumbai heavy rain yielded no mesoscalevortices
At CUSAT we are now trying to seewhether LLJ vorticity is transferredto levels below 850hPa in cases ofstrong convection in the field of LLJusing MM5 and the recently acquired WRF model
Squally weather during monsoon along the west coast
Squall : A sudden increase of wind speed by at least three stages on Beaufort’s scale,reaching at least 22 knots and lasting atleast for one minute
In a study of monsoon squalls at Goa ,Dayakrishnan et al., 1977 found that on average 105 squalls occur during amonsoon season (extreme values : 139squalls in 1971 and 74 squalls in 1969). At times more than 10 squalls occur in a single day. No well marked changes insurface temperature and pressureaccompany these monsoon squalls.
Monsoon squalls at Goa on 2 and 3 June 1971(Dayakrishnan et al., 1977)
Monsoon onset over Kerala in 1971: 27 May
.
850 hPa wind and LLJ axis on 31 June 1971
850 hPa wind and LLJ axis on 02 June 1971
.
.
850 hPa wind and LLJ axis on 03 June 1971
Monsoon squalls at Mangalore on 22 July 1989
Monsoon squalls at Mangalore on 23 July 1989
850 hPa wind and LLJ axis on 23 July 1989
.
OLR (Watts/m2) of 23 July 1989
.
Dynes PT Anemogram at Mangalore on 10 July 1989
A mechanism proposed for monsoon squalls :
The cyclonic shear north of the low level jetaxis produces intense convective clouds andrainfall in a 3-4 degree latitude belt just north of the LLJ axis. The downdrafts of theconvective clouds is hypothesised to carry the high momentum air at 850hPa level close tothe LLJ axis to the surface level causing themonsoon rain squalls.
THREE STAGES OF AN ORDINARY THUNDERSTORM
Horizontal and Vertical Profiles of Wind in LLJ on 9 July 1961 (Joseph and Raman,1966)
VERTICAL SECTION OF A SEVERE THUNDERSTORM
0 50 100 150 200 250
1.5
3
4.5
6
7.5
9
10.5
12
13.5
15H
EIG
HT
IN
KM
WIND SPEED IN KM PER HOUR
TYPICAL VERTICAL WIND PROFILE IN JETSTREAM
} VERTICALWIND SHEAR
Severe thunderstorms of north India during pre-monsoon season grow in an environment of strong vertical wind shear between surface and 500hPa.The dry air at levels 600 to 500hPa enter the storm and gets cooled by evaporation of rain falling into it which causes intense downdraft and surfacewind squalls of 50 to 100 knots. Below thesethunderstorms we get large variations in surfacetemperature and pressure. These storms are verymuch different from the monsoon convectivestorms.
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