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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