Rain Fall Quants Study

7/27/2019 Rain Fall Quants Study

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Salient Features during June 2011

1. Introduction

Normally monsoon advances over most regions of country in the month of June

except some parts of northwest India. There is large variability in the dates of advance

of monsoon over different regions and also the quantum and distribution of rainfall

which has a direct bearing on the sowing operations of kharif crops over many regions

of India.

The Indian Summer Monsoon is characterized by large spatio-temporal variability on

various scales. The pattern of variability in every year is unique. The South West

Monsoon season extends through June to September. The normal date of onset of SW

monsoon over Kerala is 1st

June. The rainfall during June accounts for 19% of the

seasonal rainfall. In this report some of the characteristic features displayed by the

southwest monsoon 2011 in the month of June are analysed and presented in the

following sections. This includes the observational aspects, anomalies in the circulation

field and synoptic systems.

2. Onset and Advance

2.1 Monsoo n onset over K erala

The arrival of southwest monsoon current over the south Bay of Bengal and south

Andaman Sea was delayed by about 10 days, due to non-conducive conditions for the

development of convection over the region. The cross equatorial flow over the Arabian

Sea, as evidenced from the speed and depth of the southwesterlies showed a rapid

strengthening from 28th

May, when the average speed became 20 knots at 925 hPa

over the area bounded by Latitudes 5-10N and Longitudes 70-80E and the depth of

westerly wind extended up to 600 hPa. The Kalpana-1 derived mean OLR values

remained below 200 W m-2

over the region confined by Latitudes 5-10N and

Longitudes 70-75E on 28th

and 29th

May (Fig.1). In association with these


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developments, the rainfall activity over Kerala increased. Widespread rainfall activity

occurred and 84% and 79 % of the rainfall monitoring stations reported more than 2.5

mm rainfall respectively on 28th and 29th May (Fig.2). Thus, there has been a sharp rise,

both in terms of spatial distribution as well as the quantum of rainfall reported by the

stations spread over Kerala, Lakshadweep and south coastal Karnataka on 28 th and

29th May. Thus, all the objective criteria specified for the Monsoon Onset over Kerala

were satisfied on 29th May and consequently onset of monsoon over Kerala was

declared on 29th May, 2011. Also, the southwest monsoon set in over most parts of

South Arabian Sea, some parts of Tamil Nadu, south Bay of Bengal and South

Andaman Sea on 29th

May 2011.

Though the arrival of monsoon was delayed by about 10 days with respect to the

normal date of 20th

May over the Andaman Sea, it was earlier than the normal date [1st

June] over Kerala by 3 days.

Fig. 1


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Fig. 2

2.2 Subsequent Advance over the country

Due to strengthening of the cross-equatorial flow over Arabian Sea and the

northward movement of a vortex in the form of an upper air cyclonic circulation along

the trough off the west coast, the monsoon further advanced rapidly and covered entire

Kerala, Tamil Nadu and Goa, most parts of Karnataka and some parts of south Andhra

Pradesh by 5th

June. However, the transformation of this vortex into a low pressure

area, its further intensification and stagnation over the east central Arabian Sea during

the period from 6th 10

thJune, caused a short lived hiatus in the advance of the

monsoon along the west coast. In the mean time, the convection over the Bay of Bengal

picked up and with the formation of a low pressure area of short life span over the

northwest Bay of Bengal, the eastern branch of monsoon advanced over some more

parts of Bay of Bengal and northeastern states, with a delay of nearly 5 days.

The well marked low pressure area over the east central Arabian Sea,

subsequent to its northward movement on 11th, intensified into a Depression on 12th and


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caused heavy to very heavy rainfall over coastal Saurashtra on 12th

June. Also, another

cyclonic circulation formed over the North Bay of Bengal resulting in enhanced

convection over the central parts of Bay of Bengal on 13 th. As a consequence, the

monsoon advanced into some more parts of Arabian Sea, parts of Saurashtra and most

parts of the Bay of Bengal and parts of coastal Andhra Pradesh and coastal Orissa on

13th June.

Subsequently, there had been a rather steady advance during 15 th 26th June in

association with formation of a Deep Depression over the northwest Bay of Bengal and

its gradual west-northwestward movement. Though this synoptic situation caused the

monsoon to cover most parts of the country outside western parts of Rajasthan and

north Gujarat state, the weakening of the Arabian Sea branch of monsoon caused

prolonged stagnation of the western limb in the Northern Limit of Monsoon (NLM).The

NLM continued to pass through Lat. 22oN/ Long.60

oE, Lat. 22

oN/ Long.65

oE, Porbandar,

Ahmadabad, Udaipur, Ajmer, Pilani and Ganganagar from 26th

- till date.

Fig.3 shows the isochrones of advance of monsoon during June 2011.

Fig.3


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The north ward propagation of convection zone from equator to 200N during the

period of 28th May to 10th June, in association of advance of monsoon, is evident from

Fig. 4.

Fig. 4

3. Major Synoptic scale systems

3.1 Depression over th e Arabian Sea (11th 12thJune)

A cyclonic circulation lay over the southeast Arabian Sea and neighbourhood off

Kerala coast on 1st June, over the southeast and adjoining east central Arabian Sea off

south Karnataka-Kerala coasts on 2nd and over the east central Arabian Sea off

Maharashtra coast on 4th & 5th. Under its influence, a low pressure area formed over the

east central Arabian Sea off north Maharashtra coast on 6th. It persisted there on 7th and

lay as a well marked low pressure area over the east central Arabian Sea and

neighbourhood on 8th

& 9th.It lay over the east central Arabian Sea off north

Maharashtra coast on 10th.

On 11th

morning, it was seen over the east central Arabian

Sea off north Maharashtra south Gujarat coasts. Subsequently, it concentrated into a

Depression and lay centred over the northeast Arabian Sea, near


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Lat.20.0N/Long.71.5E (about 180 km northwest of Mumbai) at 1730 hrs IST of 11th

. It

moved north northwestwards and crossed south Gujarat coast, nearly 25 km to the east

of Diu, in the early morning of 12 th and lay centred over Saurashtra, about 70 km south

southwest of Amreli at 0830 hrs. IST of 12 th. Gradually moving northwestwards, it

weakened into a well marked low pressure area over Saurashtra and adjoining

northeast Arabian Sea by 12th evening. It further weakened into a low pressure area

over the same region on 13th and became less marked on 14th. However, the

associated upper air cyclonic circulation extending up to 4.5 km.a.s.l. lay over

Saurashtra & Kutch and neighbourhood on 14 th and became less marked on 15th.

3.2 Deep Depression ov er the northw est Bay of B engal (16th 22ndJune)

A cyclonic circulation extending up to mid-tropospheric levels lay over the north Bay

of Bengal on 13th. Under its influence, a low pressure area formed over the northwest

Bay of Bengal and neighbourhood on 14th. It lay as a well marked low pressure area

over the same region on 15th. It concentrated into a Depression and lay centred at 0830

hrs.IST of 16th, over the northwest Bay of Bengal, near Lat.21.5N / Long.89.0E (about

150 km. southeast of Kolkata). It moved north-northwestwards, intensified into a Deep

Depression and crossed west Bengal coast, about 100 km. to the east of Sagar Islands,

between 1630 & 1730 hrs.IST of 16 th and lay over Gangetic West Bengal and adjoining

Bangladesh, near Lat.22.0N / Long.89.0E (about 100 km. southeast of Kolkata) at

1730 hrs. IST of 16th. Moving slightly northwards, it lay centred near Lat.22.5N /

Long.89.0E (about 80 km. east of Kolkata) at 0830 hrs IST of 17 th.Subsequently

moving westwards, it lay over Gangetic West Bengal, centred near Lat.23.0N /

Long.88.0E (close to Burdwan) at 1730 hrs. IST of 17th. Further moving westwards, it

lay over Gangetic West Bengal and adjoining areas of Jharkhand, centered near

Lat.23.0N / Long.87.0E, about 25 km. south of Bankura at 0830 hrs IST of 18 th. It

remained practically stationary over the same region at 1730 hrs IST of 18th

. Thereafter,

moving westwards, it weakened into a Depression and lay centred at 0530 hrs IST of

19th, near Lat.23.0N / Long.85.5E, close to and to the east of Ranchi. It further moved

northwards and lay over Jharkhand and adjoining Gangetic West Bengal, centred near


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Lat.23.5N / Long.85.5E (about 50 km. southeast of Ranchi) at 0830 hrs IST of 19th.

Moving slightly westwards, it lay centred near Lat.23.5N / Long.85.0E (about 25 km.

northnorthwest of Ranchi) at 1730 hrs IST of 19 th. It further moved west-northwestwards

and lay over Chattisgarh and adjoining areas of Jharkhand, about 50 km northeast of

Ambikapur at 0830 hrs IST of 20th. Moving slightly northwestwards, it lay centred at

1730 hrs IST of 20th, over southeast Uttar Pradesh and neighbourhood, about 150 km

south of Varanasi. Subsequently moving west-northwestwards, it lay centred at 0830

hrs IST of 21st, over east Madhya Pradesh and adjoining south Uttar Pradesh, about

100 km. east of Rewa. Then, it moved westwards and lay centered at 1730 hrs IST of

21st, over the same area close to Satna. Continuing the westward movement, it lay

centred at 0830 hrs IST of 22nd

, over east Madhya Pradesh, close to and to the west of

Panna and over the central parts of Madhya Pradesh and adjoining south Uttar

Pradesh, about 50 km. east of Sagar at 1730 hrs IST. It subsequently moved

northwestwards and weakened into a well marked low pressure area over west Madhya

Pradesh and neighbourhood in the early morning of 23rd

.It lay over northwest Madhya

Pradesh and adjoining areas of east Rajasthan and southwest Uttar Pradesh on 23rd

morning and lay as a low pressure area over the same region in the evening. It became

less marked on 24th.However, the associated cyclonic circulation extending up to mid-

tropospheric levels lay over central parts of Uttar Pradesh and adjoining north Madhya

Pradesh, tilting southwestwards with height on 24 th. It lay over east Uttar Pradesh and

neighbourhood on 25th, over south Uttar Pradesh and adjoining north Madhya Pradesh

on 26th & 27th and over east Uttar Pradesh and adjoining Jharkhand and north

Chattisgarh on 28th.

Under the influence of the above cyclonic circulation, once again a low pressure

area formed over east Uttar Pradesh and adjoining areas of north Madhya Pradesh on

29th

. It persisted over east Uttar Pradesh and neighbourhood on 30th

June and became

less marked thereafter.

The tracks of these two low pressure systems are shown in Fig. 5.


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Fig. 5

The anomalous circulation features for the month of June at 850 hPa level are

shown in Fig. 6.

Fig. 6


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The monthly mean Outgoing Long wave Radiation (OLR) for the month of June 2011

(Fig. 7) shows suppressed convection over western, northwestern and northern regions

of India,

Fig. 7

4. Rainfall characteristics

4.1 Day to Day Rainfall (All India and four homogeneous geographical regions)

Fig. 8 depicts the daily rainfall reported by the plain stations over the country as a

whole. It may be seen that during the month of June, the daily rainfall had been below

the average value on only a few periods. Fig. 9 depicts the daily rainfall over the four

homogeneous geographical regions. The rainfall over east and north east India

remained below normal during many days of June, whereas the daily rainfall over south

peninsular India was above normal during the first fortnight of June and decreased

subsequently. The rainfall statistics for the month of June are given in Table 1.


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Fig.8

Fig. 9


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RegionsActual

Rainfall (mm)Normal

Rainfall (mm)% Departure

from LPA

Country as a whole 182.0 163.5 11

Northwest India 119.1 69.4 72

Central India 187.0 163.8 14

South Peninsula 157.6 158.8 -01

East & northeast India 323.1 350.0 -08

Table 1

4.2 Week by Week Rainfall

Fig. 10 depicts the week by week progress of monsoon over the meteorological sub-

divisions during the month of June and the cumulative value of percentage departures

of rainfall in terms of excess, normal, deficient and scanty categories. The subdivisions

over southern peninsular India and north India reported excess or normal rainfall during

all the weeks of June, whereas the regions of Gujarat and northeast India reported

deficient/scanty weekly rainfall all through the month of June.

Fig. 10


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4.3 Meteorological Sub-Divisionwise Rainfall

Fig. 11 is the meteorological sub-division wise area weighted percentage departure

of rainfall for the period 1st 30th June. The rainfall is excess or normal over India

except over Gujarat state, south east peninsular India and parts of north east India.

Fig. 11


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4.4 June vs seasonal rainfall

Table 2 clearly shows that there is no 1:1 correspondence between June and

seasonal rainfall.

YEAR JUN JUL AUG SEP JUN-SEP

JUNERAINFALL

%O

FNOR

(>=100TO

=105TO=110TO

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