JOURNAL OF CLIMATOLOGY, VOL. 8. 283 -296 (1988) 55 1.553.21:55 1.577.32(548)

THE ONSET OF THE SOUTHWEST MONSOON OVER KERALA: 190 1-1980

R . ANANTHAKRISHNAN A N D M. K. S O M A N

Indian lnslirute of Tropical Meleorology. Pune 41 1005, India

Receioed 6 A U ~ U S I 1987 Rc,r>ised 28 OcIoher 1987

ABSTRACT

Utilising daily mean rainfall from dense rain gauge networks, the dates of onset of the southwest monsoon over south and north Kerala have been derived on the basis of objective criteria for the years 1901 to 1980. These dates have been compared with the onset dates as per records of the India Meteorological Department. Statistics of the onset dates are presented. The mean onset date for south Kerala is found to be 30 May and for north Kerala 1 June with a standard deviation of about 9 days in both cases. The sharp increase in rainfall that heralds the onset of the monsoon is highlighted by superposed epoch analysis. The prevailing notion that rainfall from pre-monsoon thunderstorms progressively increases and merges with thc monsoon rainfall is shown to be not valid.

KEYWORIIS Southwest monsoon Onset dates Kerala.

1. INTRODUCTION

The date of onset of the southwest monsoon over Kerala is a crucial date in the meteorological calendar of India as it marks the beginning of the rainy season for the country. Although the onset of the monsoon is associated with changes in the circulation features in the lower and upper troposphere, a sustained increase in rainfall at the observatory stations over Kerala and the island stations over the southeast Arabian Sea is an essential feature of the monsoon onset. I t is difficult to quantify this precisely and so the experience of the forecaster plays a key role in declaring the date of monsoon onset in individual years.

Some two decades ago a critical study was undertaken to examine the monsoon onset dates as per records of the India Meteorological Department (IMD) from 1901 onwards in relation to the daily rainfall from 1 May to 30 June at seven observatory stations over Kerala and the adjoining sea area (Ananthakrishnan et al., 1967). Based on this study some broad criteria were drawn up for operational use. It should be noted that in general, the forecaster works with a measure of caution. The post-mortem study of the onset dates in relation to daily rainfall distribution showed that in many years the onset dates should have been somewhat earlier than the declared dates; the discrepancies were appreciable in some years.

Apart from the small number of observatory stations, Kerala has a dense network of rain gauge stations which have been in existence from the end of the last century. It was considered to be of interest to examine the behaviour of mean daily rainfall of Kerala utilizing the data from this network in relation to the monsoon onset. The aim was to arrive at a uniform set of onset dates based on objective rainfall criteria since the existing dates of onset as per IMD records are based on subjective estimates by several forecasters over the years. In recent years there have been attempts to forecast the dates of monsoon onset using regression techniques based on past data (Kung and Sharif 1980,1982). For such studies it is desirable to have a uniform set of onset dates free from subjectivity.

2. MEAN RAINFALL SERIES AND OBJECTIVE CRITERIA FOR MONSOON ONSET

The rain gauge network over Kerala utilized in the present study is shown in Figure 1. Latitudinally the state of Kerala extends from 8" 15" to 12" 50". For the present study we have considered south Kerala and

01961748/88/030283- 14$07.00 0 1988 by the Royal Meteorological Society

284 R AKAKTHAKRISHNAN A K D M. K SOMAN

13

li

i i

Id

4

0

Figure 1. Rain gauge network over Kcrala

north Kerala separately, the dividing line being around 10 N. The division is based on the fact that the monsoon rainfall progresses from south to north along the west coast of India and also on the known difference in the rainfall characteristics between the southern and northern parts of the state. South Kerala has 44 rain gauge stations and north Kerala 31, more or less uniformly distributed over the respective regions.

Utiliing daily rainfall recorded at the rain gauge networks, daily rainfall series were constructed for south and north Kerala for the years 1901 to 1980 by averaging the rainfall at the individual stations. Examination of these rainfall series revealed alternating patterns of spells of light and heavy rain amounts of varying durations. Light rain spells are a feature of the pre-monsoon months which give place to heavy rain spells hcralding the onset of the southwest monsoon. The transition is sharp and abrupt.

January and February are the driest months in Kerala with very little rainfall. Thunderstorm activity sets in from March and progressively increases with the advance of the season. This activity is more pronounced over south Kerala. The long term (1901-1 980) normal daily rainfall of south Kerala for the months of March and April is 1.9 and 4.8 mm respectively; the corresponding amounts arc lcss than 1 mm for north Kerala. The area-averaged rainfall associated with pre-monsoon thunderstorm activity is found to be less than 10 mm per day. Averaged over individual spells whose durations are variable, the amounts are found to be between 2 and 7 mm for south Kerala and less for north Kerala. We designate these as light rain spells.

Thc long term normal daily rainfall for May and June as found from our study is 8.9 and 19.5 mm respectively for south Kerala and 7.3 and 24.0 mm for north Kerala. These are contributed by light rain spells

OKSF7' O F T H t SOIJTIIWEST MONSOOK 28 5

before the monsoon onset and heavy rain spells during and after the monsoon onset. The observational data showed that during such heavy spells the daily area-averaged rainfall exceeds 10 mm per day on most 0 1 the days while the rainfall averaged over the spell duration varies between 15 and 50 mm per day. In view of this, a limit of 10 mm per day was arbitrarily chosen as the demarcation between light and heavy rain spells.

Since we are dealing with the average rainfall from a large network of stations distributed over the regions, i t is obvious that the heavy spells arc associated with widespread moderate to heavy rainfall while light spells imply scattered rainfall of small amounts with perhaps isolated heavy falls. This distinction in the nature of the two types of rain spells is important. I t implies that the physical mechanisms associated with them are different. Heavy rain spells imply deep convection in association with synoptic scale systems indicating ITCZ activity; light spells imply sporadic convective activity associated with prc-monsoon thunderstorms or weak monsoon conditions after monsoon onset.

The date of monsoon onset is taken as the first day of the transition from light to heavy rain spell category with the proviso that the average daily rainfall during the first 5 days after the transition should not be less than 10 mm. In several years the heavy rain spell which heralds the onset of the monsoon continues for more than 5 days with daily rainfdl in excess of 10 mm on all days. There are, however, some years in which the daily rainfall on 1 or 2 days during the onset spell may fall below 10 mm, the average still remaining in cxccss of 10 mm per day. This is associated with the pulsatory nature of monsoon activity.

As an illustration, the daily rainfall sequences from 1 May to 30 June for south Kerala for the years 1917, 1918, 1979 and 1980 are shown in Figure 2 in which the monsoon onset dates fixed by the above-mentioned criteria are also indicated. The average daily rainfall for the first 5 days including the onset date in these years is 18.1, 19.1, 27.7 and 17.3 mm respectively. I t may be noted that in the first 3 years the daily rainfall was less than 1 0 nim on 1 day during the first 5 days following the onset.

The rainfall sequences associated with the monsoon onset over Kerala and the further northward advance of the monsoon vary from year to year. In some years the onset spell of heavy rains is followed by a prolonged spell of light rainfall with subsequent revival of rain activity. In such years it has been customary in the IMD to regard the first onset as a temporary phase and consider the revival date as the onset date. We have adopted the same convention in the present study.

Figures 3 and 4 illustrate the nature of the rainfall transition associated with the onset and advance of the monsoon over Kerala for two years, 1936 and 1951. In these diagrams, the distribution of rainfall over the station network is shown for 4 consecutive days starting from the date prior to the onset. The rainfall is plotted correct to 1 mm over the station locations shown in Figure I . Stations with zero rainfall are left blank.

In 1936 the monsoon set in over south Kerala on 20 May. On the previous day the rainfall over south Kerala was sporadic with the daily mean of 8.7 mm. By the next day widespread heavy rainfall with an average amount of 41.6 mm was recorded. The monsoon advanced into north Kcrala on 21 May and by the next day widespread heavy rainfall occurred all over the State with average amounts of 61.4 mm and 80.3 mm respectively over south and north Kcrala. In 1951 the onset over south Kerala occurred on 30 May and over north Kerala on 1 June. The spectacular increase in rainfall over north Kerala from 31 May to 1 June may be noted.

3. ONSET DATES OVER SOUTH AND WORTH KERALA (1901 1980)

Following the criteria discussed in the previous section, the dates of monsoon onset over south and north Kerala were fixed for the individual years 1901 1980. These dates are listed in Table I in which the monsoon onset dates for Kerala as per IMD records are also given. The IMD records give only a single date for Kerala as a whole. The onset dates vary between 7 May and 22 June. Comparison of the corresponding onset dates for south and north Kerala show that, in many years, the onset over north Kerala was either simultaneous or I to 3 days later than over south Kerala. In 1913, 1952, 1953, 1965 and 1977 the northward progress of the heavy rain spells was slow, resulting in delayed onset of the monsoon over north Kerala by 10 days o r more compared with south Kerala.

Table I shows that for the six years 1932, 1943. 1949, 1955, 1957 and 1959 the IMD onset dates were later by 10 days or more than the dates we have found from the rainfall time series: in 1969 the IMD onset date

286 R ANANTHAKRISHNAN A N D M. K. S O M A N

m m +-MAY- 2; -*loJu,:' 4ol-++-!+4--- T--- r - l - -+, I 30 c I 9 * O

3 0 I

2 0 -

10-

o - d l l . . l l l ' I * * A l

30 -

20-

0 -

10-

2 0 I 9 l 7

I

I

L O

20

10 I I

I '

3 0

2 0

' I 6 I1

k - - - - -

Figure 2. Mean daily rainfall over south Kerala from 1 May to 30 Junc for selected years showing onset dates of the southwest monsoon

was 8 days earlier. Scrutiny of the rainfall data shows that the IMD onset dates have to be revised for all these years. The daily rainfall sequence from 1 May to 30 June for some of these years is depicted in Figure 5. The mean daily rainfall during the first 5 days including the onset dates for the ycars 1932, 1943, 1959 and 1969 are 19.5, 16.2, 14.2 and 13.4 mm respectively, satisfying the onset criteria.

Table I1 gives the number of onset years in 3-day intervals for south and north Kerala through May and June. For the 80-year period the onset dates are equally distributed between May and June for south Kerala; for north Kerala the corresponding numbers are in the ratio 35.

The mean, median and standard deviations of the onset dates are given in Tablc 111. The mean onset date for south Kerala is 30 May and for north Kerala 1 June with a standard deviation of about 9 days in both cases. The median onset dates are found to be a couple of days after the mean dates. The mean onset date for Kerala as a whole as per IMD data in Table I is 2 June which is later by 3 days and 1 day respectively from the mean onset dates for south and north Kerala of the present study.

4. SUPER-POGED EPOCH ANALYSIS OF THE MEAN DAILY RAINFALL SERIES IN RELATION T O THE MONSOON ONSET

I t is often stated that the pre-monsoon thunderstorm rains over Kerala progressively increase and merge with the monsoon rains on account of which the fixation of the monsoon onset date is rendered difficult

K E R A L A

DAILY RAINFALL Imml

2 2 MAY 1936

A R A B I A N

S E A

Figure 3. Rainfall sequencc associated with the onset and advance of the southwest monsoon over Kerala (1936)

K E R A L A

D A I L Y R A I N F A L L ( m m ) 2 9 M A Y 1951

I 0

A R A ~ I A N

S E A

IS

7-

12.

I 1

I0

09

75

* 157

K E R A L A

D A I L Y R A I N F A L L l m m ) I J U N E I S 5 1

123 39

10

A R A B I A N

S E A

0 ,

Figure 4. Rainfall sequence associated with the onset and advance of the southwest monsoon over Kerala (1951)

Tabl

e I.

Ons

et d

ates

of

the

sout

hwes

t m

onso

on o

ver

Ker

ala

(190

1-19

80).

Yea

r SK

N

K

IMD

Y

ear

SK

NK

IM

D

1901

19

02

I903

19

04

1905

19

06

1907

19

08

1909

19

10

191 1

i9

12

1913

19

14

1915

19

16

1917

19

18

1919

19

20

33

36

36

37

39

43

33

32

37

39

35

44

31

37

40

41

32

33

36

37

26

35

35

36

25

35

33

35

45

42

27

28

27

30

07

08

28

35

33

34

38

1921

37

19

22

43

1923

38

19

24

41

1925

45

19

26

39

1927

42

19

28

33

1929

33

19

30

37

1931

39

19

32

33

1933

35

19

34

46

1935

33

19

36

31

1937

11

19

38

34

1939

34

19

40

38

31

36

31

27

36

23

34

32

36

31

14

21

37

45

20

33

32

36

43

37

31

41

32

27

38

27

34

32

38

30

15

22

39

45

21

34

26

37

45

33

31

42

33

27

37

27

34

29

39

35

33

22

39

43

19

35

26

36

45

Yea

r SK

N

K

1941

22

22

19

42

32

41

1943

12

13

19

44

33

34

1945

32

36

19

46

29

34

1947

33

33

19

48

41

40

1949

13

13

19

50

27

27

1951

30

32

19

52

20

33

1953

37

48

,1

954

28

32

1955

17

16

19

56

18

20

1957

18

18

19

58

43

44

1959

12

15

19

60

14

15

IMD

Y

ear

SK

NK

IM

D

23

1961

18

41

19

62

10

29

1963

36

34

19

64

36

36

1965

24

29

19

66

31

34

1967

39

42

19

68

38

23

1969

25

27

19

70

25

31

1971

25

20

19

72

53

38

1973

34

31

19

74

23

29

1975

32

21

19

76

30

32

1977

27

45

19

78

27

31

1979

42

14

19

80

31

20

10

35

35

37

31

40

40

32

26

25

53

37

23

31

31

38

29

43

31

18

17

31

37

26

31

40

39

17

26

27

49

35

26

31

31

30

28

44

32

I

C

3 z

SK, O

nset

dat

e ov

er s

outh

Ker

ala;

NK

, Ons

et d

ate

over

nor

th K

eral

a; I

MD

, Ons

et d

ate

for

Ker

ala

as p

er I

MD

reco

rds.

D

ates

are

giv

en s

eque

ntia

lly a

s fo

llow

s: 1

May

=Ol

. . .

31 M

ay=3

1; I

Jun

e=32

. . .

30

June

=61.

290 R . ANANTHAKRISHNAN A N D M. K. S O M A N

1. -MAY JUNE mm 6 1 1 16 21 26 311 5 10 I5 20 25 30 mm

1 1 I I S I 1 1 I - 4 0

- 30 40,+ I

- 20

- 10

0

::[ Y I I

0

' I I ,

. I I

10

0

- 30

- 20 - 10

0

- 30

- 2 0

- 10

I- 0

-30

b- - 20

Figure 5. Mean daily rainfall over south Kerala from 1 May to 30 June showing IMD and refixed onset dates.

(IMD, 1943). Our study shows that the types of rainfall associated with the two events are distinctly different, as stated in Section 2. Merging occurs when the rainfall data for several years are averagcd for calendar dates because of the dispersal in the onset dates over a period of years. Rainfall curves constructed by such averaging give the impression of the merging of pre-monsoon and monsoon rainfall.

To highlight the abrupt nature of the rainfall transition during the onset, the daily mean rainfall amounts for the 80-year period were composited in two ways:

(a) by averaging the rainfall amounts for calendar dates (Figures 6a, b). (b) by averaging the daily rainfall amounts after superposing the onset dates for the individual years

Figures 6a, b present the variation of the calendar date averaged daily rainfall for south and north Kerala from the third week of March to the end of August. The mean onset dates are indicated on these diagrams. I t

(Figures 7a, b).

ONSET OF THE SOIJTHWEST MONSOON 29 1

Table 11. Number of onset years in 3-day intervals

Interval S.K. N.K. IMD Interval S.K. N.K. 1MD

May 5-7 8-10

11-13 14-16 17 19 2&22 23-25 26 28 29 31

0 0 2 0 2 1 4 1 1 4 5 3 2 2 6 9 7 14

June 1-3 4- 6 7-9

10 12 13-15 16 18 19 21 22-25

15 15 16 13 16 8 5 8 9 4 6 7 2 4 5 0 1 1 0 0 1 1

Total 40 29 34 40 51 46

SK, South Kerala; NK, North Kerala.

Table 111. Mean, median and standard deviation of onset dates

Parameter S. Kerala N. Kerala IMD

Mean date 30 May 1 June 2 June Standard deviation 8.8 days 9.2 days 7.8 days Median date 1 June 3 June 2 June Earliest onset date 7 May 1918 8 May 1918 1 1 May 1918 Latest onset date 22June 1972 22 June 1972 18 June 1972

may be noted that from about the second week of May till the onset date there is a progressive increase in the mean daily rainfall. This arises from the superposition of the monsoon rainfall in the years of early monsoon onset on the pre-monsoon thunderstorm rain of the remaining years. After the mean onset date, there is a further increase in the mean daily rainfall which is more pronounced over north Kerala.

The superposed epoch diagrams, Figures 7a, b, bring out a sharp and spectacular increase in rainfall accompanying the monsoon onset, compared to the pre-onset phase. For south Kerala the level of daily rainfall prior to the onset is around 5-6mm which is due to pre-monsoon thunderstorm activity. It is important to note that the pre-monsoon thunderstorm rain does not progressively increase but levels off by the middle of April. On the onset date the rainfall abruptly jumps up to the level of 17 mm and continues to increase reaching a peak value of about 28 m m pcr day during the next 5 or 6 days. This is followed by a gradual oscillatory decline. The mean pre-monsoon rainfall level for north Kerala is about 4 to 5 mm per day which shoots up to a little over 20 mm per day during the onset with further increase during the next 5 to 6 days.

I t should be emphasized that the mean rainfall variation depicted by Figures 6a, b is not a correct representation of the actual rainfall variation in individual years. This is more correctly represented by Figures 7a, b. Thus, the present study categorically shows that the prevailing notion that the pre-monsoon thunderstorm rainfall over Kerala progressively increases and merges into the monsoon rain resulting in a gradual transition is not in conformity with facts. The onset of the monsoon is heralded by a sudden and spectacular increase in the rainfall from the pre-monsoon level.

5. CHARACTERISTICS O F THE ONSET SPELL

The onset spell has been defined as a heavy spell of rain with a duration of 5 days or more and mean rainfall in excess of 10 mm. We have examined the duration and average daily rain amount associated with the onset

mm 1 3 5

mm 3 51

-

-

3 0

2 5

D A I L Y MEAN RAINFALL OF SOUTH KERALA : 1901-1980 CALENOAR DATES : M E A N ONSET DATE 30 MAY

30

2 5 -

-

m m 35

3 c

2 5

20

I 5

I

mm 35

DAILY MEAN RAINFALL OF N O R T H KERALA : 1901- 1980 CALENDAR DATES: MEAN O N S E T DATE I J U N E

M E A N ONSET

5

0

Figure 6. (a) Mean daily rainfall over south Kerala averaged for calendar dates for the years 1901 -1980 showing mean onset date of the southwest monsoon. (b) Mean daily rainfdl over north Kerala averaged for calendar dates for the years 1901-1980showing mean onset

date of the southwest monsoon

m m m m

DAILY MEAN RAINFALL OF SOUTH KERALA: 1901-1980 SUPERPOSED EPOCH : ONSET DAY 0

--I DAILY MEAN RAINFALL OF NORTH KERALA ( 1 9 0 1 - l 9 8 0 )

b- D A Y S DEFORE ONSET +DAYS AFTER Ol(fET

Figure 7. (a) Mean daily rainfall over south Kerala after superposing onset dates of the southwest monsoon for the period 1901 ~ 1980. (b) Mean daily rainfall over north Kerala after superposing onset dates of the southwest monsoon for the period 190-1980

294 R. ANANTHAKRISHNAN A N D M. K. S O M A N

spell for the individual years of the 80-year period for both south and north Kerala. The relevant data for south Kerala are shown graphically in Figure 8. Here the vertical pillars indicate the spell duration in days while the thick dots (joined by the broken curve) denote the mean daily rainfall for the respective spells. Note that both the duration as well as the rainfall associated with the onset spells show wide dispersion.

Statistical data relating to the analysis of the onset spells are given in Table IV (a), (b). The spell duration in individual years varied from 5 days (1976) to 50 days (1922) with a mean duration of 15.4 days. The onset spells were generally of longer duration during the first-half of the period compared with the second half. The mean daily rainfall associated with the onset spell is 26 mm. This quantity showed a wide range from 1 0 3 mm in 1976 to 495 mm in 1933. It may be of interest to note that 1976 was the year of lowest rainfall for Kerala during the 80-year period covered by the present study. The data in Table IV provide justification for our criteria to fix the onset date of thc southwest monsoon on the basis of the mean daily rainfall over the rain gauge network over south Kerala. The data for north Kerala show similar behaviour.

DAYS I Imm

n r

Y E A l l - - r

Figure 8. Duration and averagc rainfall of onset spells over south Kerala (1901 -1980)

Table IV. Statistical data pertaining to onset spells (a) Spell duration

Duration (days) 5-9 10-14 15-19 20-24 25-29 3 0 34 2 3 5 Total

Frequency 24 23 14 7 6 3 3 80

Mean spell duration

Standard deviation (days)

1901 -1940 1941 1980 1901-1980

17.9 12.9 15.4 10.9 7.0 9.5

(b) Mean daily rainfall

Frequency 2 18 19 18 13 5 2 3 80

Mean daily spell rainfall (mrn) Standard deviation (mm)

1901 --I 940 1941 - 1980 1901 -1 980

27.6 24.4 26.0

8.0 7.2 7.8

ONSET OF THE SOUTHWEST MONSOON 295

6. DISCUSSION

In an earlier study by Ramdas et al. (1954) the mean daily rainfall data over south Kerala (referred to as Travancore Cochin in their paper) for May-June have been made use of to fix the date of establishment of the southwest monsoon for the period 1891-1950. Visual examination of the rainfall curve depicting the rainfall sequence was the basis on which the dates were arrived at. The date of ‘commencement of persistent heavy rainfall’ was taken by them as the date of establishment of the monsoon. Neither the threshold value of the rainfall nor the minimum duration of persistence has been specified. We have found it possible to fix such limits in the present study. The onset dates for south and north Kerala in Table I furnish a set of dates based on uniform criteria with the least measure of subjectivity. The delay in the mean onset date by 2 days between north and south Kerala seen in Table 111 is in accordance with the average rate of northward advance of the southwest monsoon along the west coast of India.

Examination of the dates of monsoon onset for the first and second halves of the 80-year period shows some interesting features, which are shown in Table V. During the period 1901-1940 the monsoon onset was more often in June than in May while the reverse feature occurred during 1941-1980. This feature is reflected in the mean onset dates for the two periods which differ by 4 days.

A significant feature brought out by the present study is the clear demarcation between the rainfall associated with the pre-monsoon thunderstorm activity over Kerala and the rainfall that heralds the onset of the monsoon. The super-posed epoch rainfall diagrams (Figures 7a, b) show that the average daily rainfall from pre-monsoon thunderstorm is of the order of 5 mm. This limit is reached by about the middle of April and there is little increase thereafter. Following the monsoon onset the mean daily rainfall steeply increases to about 30 mm. The limit of 10 mm chosen for demarcating the light and heavy rain spells roughly corresponds to twice the average pre-monsoon rainfall level.

In two recent papers, Subbaramayya et al. (Subbaramayya and Bhanukumar, 1978; Subbaramayya et ul. 1984) have discussed various features associated with the onset and advance of the southwest monsoon over India. For the 25-year period 1956- 1980 these authors have, in one of their diagrams, indicated a mean onset date of 20 May over Kerala. This is totally at variance with the mean onset dates for the same period as per IMD records (30 May) and as per the present study (29 May for south Kerala and 30 May for north Kerala). It is obvious that these authors distinguished not between the pre-monsoon thunderstorm rains and the monsoon rainfall over Kerala.

7. CONCLUSIONS

The main points brought out by the present study are: (i) The mean daily rainfall series for south and north Kerala enable formulation of objective criteria for

fixing monsoon onset dates in individual years. From the dates thus arrived at, the mean onset dates for south and north Kerala are found to be 30 May and 1 June respectively for the period 1901-1980 with a standard deviation of about 9 days.

Table V. Comparison of monsoon onset over Kerala during 1901- 1940 and 1941 -1980.

~

1901 1940 1941 -1980 Item S.K N.K IMD S.K N.K IMD

Mean date 1 June 3 June 4 June 28 May 30 May 31 May Std. Dev. 7.5 7.9 7.0 9.6 10.1 7.9 (days)

May onset (No. of years) 15 1 1 9 25 18 25 June onset (No. of years) 25 29 31 15 22 15

296 R. ANANTHAKRISHNAN AND M. K. SOMAN

(ii) The mean onset date for 1901-1940 is later than for 1941-1980 by 4 days. (iii) Super-posed epoch analysis of the mean daily rainfall reveals a sharp and spectacular increase heralding

(iv) The statement often made that the pre-monsoon thunderstorm rain over Kerala progressively increases

(v) The spell that heralds the onset of the monsoon has a mean duration of about 15 days and the associated

the monsoon onset.

and merges with the monsoon rains resulting in a gradual transition is not valid.

daily mean rainfall is 26 mm. These have large variations in individual years.

ACKNOWLEDGEMENTS

We wish to thank the Director, Indian Institute of Tropical Meteorology for his interest in this study and for providing the necessary facilities. We also wish to express our thanks to Shri R. M. Soni for preparation of fair copies of the diagrams, Shri R. P. Mali for photographic assistance and Miss S. R. Kamble for typing the manuscript.

The data utilised in this study were kindly made available by the National Data Centre of IMD. We arc grateful to the India Meteorological Department for a research grant.

REFERENCES

Ananthakrishnan, R., Acharya, U. R. and Ramakrishnan. A. R. 1967. 'On the criteria for declaring the onset of the southwest monsoon

India Meteorological Department. 1943. CIimarological Atlas j i ir Airmen. Kung. E. C . and Sharif, T. A. 1980. 'Regression forecasting of the onset of the Indian summer monsoon with antecedent upper air

Kung, E. C . and Sharif, T. A. 1982. 'Long range forecasting orthe Indian summer monsoon onset and rainfall with upper air parameters

Ramdas, L. A., Jagannathan. P. and Gopal Rao. S. 1954. 'Prediction of the date of the establishment of southwest monsoon along the

Subbaramayya. 1. and Bhanukumar, 0. S. R. U. 1978. 'The onset and the northern limit of the southwest monsoon over India', M e f .

Subbaramayya, I., Babu, S. V. and Rao, S. S., 1984. 'Onsct of the summer monsoon over India and its variability', Met. Mag., 113,

over Kerala', I M D , Forecasting Manual, No. IV-18.1.

conditions', J . A p p l . Me!., 19, 37C380.

and sea surface tempcratures', J . Mer. Soc. Japan, 60, 672481.

west coast of India', Ind. J . Mer. Geophys. 5, 305 314.

M a g . . 107, 3744.

127-135.