A study of major rainstorms of Assam

ABSTRACT

More than 100 rainstorms of durations ranging from 2 to 7-day during the period 1901-1960 over the Brahmaputra catchments in Assam plains have been studied by the Isohyetal Method. Enveloping curves of maximum depth for various durations are presented and discussed in this paper. The synoptic situations associated with some of the heavy stomas are also discussed.

Introduction
The southwest monsoon is responsible for a large bulk (about 70 per cent) of the annual rainfall of India. In the case of Assam the normal monsoon rainfall (June to September) is 164 cm and forms 66 per cent of the annual rainfall. It is also interesting to note that Assam is one of the regions in India with low variability of the seasonal rainfall, being 10 per cent. If we consider the individual months of the monsoon season the variability is 19.3 in June, 18.4 in July, 18.2 in August and 24.4 per cent in September (Parthasarathy 1960).

One of the interesting features of the monsoon rainfall in Assam, as well as in other parts of the country is that the rainfall during each month is made up of a few spells of above normal rainfall with normal or even below normal rainfall during the rest of the month. This type of distribution of the monsoon rainfall causes floods in certain periods of the month even when the total monthly rainfall itself is just normal or even below normal.

The purpose of this study is to examine the characteristics of some of the heavy rainstorms that occurred over this area during the 60-year period 1901-1960. Attention has been given to the Brahmaputra catchment lying within Assam and its neighbourhood, since we are primarily interested in rainstorms that are likely to cause floods in the Brahmaputra.

Area selected and data used for storm study

The present study is confined to the Assam plains. The area considered is marked in all the relevant figures and is about 37,000 sq. miles.

The Daily Rainfall Volumes for the period 1901 to 1960, compiled by the India Meteorological Department, have been taken as the main source of the data for this study. Tire data of the raingauge stations in the neighbourhood of the area under study have also been utilized for preparing isohyetal maps.

Normal features of rainfall over Assam

Monsoon sets in over Assam in the first week of June and withdraws in the second week of October. The rainfall during the month of June is highest for the season and it gradually decreases thereafter. Another interesting feature is that even before the monsoon sets in there is considerable thunderstorm activity in this region in the month of May and the rainfall caused by these thunderstorms (33.8 cm) is comparable in magnitude to the rainfall of any of the monsoon months. Hence, for purposes of storm selection and study, May is equally important.

The normal annual isohyetal pattern for Brahmaputra catchment in Assam is shown in Fig. 1. The monthly isolyetal maps from May to October have also been examined in conjunction with the Normal Annual Isohyetal Maps and one significant feature found to be that the patterns in all of them have strong resemblance to each other and the regions of high and low rainfall are more or less the same. This is mainly because the rainfall in Assam is largely determined by orographic features and the direction of monsoon winds with respect to these features. Further, about 70 per cent of the annual rainfall is made up by the monsoon rainfall itself. Also the maxima of rainfall occur around Mawsynram, Tamenglong and the north-western and northeastern parts of Assam withmarked minimum between Lanka and Dimapur .

Storm studies

In order to limit our study to relatively heavy rainstorms only such storms which gave an average depth of at least 2.5 cm per day over the area under study and extended at least over 2-day have been considered. According to this criterion, during the period 1901 to 1960 there were 103 storms. A list of these storms together with the average depth of precipitation and the synoptic situations associated with the storms are given in Appendix.

Frequency of rainstorms

The distribution of the rainstorms according to their duration and the month of occurrence is given in Table I. It will be seen that rainstorms are most frequent in this area in the month of June (41 out of 103) with next highest frequency (22) in July. They are equally frequent (15) in the month of May and August. In the month of September and October they are infrequent. There are equal number of 2 and 3-day storms whereas storms of 4-day duration are relatively less (23 out of 103). Storms of duration of more than 4 days are rare. Also rain-storms are rare in the month of October over Assam. The number of rainy days in this month are 7 and only one storm of 2-day duration occurred during the entire 60-year period under study.

It will also be interesting to examine the different types of meteorological situations that are responsible for these rainstorms in different months. In order to study this, the bivariate frequency distribution of the 103 storms considered here, according to the month, of occurrence and associated meteorological situations, is presented in Table 2.

The significant feature of this distribution is that, considering the period May to October as a whole, the one synoptic situation which most frequently causes rainstorms in Assam is the extension of the eastern end of the monsoon trough over Assam.

It may, in this connection, be mentioned that normally the monsoon trough dips into the head Bay of Bengal and it is pulled up towards Assam either under general 'break monsoon' conditions over the country or under the influence of disturbances in the westerlies which affect the northern portions of Assam. It is noticed that the well-known break monsoon conditions occur after monsoon has established over the country as a whole whereas the changes in position of the eastern end of the monsoon trough take place as soon as the monsoon sets in over northeast India. It will be seen from Table 2 that maximum number of rainstorms due to the extension of trough over Assam occur in the month of June whereas those associated with the break monsoon conditions occur in the mouth of July. Next in importance is the pre-monsoon thunderstorm activity over Assam. The pre-monsoon thunderstorm rain in the month of May is comparable in magnitude to the rainfall received during any of the monsoon months. The rainstorms which occur as a result of thunderstorm activity are most frequent in the month of May as should be expected.

Low pressure systems that formed over the Bay of Bengal also cause rainstorms over Assam when they are either directly over Assam or when they have moved westnorthwestwards from the head Bay and lie to the west of Assam. It will be seen that the number of low pressure systems which lie directly over Assam and cause rainstorms are relatively less than those which move westnorthwestwards and cause rainstorms over Assam when they are located west of Assam.

Table 3 shows whether rainstorms of particular duration have a tendency to occur more frequently in association with a particular synoptic situation. It is seen that most of the storms are of 4-day duration or less. It is also seen that with each synoptic situation storms of different durations have occurred thereby indicating that there does not seem to be any apparent relation between the synoptic situation and the duration of the storm.

Return period Storm rainfall magnitude(cm)
(yr)
6 5.7
10 6.8
25 7.8
50 8.8
75 9.6
100 10.2


Another characteristic of the rainstorms that will be of utility is the relation between the depth of precipitation and duration of storms. The study reveals that only 25 per cent of the 3-day storm cause average depth of precipitation greater than the depths caused by 2-day storm and only 14 per cent of the 4-day storm cause depth greater than that yielded by 2 and 3-day storms.

Conclusions

Rainstorms are most frequent in the month of June in the Brahmaputra catchment lying largely in Assam. Most of the storms are of duration 2-4 days.

2. The synoptic situation which is associated with, maximum number of rainstorms is the extension of the eastern end of the monsoon trough over Assam either in association with a general break in the monsoon or otherwise.

3. Storms of duration greater than 4-day are not only rare but do not contribute substantially more to the depth of precipitation than those of 4-day duration,

4. The storms of 18-21 June 1934 and 9-12 August 1902 are the heaviest on record and have made maximum contribution to the enveloping curves for durations 2 to 4 days. It is interesting to note that both these heavy storms occurred in association with shift of the eastern end of the monsoon trough over upper Assam.


Parthasarathy, K., 1960 Monsoon's of the World, India met. Dep.,pp 185-191.
W. M. 0.1965 Guide to Hydrometeorological Practices, WMO-1968
Raman P.K. and Dhar 0. N. 1966 Indian J. Met. Geophys. 17 Spl. No,pp.87-90.