Northeastern India (east of 88o E and North of 21o N) has distinct precipitation and drainage patterns due to its unique location and orography. Empirical Orthogonal Function (EOF) analysis of southwesterly monsoon rainfall over India shows that seasonal rainfall patterns over the Northeastern region contrasts to that over the rest of the country(Neelima A. Sontakke). Between March and May in Northeast India, thunderstorms contribute about 20% of annual rainfall totals. During the months of June to September, southwesterly monsoons supply 70% of the annual precipitation. Another 8% of annual rainfall between October and November is associated with northeasterly monsoons. During the southwesterly monsoon season, frequent floods responsible for both human casualties and property damage occur as the powerful Brahmaputra River flows are constricted through the narrow Assam valley, fed by torrential rains and snowmelt from Himalayan ranges and laden with silt from actively eroding steep headwater slopes.

Seasonal variation of rainfall is the most distinguishing feature of the monsoonal regions of the world. A major portion of the annual rainfall over most of the Asian domain occurs during the summer monsoon period (June – September). The year-to-year variability in the monsoon rainfall occasionally leads to extreme situations of droughts and floods affecting the agricultural output and the national economy. Hence the variations in seasonal monsoon rainfall may be considered a measure to examine climate variability/ change over the Asian monsoon domain.

Synoptic climatology of northeast India during the pre-monsoon season

It is well known that the thunderstorms, a mesoscale phenomena, are born of synoptic scale disturbances which in turn develop out of distinct large scale features. It is, therefore necessary that the large and synoptic scale features that generate these mesoscale disturbances are identified. As a part of the global patterns of pressure distribution, the hot weather or the pre-monsoon season witnesses the gradual weakening of the intense winter anticyclone over central Asia and also of the gradual northward shift of the equatorial trough(TCZ) from its winter position.

Seasonal synoptic features

Examination of Weather charts, surface and upper air for the days of thunderstorm activity over Assam during the pre-monsoon months of April and May reveal that incursion of moist air in the lower level over Assam influenced by the extension of the seasonal trough, existence of wind discontinuity in lower levels and existence of upper divergence associated with passage of upper troughs in the westerlies or occurrence of jet maxima constitute favourable conditions for occurrence of thunderstorms in Assam and adjoining areas. ( which can be detected on the streamline charts for 30,000—40,000 ft levels, provided adequate wind observations from such levels are available.)

The month of February-March marks the transition from the -winter to summer over north east India, when the prevailing anti-cyclonic circulation of the winter season begins to give way to a shallow low pressure area, with its central region over Bihar and Gangetic West Bengal. Occasionally, the low gets accentuated due to the passage eastward of a secondary western disturbance, across the region, or as a result of the progressively moving wave of low in the upper air westerlies during this period. Under the influence of the surface low an incursion of relatively moist air tends to occur over the region, although in many instances the moist air flow is restricted to a height below 1.5 km or even less. As the season progresses, the low pressure system becomes somewhat deeper, and the resulting inflow of moist air extends to a height of some 1.5 to 2.0 km. . Quite often from extreme northeast Assam, Easterly current, which is relatively dry and somewhat cooler blows westwards across the foot of the Himalayas. The existence of this ENE/E-ly air over extreme north Assam and warm moist SW/S-ly air from the south results in an E-W line of wind discontinuity over upper Assam. It seems that this wind discontinuity shows some north-south oscillation in its day-to-day position.*

The wind flow in the upper air brings in air of continental character, with a fairly high lapse rate of temperature approaching dry adiabatic. The over-running of the convectively unstable moist air below, by dry air above results in a certain degree of latent instability in the atmosphere, favoring development of thunderstorms which may sometimes be quite severe. Rainfall accompanying the thunderstorm provides a welcome relief to the rather prolonged drought conditions, or scanty rainfall during winter months, and helps bring down the day temperature to a good extent.

Pressure distribution

As a part of the global patterns of pressure distribution, the hot weather or the pre-monsoon season witnesses the gradual weakening of the intense winter anticyclone over central Asia and also of the gradual northward shift of the equatorial trough from its winter position (Ananthakrishnan, Srinivasan, and Ramakrishnan 1968).

The main feature in the pressure distribution over north India during the Nor'wester season in NE India is the trough of low pressure extending in a W-E direction across the plains. The orientation of its axis and its intensity is affected by the passage of western disturbances, in the form of low pressure waves. There is also a marked diurnal variation, along with insolation accentuating the trough in the afternoon. The upper winds during this season indicate a layer of southerlies from the Bay(SW,S or SE) up to about 1-2 km and westerlies or northwesterlies from the north Indian plains aloft. The extent, direction and depth of the moist current from the Bay is generally determined by the orientation and intensity of the surface trough of low pressure.

Let us take a day when the trough is so oriented that the moist air mass flows towards west Bengal and Chota Nagpur, as indicated by the upper winds. In the morning, the moist air is usually confined to a kilometer or less in the southern districts of Bengal. With the advance of day the trough is accentuated and the moist current flows further inland and to a greater depth, towards North East India. The afternoon insolation in this region and the uplift provided by the hilly topography seems to provide the trigger action for the release of latent instability and the first line of thunderstorms starts more or less at the boundary of the moist current.

Over India continuous and rapid rise of temperature and fall of barometric pressure give rise to the evolution of a trough of low pressure which from a weak beginning in March is most intense in May.

The area of lowest pressure and highest temperature in May lies in northwest Rajasthan, the trough extending there from up to Bihar Plateau- these pressure patterns bring likewise changes in the wind fields at the surface and at the lower levels and, therefore, in the horizontal and vertical extent of moisture incursion on land.

The study of synoptic charts of Northeast India for a number of days reveals that widespread pre-monsoon thunderstorm activities in the north east Indian states is generally associated with the easterly movements of low pressure waves or secondary western disturbances along the foothills of the eastern Himalayas. Intense thunderstorm activity is also favored by the appearance of “lows” in the central parts of Bengal and Chotanagpur area – stationary or with little easterly or northeasterly movements.

From past record of both surface and upper air data it has been observed that the occurrence of pre-monsoon thunderstorm in Northeast India is most favored on those days when latent instability is present in the atmosphere. Operation of certain trigger is necessary to release this latent energy which in its turn helps the development of the cumulonimbi cloud. Solar heating in the lower layers of the atmosphere serves the triggering action though it is not always sufficient (Choudhury, 1961). Low level horizontal convergence of air, which through its vertical spreading action serves to steeper the lapse rate can also render the warm air mass more unstable and can thus favour the lifting (Namias.1940), In addition to the location of the "lows" on the surface charts of the associated cyclonic circulation in the upper air, the locations of the zones of divergence and convergence in the lower layer of the atmosphere can provide vital information for thunderstorm forecasting over the region.(Choudhury,1961).

Weather charts, surface and upper air for the days of thunderstorm activity over Assam during the pre-monsoon months of April and May were carefully analyzed. Preliminary examination revealed that incursion of moist air in the lower level over Assam influenced by the extension of a seasonal trough, existence of wind discontinuity in lower levels and existence of upper divergence associated with passage of upper troughs in the westerlies or occurrence of jet maxima constitute favorable conditions for occurrence of thunderstorms in Assam and adjoining areas.

The atmospheric pressure as registered by a barograph at the ground undergoes rapid fluctuations in the neighbourhood of a thunderstorm. For example, during the growing stage of a thundercloud, there is a feeble pressure "low" in the vicinity of the cloud. During the precipitating stage, there is a relatively strong pressure "high". In respect of the thunderstorm 'high'', in the development of which several static and dynamic factors have been supposed to play a part.

In a later phase we plan to present detailed quantitative estimates of different contributions to this high, taking into account temperature fluctuations and also changes in the quantity of raindrops and hailstones. The computed values are to be compared with observations.

In the upper troposphere, westerly wind prevails over north India. Passage of upper troughs in the westerlies or jet maxima across upper Assam can be detected on the streamline charts for 30,000—40,000 ft levels, provided adequate wind observations from such levels arc available.

Wind flow pattern over Brahmaputra Valley

During the pro-monsoon season the pressure distribution over India changes resulting in certain changes in the air flow-over this region greatly modified by the local orographic influences.

The air that streams across the greater part of the Indian sub-continent during this season is tropical continental (Tc), with source region at south west Asia. Above the surface layers wind direction over the region is westerly above about 3 km above mean sea level in January and southwesterly and then westerly above 1 km m.s.l in April. . Throughout the year, the low-pressure area is located towards the southwest of the region and the katabatic wind coming from the Eastern Himalaya and Tibetan plateau blows along the down slope of the valley towards the low pressure area. Thus, the cold air from the region is drained out. The wind flow pattern in the region is also greatly affected by the surrounding hills of the region specially, the Garo, Khasi, Barail, and Patkai hill range greatly influences the wind flow pattern and hence on the spatial distribution of rainfall in the region. That these easterlies should be dry and cooler in comparison to the southerlies is backed by the theory of origin of the easterlies at the lower levels. The southerlies approaching the Brahmaputra valley originate from the Bay of Bengal; so they are warm and moist. On the contrary, as mentioned earlier, the easterlies originate from the Tibetan plateau and are, therefore, cool and dry.

Easterlies in the lower levels:

The easterlies over the Brahmaputra valley during winter are mountain winds on a seasonal scale. Once the easterlies set in during winter they should continue to prevail till the Tibetan plateau and the Himalayan ranges get sufficiently heated up to make up the difference of their temperature from that of the adjacent air over the valley. The Tibetan plateau may get heated up in two processes, viz., insolation and the transport of heat by air flowing over it. As the Tibetan plateau is mostly snow covered during winter it will reflect back most of the solar energy incident on it. The little portion of the energy that will be absorbed will be used up as latent heat in converting snow into water. The Tibetan plateau slopes down gradually from west to east. The prevailing westerlies will have a long travel over the western side of the plateau before reaching to the north of the Brahmaputra valley. These westerlies will have contact with the snow covered surface for a long time and will, therefore, be considerably cooled before reaching the part of the plateau with which we are concerned. The Brahmaputra valley on the other hand will absorb more Solar radiation. There is no cooling process operating on the westerlies reaching the Brahmaputra valley at the level of the plateau. It follows, therefore, that the Tibetan plateau will continue to remain colder than the air over the Brahmaputra valley at the corresponding level. Hence the easterlies over the valley, shown to be mountain winds on a seasonal scale, will continue to blow during the pre-monsoon season.

Southerlies entering the valley:

The main seasonal feature over north India in April and May on the lower level charts up to 0.9 km is the existence of a trough of low pressure over Uttar Pradesh and north Bihar, with its axis or trough line running in a WNW—ESE direction. Around this trough, WSW/W-ly current normally blows over Assam in the lower levels. With the passage of western disturbances or otherwise, the seasonal trough often gets accentuated and extends south-eastwards into the Gangetic West Bengal and neighbourhood or eastwards into north Bengal causing incursion of moist southwesterly to southerly air from the Bay of Bengal into Assam. Quite often from extreme northeast Assam, ENE/E-ly current, which, is relatively dry and somewhat cooler blows westwards across the foot of the Himalayas" (Sen. and Basu; 1961).

Orographic effect on air flow:

The presence of Naga-Khasi-Jaintia-Garo hill range greatly influences the air flow over the Brahmaputra valley. This range runs from east to west from 95°E to 90°E longitudes between 25°N and 26°N latitudes. On its east, run the mountains of Burma almost perpendicular to it. Southerlies approaching the Brahmaputra valley, are, therefore, obstructed by these ranges. The easterlies originating from the Tibetan plateau and blowing over the Brahmaputra valley are also protected from mixing with the southerlies. As the average height of the range is less than 1.5 km a.s.l., this sort of protection should be confined to a level not more than 1.5 km a.s.l. It is normally supposed that the existence of these ENE/E-ly air over the extreme north Assam and warm moist southwesterly to southerly air from the south results in an E-W line of wind discontinuity over upper Assam.

This wind discontinuity is apparent on weather charts by the separation of two air currents of different origin by the Naga-Khasi-Jaintia-Garo range. Strong southerlies can, however, cross over the barrier at levels 1.5 km and above. These are expected to remain above the easterlies over the Brahmaputra valley as these are warmer and moister (with the progress of the summer season the easterlies are likely to weaken) and should not normally descend unless forced by some mechanism to do so. At night, when the Garo-Khasi-Jaintia hills cool down more rapidly than the plains katabatic wind blows down the slope towards the valley. This, in turn, brings down the moist air to the valley. Therefore, there will be a front like structure during night, having two different types of air on - either side. This is probably the reason why the pre-monsoon thunderstorms occur over the Brahmaputra valley mostly during night.

SOME FEATURES OF PRE-MONSOON THUNDERSTROMS OVER NORTH EAST INDIA

Thunderstorms are localized phenomena which have been defined as storms produced by a cumulonimbus cloud always accompanied by lighting and thunder. They are usually of short duration, seldom over two hours. Generally they are accompanied by wind gusts, heavy rain and sometimes hail. Thunderstorms are serious weather hazard to aviation, A large percentage of aircraft accidents occur due to it. So, the aviators dread thunderstorm and tried to keep it away. Therefore, this severe weather phenomenon has always been considered as an important subject to be investigated by
the weather scientist.

Atmospheric instability, lifting of potentially unstable air and sufficient supply of warm moist air are the main factors favouring the formation of a thunderstorm (Glode, 1977). There are marked seasonal and latitudinal variations in the height, horizontal extent and duration of thunderstorms. Generally, they range from 4 km to 20 km in height above mean sea level. The maximum height is observed in tropical region during warm season. Thunderstorms may cause destruction of properties rendering thousands of
people homeless, over rooting trees and damaging standing crops in the fields.


In Northeast India, thunderstorms (Nor’westers) are very common phenomenon during the period February to May. The areal distribution of thunderstorm activity in India during the pre-monsoon months (February to May) is depicted in Figure 4, f and 4.2 (Rao,1983). The figures show that during pre-monsoon months the areas with marked thunderstorm activity are North eastern part of the country including Northeastern states, Bengal and Orissa, some parts of central India and extreme south of the subcontinent.

Thunderstorms have diurnal variation. In Northeast India, generally they occur during afternoon hours. During night and early morning hours also the region experiences considerable thunderstorms events. Pre-monsoon thunderstorms are sometimes accompanied by hail usually in and near sub-montane regions.


Analyses of IMD Tephidiagrams show that there are no days during the Nor’wester season in the region without latent instability being present in the atmosphere in some form. There is usually an inversion between 1 to 2 km(at Guwahati airport), and if the necessary trigger action becomes available, the energy that can be realized on most days is considerable. If the whole of the energy was realized, the velocity of the squall will exceed 75 mph on many occasions. (IMD 1944)

On most occasions the energy has only been partly used up and on a few occasions the realizable energy has shown an increase after thunderstorms, which suggest maintenance of intensification of state due to air movements. In vertical sections θs (saturation potential temperature) decrease from about 330K to about 292K at 5km, and thereafter increases, reaching the surface value at about 13km. Therefore convection maybe expected to extend up to about 13 km and convective clouds to reach that level. The vertical sections as well as winds on isentropic surface indicate in a general way both upward and downward currents and after a thunderstorm θs at the surface has nearly the same value as the minimum θs in the upper air suggesting a descent of air from about 5 km. It is also noticed from an examinations of all available thermograms that the value of the wet bulb is, after a squall, of the order of 294 °K, irrespective of locality.

From past records of both surface and upper air data it has been observed that the occurrence of pre-monsoon thunderstorm in Northeast India is most favored on those days when certain latent instability is present in the atmosphere.

The main feature in the pressure distribution over north India during the Nor'wester season is the trough of low pressure extending in a W-E direction across the plains.

There is no cooling process operating on the westerlies reaching the Brahmaputra valley at the level of the plateau. Therefore, the Tibetan plateau will continue to remain colder than the air over the Brahmaputra valley at the corresponding level.

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.