Friday, May 21, 2010

Cyclone Laila batters Andhra - Addanki receives 522 mm & Maddipadu 510 mm

Andhra Pradesh heaved a sigh of relief after cyclone Laila weakened ahead of landfall about 30 km from Bapatla in Guntur district on Thursday afternoon but still left 16 people dead, flooded thousands of homes, snapped power supply in many areas and threw rail and road traffic out of gear in the coastal region of the state.

The cyclone hit the coast near Bapatla around 1.40 pm and moved on northwards by 5.30 pm. "The system is likely to weaken gradually and move initially in a northerly direction and then recurve in northeasterly direction towards south Orissa. There is a possibility of the cyclonic system emerging in the north Bay of Bengal and intensifying again over sea," the Indian Meteorological department warned and called for vigil across the region for at least another 24 hours. Despite the cyclonic system moving away, the coastal region of AP is expected to receive heavy to very heavy rains on Friday as well.

The swollen Addavagu at Madhiralapadu surging with flood waters in Ongole after Cyclone Laila hit. Photo: Srinivas Kommuri
The swollen Addavagu at Madhiralapadu surging with flood waters in Ongole after Cyclone Laila hit.

Even as the cyclone-battered Ongole town is limping back to normality, the heavy rain received in the last two days, has made rivulets like Gundlakamma, Addavagu, Pothurajukalva swollen.

Addanki received the highest rainfall of 522 mm followed by Maddipadu 510 mm and Kothapatnam 258 mm in the last 24 hours. Ongole has got 320 mm on May 20th and 142 mm in last 24 hours. Three irrigation tanks suffered breaches People in several villages like Maddiralapadu, Chevella, Karavadi, Gundayapalem, Chintala were marooned on Friday.

Sunday, May 16, 2010

Rainfall on the Meghalaya plateau in Northeastern India

Monthly and daily variations in rainfall over Cherapunjee and Mawsynram on the Meghalaya plateau of northeastern India are analysed. Cherapunjee and Mawsynram are well known as two of the places with the heaviest rainfall in the world. The daily rainfall variation is attributed to the influence of synoptic scale disturbances, with a periodicity of 10–20 days, and the orographic interaction. The annual and monthly highest rainfalls over Cherapunjee during the 31 years from 1973 to 2003 were much larger than mean values.

The Meghalaya plateau is located on the northeastern part of the Indian subcontinent and consists of relatively high hills. The highest hill peak of the region is about 1,965 m above mean sea level (Fig. 1). Meghalaya means ‘‘adobe of clouds’’. Cherapunjee is well known as the place with the world record for the highest rainfall of 26,461 mm, during August 1860 to July 1861. It is located on the southern slope of the Meghalaya plateau and inside a deep canyon with steep cliffs. Four seasons are experienced in Cherapunjee, viz., winter (December–February),pre-monsoon (March–May), monsoon (June–September) and post-monsoon (October–November). It was opined that Mawsynram might be the rainiest station in India. Mawsynram is the neighbouring village of Cherapunjee, and a rain gauge was installed in 1941 and hence the mean annual rainfall over Mawsynram and Cherrapunjee was compared simultaneously for 10 years and the annual mean rainfall over Mawsynram was larger than that over Cherrapunjee. However, they indicated the difficulty of this comparison, because of the limited data of 10 years. The rainfall records for Cherrapunjee and Mawsynram are available for 100 and 60 years, respectively. After comparison of annual mean rainfall over both stations, Cherrapunjee still has the maximum rainfall over 24 hours, 1,036.3 mm in 1867, in India. On the basis of hourly rainfall analyses for 4 years, indicated that rainfall over Cherrapunjee occurred mostly in the early morning (0100–0700 IST). It was speculated the existence of the frontal zone with dry easterly or northerly to the north and maritime wet southerly to the south to explain the morning rainfall and the effect of topography along with the front near Cherrapunjee caused the extraordinary rainfall. Recently, observations using the tipping-bucket rain gauge revealed that severe hourly rainfall occurred during the night.

Analyzing the variations in annual, monthly and daily rainfall over Cherrapunjee was done, Cherrapunjee is located on the southern slope of the Meghalaya plateau, most of the rain flows directly down into Bangladesh; the amount of rain falling in the catchment area upstream of rivers is important for the forecast of severe floods in Bangladesh. By comparing the rainfall over Cherrapunjee and the water level in adjoining rivers in Bangladesh, The water level in Bangladeshi rivers is significantly related to rainfall over Cherrapunjee.

By utilizing the monthly rainfall data from January 1973 to December 2003 and daily rainfall data from June 2003 to July 2004, supplied by IMD. Figure 2 shows the annual rainfall variation at Cherrapunjee for 31 years from 1973 to 2003. The maximum rainfall was 24,555.3 mm in 1974 and the minimum was 6,950.3 mm in 1978. The average annual rainfall during this period (referred to as l) was 11,987 mm with a standard deviation (referred to as r) of 3,324 mm. Most of the rainfall occurs during the summer monsoon season, and the amount of rain in winter is much less. Rainfall begins to increase in March, with 300 mm, and the rainy season continues until October. The peak of monthly rainfall comes around July, at more than 3,000 mm. The variation of monthly rainfall during 1974 (the year of highest annual rainfall) is shown as a dashed line. The monthly highest rainfalls during the 31 years under consideration were recorded in the 4 months of April, July, September and October in that year.

Source:Fumie Murata, Taiichi Hayashi, Jun Matsumoto & Haruhisa Asada for Nat Hazards (2007)
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Saturday, May 15, 2010

Will there be a cyclone?? Will it cross Chennai around 21st May 2010

The US Centres for Environmental Prediction (NCEP), too, shows southwest Sri Lanka being hit during the week ending May 21. The NCEP has gone on to signal heavy precipitation over the Tamil Nadu coast and over the north-eastern States concurrently during May 21 to 28.

Meanwhile, the CPC points towards the possibility of enhanced convective phase of the Madden-Julian Oscillation (MJO) wave and numerical weather forecast guidance forcing tropical storm formation for the Arabian Sea during next week. The CPC outlook also said that the active MJO phase would trigger enhanced rainfall for the eastern Indian Ocean and the maritime continent during this phase as well as the southern tip of Sri Lanka. Interestingly, it sees possible tropical storm spinning up over the southwest Bay of Bengal thereafter.

This is exactly the scenario being envisioned by Prof Paul Roundy, a leading US-based ocean weather forecaster, to unfold around May 25.

The European Centre for Medium-Range Weather Forecasting (ECMWF) has predicted that the South-West monsoon may hit Sri Lanka around May 20, a week before schedule.

Source:Business Line
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Friday, May 14, 2010

Wettest places in India


By incorporating data from unofficial, non-reporting raingauge stations a list of locations with an average annual rainfall exceeding 5000 mm can be compiled for India. All stations are located along the westward exposed escarpment of the Western
Ghats at a distance of not more than 100 km from the Arabian Sea. Here the superimposition of la~'ge-scale dynamic and static, locational "controls" during the Southwest Monsoon period (June-September) results in annual, monthly and daily rainfall amounts that are only surpassed at a few stations outside the peninsula in Northeast India. The fourteen rainfall stations (see Fig. 1) whose mean annual rainfall is 500 cm (i.e. 200 inches) or more.Among these fourteen stations, there are only two stations (viz. Cherrapunji and Mawsynram) whose mean annual rainfall is more than 1100 cm. These fourteen stations have been called the rainiest stations of this country.

In addition to the heavy rainfall stations listed 7 other locations(with a minimum of 10 and up to 39 years' continuous records) were found to receive over 5000 mm of rain per year on an average. However the figures are not strictly comparable since they do not relate to identical observation periods; nevertheless, they indicate those areas in Peninsular India where rainfall of such quantity and intensity can be expected.

The most conspicuous feature regarding the spatial distribution of the 17 heavy rainfall stations in Peninsular India (Fig. 1) is their alignment along the westward exposed slopes of the Western Ghats (or Sahyadri Range) within a distance of not more than 100 km from the Arabian Sea and in close proximity to the (asymmetric) subcontinental "divide" or watershed which runs parallel to the west coast of the peninsula. While 9 stations are located at the western edge of the Deccan in Maharashtra and Karnataka, the remaining stations represent heavy rainfall areas in the "mountain blocks" of 'Tamil Nadu and Kerala further south: 4 in the Nilgiris, the junction of the Western and Eastern Ghats culminating in Dodabetta (2636 m), 3 in the High Range (or Kanan Devan Hills) next to Anaimudi (with 2695 m the highest peak in India south of the Himalayas) and finally Peermade in the Cardamom Hills, about 75 km from the Kerala coast.

Another characteristic of all stations falling into the "heavy rainfall" category as defined above is revealed by studying their annual rainfall pattern: a long dry season from November to April/May in the north and from December to March/April in the south, i. e. during the Northeast Monsoon period when "tropical easterlies" (or "trade winds") with large-scale subsidence and stability in the lower and middle troposphere prevail and when depression or cyclonic influences hardly ever reach these tracts of India. Rainfall during the dry season constitutes less than 5% of annual precipitation. At Mahabaleshwar, for instance, the average from November through April, for half the year, just reaches 100 mm and accounts for only 1.5% of the annual total.

Thus precipitation in the heavy rainfall areas in Peninsular india is mainly based on rainfall during a comparatively short rainy season. This "seasonal concentration" is the result of dynamic (weather) "controls" and locational factors interacting during the course of the annual monsoonal rhythm, the alternation of dry weather conditions during the Northeast Monsoon and the "rain-producing" synoptic patterns during the Southwest Monsoon.

Source:H. J. von Lengerke, South Asia Institute, University of Heidelberg, Germany
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Monday, May 10, 2010

Rainfall variation in Windward Vs Leeward sides of the Cardamom Hills (Western Ghats)

The data from Elappara (Windward side) and Bodinayakkanur (Leeward side) were used in this study. The first is instrumental data from the Elappara during 110 years (1896-2005), representing high altitude humid tropics in the Elappara range of cardamom hills (part of Periyar basin in Kerala facing the Arabian Sea). The station is one of the few hill stations in southern India, which has a record of more than 110 years of rainfall data. The station is located in high altitude (1500 msl) mixed tropical forest, which is a major type of forest in Western Ghats, where there is no clear seasonality in environmental factors such as rainfall and temperature. The other data set is from the Bodinayakkanur station located in Tamil Nadu, on the eastern slopes of the cardamom hills facing the Bay of Bengal (falling under semi-arid tropics) and provided 100 years of rainfall data (1906-2005). This rain gauge station is attached with TWAD (Tamilnadu Water and Drainage Board) board, Bodinayakkanur, Tamil Nadu. The Elappara range forms the western boundary of the cardamom hills' reserves (CHR). The Bodinayakanur range forms the eastern boundary of CHR in Tamil Nadu. The Chokkanadu and Kolukku Malai form the northern boundary. To the west of Elappara range, tropical rainy climate prevails. As one moves eastwards, the climate gets more arid, till it is regarded as completely arid in the region between Saptur and Usilampetti ranges. The CHR thus constitutes an area of 334 square miles, with Bodinayakanur and the Elappara are the representative stations for the two extreme ecosystems the humid and semi-arid tropics (Fig 1) respectively.

LEEWARD SIDE

Semi-arid slopes and the plains of Bodinayakkanur are characterized by a high temporal variability of seasonal and annual rainfall. The data show a drastic change in the rainfall pattern from seasonal to decadal scales (Fig 2), but no significant periodicity appears to be present. Over the last hundred years, the contributions of the ISM, NEM and BR (Blossom Rainfall) were 21.3%, 48.1% and 30.6% respectively. The highest rainfall of 2270.4mm was recorded in the year 1958 and the lowest rainfall of 328.5mm was reported in 1983. Mean annual rainfall for entire period was 767.1mm with a coefficient of variation (CV) of 36.9% and standard deviation of 282.8mm. Highest CV was reported for ISM (108.6%) followed by BR (44.30%) and NEM (40.3%). ISM rainfall ranged from19.82 to 1327.8mm while NEM rainfall varied from 83.8 to 773.3mm. BR also varied greatly from 32.9 to 665.4mm. Since CV was higher than 30% for all the seasons, the region experienced more frequent and severe droughts during the period. Higher Precipitation Concentration Index (PCI) (>20) was recorded only in the recent past three decades, which implies very long dry periods for up to 3-5 months. The PCI denotes uniformity of monthly distribution in a year over a particular station or the inter-annual variability. Therefore year-to-year variation in rainfall was considerable and showed definite decreasing trend during the last three decades. The index value was always below 20 for the previous decades.

The mean rainfall of the recent past three decades was well below the centennial average rainfall, clearly indicating decreased monsoon activity. This could be related to increase in tropical deforestation and land use change occurring in the areas surrounding this semi-arid ecosystem, like the Montane cloud forest high waives (Mehamalai) and the cardamom hills during the report period. No alternating pattern of increasing and decreasing rainfall in the decades is seen and no epochal trends in ISM as observed for the rest of the country. The 20, 30, 40 and 50 year running means showed declining trends (2.7 to -3.4 mm per year, with R2 values ranging from 0.42 to 0.85, significant at 95%). This means the future rainfall in the rain shadow slopes of the cardamom hills is likely to decrease. Though historical records say that the eastern slopes have enjoyed good rainfall, in the last century the slopes had lost their thick evergreen forest (90% of primary forest). Whether this has triggered or will trigger changes in rainfall pattern, questionable and sensitivity analysis performed elsewhere showed some teleconnected effects of deforestation in the precipitation change of south East Asia. El Nino and La Nina events have profound influence on the rainfall of this region. The most intense El Nino year in the last century was 1997, when the rainfall was well above the average. Nearly 58% of the El Nino years had below average rainfall. On the other hand 42% of the El Nino years accounted above normal rainfall. Amazingly the lowest rainfall of the last century was in 1983(328.5mm), which happened to be a typical La Nina year, and the effect was spectacular. 70% of the 17 La Nina years brought about below average rainfall and only 30% of the La Nina years experienced more than average rainfall. This shows a changing relationship between semi-arid rainfall climatology and ENSO in this region. Up to many decades ago, the entire area was under cardamom cultivation. For the past 8-10 years, the actual area under cardamom cultivation has shrunk to about 40,000 hectares. Relatively drought resistant crops like coffee and silk cotton are cultivated in the remaining areas. This is indicate the insufficiency of rainfall on the eastern and northern borders of CHR, and the severity of aridity is advancing towards up in the cardamom hills' reserves into the state of Kerala.

WINDWARD SIDE

In our analysis for the period (1896-2005) (Fig 4) the highest rainfall of 7000 mm (1961) was reported. This could be the second highest rainfall in the leeward hill slope of south-Western Ghats in peninsular India after Shimoga in Karnataka where in 8000 mm in a year was received. Careful study of the long term climate record has shown that even seemingly modest fluctuations in rainfall can create havoc in vulnerable societies. Orographic lifting favors the windward to the leeward side ofmountains for convection. However, a close examination of other gauge stations data of the cardamom hills (10-20 years data) indicate that the constant high rainfall (>5000mm in a year) is received near Anaimudi peak (2694 msl). The putative reason for the rainfall maximum in these mountains could be attributed to the strong convection, which undergoes diurnal cycle in which these mesoscale mountains play important role. However, the studies on the reasons for local rainfall maximum need careful observation and analysis besides understanding the physics behind convection-circulation interaction in the mesoscale mountains. The inter annual variations in the monthly rainfall values are very large; for example the values for January and February changed by as much as a factor of two or more from one year to another. The lowest rainfall during the period was reported in 1987 which was one of the two most intense El-Nino years in the world.

There was a strong association of El-Nino phenomenon with below average rainfall (67% of the El-Nino years), indicating a good ENSO relationship. In the case of La-Nina years, fifty per cent of the years enjoyed excess rainfall and rest of the years were low rainfall years. However, the impact of El-Nino phenomenon was not severe during the first pentad of this century in this mountain range. This shows the relationship seems to have weakened in the recent past. Considering these, the scientists concerned should take precautions in forecasting local climate. The coefficient variation of rainfall (CV) for the entire series ranged from 9 to 27%. The decadal averages for the first three decades (1896-1925) and the last three decades (1976-2005) studied, reported values less than the annual mean (5000 mm) for the whole period (1896-2005). Only during the middle period (1926-1975), the decadal average rainfall did exceed the series annual mean. Interestingly the strongest El-Nino years (1997-1998) had received (5120 mm) fairly well above the annual mean. Since 1896, of the total El Nino years, only ten years each in El-Nino and La-Nina years had envisaged less than average. Otherwise all other El-Nino and La-Nina years found to be more than the annual average (5000 mm). These results are found to be for and contradictory to the results reported for many regions of varied environment in India Therefore, extreme rainfall anomaly can occur even in the absence of strong ENSO events, thus uncertainty in ENSO effects is quite large from one environment to another even that are close to each other. The production trend of major spices and plantation crops shows a negative trend for the first one and half a pentads (2000-2007) of this century. Cardamom reported a maximum percentage change of -12.5.1% followed by black pepper (-10.0%) in CHR. In particular, tea production in the cardamom hills has gone down significantly reporting a percentage change of -20% during the recent past seven years. Therefore, this unhealthy trend of production of specific crops is certainly due to the change in the climatic condition which affects both quantity and quality of crops.

Though at this point in time the cardamom hills receive more rainfall than the crops require (for the successful production of tea and cardamom), in the near future the cardamom and tea ecosystem hydrology may get affected as the demand for water is expected to raise many fold. Already many stream flows have been found to decrease and become short lived. Thereby, the ecosystem degradation could be more pronounced at than the present level because cardamom is very sensitive to drought conditions. The most semiarid eastern slopes of the cardamom hills are experiencing down ward march of rainfall. Under the decreasing rainfall pattern coupled with continuing atmospheric warming scenario, the near future could jeopardize the agriculture and sustainable development in this resource (water) poor semi-arid ecosystem, and the coming years could be highly challenging to climate and agriculture scientists as the demand for water would increase by 20-80% in this densely populated valley. Already half a million people from the eastern slopes have moved to western slopes in search of livelihood and the eastern slopes now look like a desert, with at least100 sq.km area (Thevaram- Bodi tract)which have been rendered into a desert like system. Immense ecological degradation and dangerous climatic variability of cardamom agriculture has already been an immutable truth elsewhere in the world. Therefore, understanding rainfall variability in these limited ecosystems is important.

Source:M.Murugan, V.Mukund, R.Ramesh, M.B.Hiremath, A.Josephraj kumar and P.K.Shetty for Caspian Journal of Environmental Sciences, University of Guilan, Iran
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Saturday, May 8, 2010

Rainfall around Bhavani River basin

The Bhavani and Moyar rivers flow together into the Bhavanisagar reservoir after which it becomes the Bhavani River (Fig. 1). The Bhavani River is a tributary of the Cauvery, one of the largest rivers in India. This study refers to the combined catchment area of the Bhavani and Moyar rivers, which are referred to as the Upper Bhavani river basin. The basin (4100 km2) is a high altitude area at the confluence of the Eastern and Western Ghats, bounded to the north by the state of Karnataka,to the west by Kerala to the east by the Bhavanisagar reservoir and to the south by the Coimbatore plateau. The elevation of the river basin ranges from 300 m a.m.s.l. on the plains to 2600 ma.m.s.l. on the Nilgris plateau. Accordingly, the annual rainfall varies from 700 mm on the lowlands to nearly 3000 mm in the hills. The topography is undulating in the lower plateau and uplands while rugged in western parts. The vegetation coverage is a mosaic of grassland, deciduous and coniferous mountainous forests, plantation and agricultural crops.

The main part of the basin has a humid equatorial environment, although the lowland plains are sub-humid. The weather is dry from January to May whereas heavy rains are common during the southwest (SW) monsoon (June-September), and the northeast (NE) monsoon (October-December). The SW-monsoon dominates in the western part of the river basin while the NE-monsoon prevails in the eastern region, due to the hills running in a north-south direction which act as a barrier to both monsoons. Both the western and eastern areas can therefore be regarded as rain shadow areas during one of the monsoon periods.



Source:Julie Wilk & Lotta Andersson,Department of Water and Environmental Studies, Link├┤ping University,Sweden

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Wednesday, May 5, 2010

Northeast Monsoon in Tamilnadu

Tamil Nadu, located in southeast Peninsular India receives the major part of its annual rainfall during the northeast monsoon(NEM) season (the three-month period from October to December). While coastal Tamil Nadu receives about 60% of its annual rainfall, interior Tamil Nadu receives about 40–50% of annual rainfall during NEM season (IMD 1973). In comparison with Indian Summer Monsoon (ISM), the NEM is characterized by limited aerial extent and average lesser rainfall amount. During NEM season, Tamil Nadu generally receives rainfall due to the formation of trough of low, cyclonic circulation, easterly waves, low pressure area, depression and cyclonic storm over Bay of Bengal. Because the NE monsoon season is the major rainy season, the vicissitudes of the rainfall of Tamil Nadu state has led to considerable and widespread interest among the public/farmers and in government circles in recent years, in view of the frequent failure of northeast monsoon rainfall (NEMR) over Tamil Nadu and the consequent water scarcity condition.

The NEMR series of Tamil Nadu during 1901–2004 was prepared as follows: The daily rainfall data of 173 stations in Tamil Nadu for the period 1901–1996 was obtained from National Data Center, Pune. Using this daily data, the seasonal rainfall series of Tamil Nadu subdivision for the period 1901–1996 was constructed. The data for the remaining part of the series was collected from the weekly weather report of Area Cyclone Warning Centre, Chennai (Tamil Nadu). As per IMD criteria, excess (deficient) monsoon years for subdivision is defined as whenever the percentage departure of seasonal rainfall from normal of subdivision is more (less) than +19% (−19%). Using this criteria, the excess and deficient NE monsoon years during the period 1901–2004 were identified. During this period there are 22 excess NE monsoon years and 25 deficient NE monsoon years and these are listed below.


Tamil Nadu receives mean rainfall of about 450mm with coefficient of variation 28% during NEMR season. Figure shows the mean rainfall distribution over Tamil Nadu during NEMR season. During the entire NEM season, northeast coastal Tamil Nadu receives rainfall exceeding 600mm. Generally rainfall decreases westwards from coastal area to inland and from north to south. Figure shows the NEMR of Tamil Nadu during the period 1901–2004 expressed as percentage departure from long term normal. Out of 104 years of present study, NEMR over Tamil Nadu was below normal for 54 years (i.e., about 53%). The NEMR over Tamil Nadu was continuously below(above)long term average for a maximum of 9(4) years. Ropelewski and Halpert (1987) found that NEMR over south Tamil Nadu is enhanced (diminished) during the well-known El-Ni˜no episodes that refer to the events of warming (cooling) over central and equatorial Pacific Ocean. Out of 24 years of El-Ni˜no during the period of study, NEMR was normal or excess during 23 years (except during the year 1951).

Source:S Balachandran, R Asokan and S Sridharan, Regional Meteorological Centre, Chennai.
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