Download 1 CHAPTER 1: INTRODUCTION Atmosphere is an

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CHAPTER 1: INTRODUCTION
Atmosphere is an envelope of gases surrounding the earth. Due to the effect of
insolation there is a constant change in the atmosphere and the manifestations of the
changes in atmospheric state is the weather. The effect of weather on human activities is
very significant. Weather can affect the human comfort and also his planned activities.
The history of weather is called ‘Climatology’. Rainfall is essential for agricultural
production and food . Solar radiation affects humans, animals, plants and the entire
biosphere. In fact sun is the source of energy for earth. The winds are responsible for
mixing of moisture and moisture content affects humans, food items etc. The structural
design of houses, location of industries, airport and harbour activities are decided by the
prevailing weather and climate conditions. Hence man from time immemorial has shown
interest in weather changes and knowing about the atmospheric conditions. Therefore
meteorology is one of the oldest sciences of the world., one can say, like medicine. That’s
the reason why national weather forecasting service is regarded to be one of the essential
services in all the countries.
1.1. WEATHER FORECASTING
. Weather forecasting requires knowledge of meteorology, which comprises of the
principles and methods of Mathematics, Physics and Chemistry. Weather forecasting can
also be said as an art and science because of its complicated nature. Atmosphere is a
dynamic medium and its state is constantly changing due to the mixing process
(Petterssen 1958). The energy changes due to the atmospheric dynamics manifest
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themselves in different forms of weather phenomena (Holton 2004). Particularly in the
tropical region of the earth (region encompassed between about Lat.30 N and Lat 30 S)
the atmospheric changes are comparatively more and fast (Asnani 1993; Buyers 1994;
Richard 1981).
Weather is a global phenomenon. Various parameters are there to describe the
weather conditions. The important meteorological or weather parameters are:
temperature, pressure, winds, humidity etc. Meteorological stations measure the
parameters on a real time basis daily to get the overall field distribution for the
parameters. These measurements form the input to the weather charts of the
meteorologists. By analyzing these charts professional meteorologists study the condition
of weather, identify weather systems and forecast future weather.
Indian subcontinent has a vast area and it has a big peninsular area in south
coming well within the tropical regions. Weather forecasting is done in India as well as in
other parts of the world based on ‘synoptic models’ (Bosart 1985).
That is,
meteorological measurements are taken at different observations spread over the country
or continent at the same time. They are then viewed together on the weather charts.
Weather systems are identified and from the knowledge of the dynamics and
characteristics of the weather systems, forecast are issued for the region or the country.
After the advent of computers numerical models are used which also needs large
observational input (Lynch and Peter 2008).
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1.2. SPOT SPECIFIC FORECAST
The operational forecast is issued on qualitative terms like ‘isolated’, ‘scattered’,
‘fairly widespread’ and ‘widespread’ occurrence of a weather event. Isolated means the
probability of the occurrence of weather event could be 1-25 % of the region and the
probability of the occurrence would be 26-50 %. of the area for’ scattered’. The fairly
widespread forecast would occur 51-75 % and the widespread weather forecast will occur
over 76-100 % of the area based on their synoptic situation. Let us take a forecast that
“scattered rain likely over a particular region”. This means 26-50 % of region or the
stations belonging to the concerned territory are to get rainfall. So even when three out
of eight stations reporting rain, the forecast is theoretically correct. But this may not
appeal for the commoner in the remaining five stations of concerned region where there
is no rain at all. Hence a method is needed to give location specific and date specific
forecast of event like the occurrence of rain or not at say, Chennai or Trichy on a
particular date etc., That will be more meaningful and useful practically. Such forecast
is being tried for a few major cities (www.weather-forecast.com).
The necessity for such spot specific forecast is increasing day by day. For
example test matches or sports activities are planned over a place and forecast for exact
town for a particular day is required. Tourism is a growing industry day by day. To plan
effectively weekend trips to places of tourist or religious importance, picnic spots etc
meteorological forecast is sought. Nowadays planning of special campaigns, visits of
government dignitaries etc also need weather information. Weather forecast is also
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required for arrangements during national day celebrations like Independence day at the
capitals, special carnivals at selected locations and religious congregations over
pilgrimage centers. Hence a method of forecast of rain or cold/heat wave at specific
locations is the need of the hour.
1.3. MONSOONS IN INDIA
India is a monsoon country. Though ‘monsoon’ means ‘a reversal of wind
direction it has got its own significance (Das 1995; Ramage 1971). Major rainfall is
realized during the monsoon season (Attri and Ajit Thyagi 2010). It is important not only
for agriculture but also for flora and fauna and the collection of water necessary for dayto-day living. According to the India Meteorological Department, it is the seasonal
reversals of wind direction along the shores of the Indian Ocean, especially the Arabian
Sea (Rao 1976). It blows from the southwest (SW) during half the year and from the
northeast (NE) during the other half. The southwest monsoon in India is categorized as
the Arabian Sea branch and the Bay of Bengal branch (Ananthakrishnan et al 1968). The
monsoon is a typical tropical phenomenon, with the Indian subcontinent lying northwards
of the equator. Huge air mass from southern hemispheric Indian ocean crosses equator
and due to the coriolis force moves towards the Indian landmass from June to September.
These are rain-bearing winds. When they touch the southern most tip of the Indian
peninsula they split into two branches, such as the Arabian Sea branch and the Bay of
Bengal branch. Then around October, these winds change direction and begin to blow in
the northeasterly direction. This is now a land to sea flow that is from the subcontinent to
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the Indian Ocean. At this point they carry less moisture and bring rain to only certain
parts of the subcontinent, that is coastal Andhra Pradesh and Tamil Nadu states of India.
This is the North East monsoon and is responsible for maintaining the rice bowls of south
India.
There are some essential factors required to cause these rains. The most important is
the unique geographical features that the subcontinent enjoys. The presence of abundant
water bodies around the land mass is essential. For that we have the Arabian Sea, the
Indian Ocean and Bay of Bengal. These help in accumulating moisture in the winds
during the hot season. Southwest monsoon brings in the rainfall to most parts of the
country. Tamilnadu state and adjacent areas are in rain shadow region of Western Ghats
and hence they get only less rain during the period. But during October to December
when the wind reversal takes place, northeasterly winds prevail over Tamilnadu and most
parts of the peninsula (Duraisamy 1946; Ramaswamy 1972; Rao 1963). This period is
called northeast monsoon season. Troughs and low-pressure areas from Bay of Bengal
cross over the east coast and affect Tamilnadu and nearby region. Hence Tamilnadu gets
good amount of rainfall during the period (Srinivasan and Ramamurthy 1973). The
agricultural operations have to be suitably planned due to this rain in this part of the
country. A lot of work has been done on south west monsoon (Agnihotnri et al 2002;
Alexander et al 1978; Anandakrishnan 1968, 1981, 1988; Basu 2007; Bhalme 1986;
Cadet 1979; Chiew et al 1998; Gadgil 1988; Gowarikar et al 1989,1991; Hastenrath
1987; Koteswaran 1958; Lau 1992; Majumdar et al 2001; Mooley 1983; Mukherjee and
Sivaramakrishnan 1976, 1982a, 1982b; Raghavan et al 1983, 1987; Raj and Jamadar
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1990; Ramage 1971; Shukla and Pavolino 1983; Sivaramakrishnan 1982,1992).
The
onset dates have been established for different parts of the country. The mechanism of
rainfall has been understood and the synoptic situations have been established. There are
also several models to forecast the seasonal total rain of SW monsoon. Correlations with
global events like ElNino, ENSO, Siberian High pressure cell etc have also been worked
out (Gowarikar et al 1991; Parthasarathy et al 1996; Vinayachandran 2004). However
being a small level circulation not much work has been done for NE monsoon. A few
attempts on the rainfall analysis, occurrence of heavy rain and the cyclonic storms have
been made (Balachandran et al 2006; Dhar and Rakheja 1983; Kripalani and
Pankajkumar 2004; Nayagam Lorna 2009; Pankaj Kumar et al 2007; Seetharam 2009;
Sivaramakrishnan et al 1983, 1985; Sivaramakrishnan 1989; Sivaramakrishanan and
Sridharan 1987; Sivaramakrishnan and Suresh 1997; Sridharan and Muthuchami 1990;
Zubair and Ropelewski 2006).
Y.E.A. Raj (2003) has investigated the onset and
withdrawal of monsoon along the east coast during the last century while Krishnan
(1984) has studied the long term trends in rain. Hence if some forecast regarding the
seasonal rain amount of NE monsoon is possible that will go a long way for planners in
agriculture and water managers, since the rainfall distribution shows a large variability.
1.4. CAUVERY DELTA
Cauvery is an important river through Tamilnadu irrigating major portions of
Thanjavur district, the rice bowl of Tamilnadu. This flows from west to East. Once prior
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to construction of large dams copious water was flowing during SW monsoon from
neighboring Karnataka state. But with increase in the number of dams along the river and
urbanization in the upper parts of the riverbanks, copious flow is not guaranteed which
has made Cauvery delta farmers to now depend much upon the NE monsoon rain and
plan their agricultural operations. Predicting the NE monsoon rainfall will be useful to
sowing and harvesting operations in agriculture fields and also for water managers to
plan urban drinking water supply schemes. It will also help in water conservation if
monsoon is not likely to be upto the mark.
1.5. RAINFALL AND TEMPERATURE
The predisposition of a climate to storms producing excessive precipitation is the
fundamental basis of the flood hazard. In some climates flood-producing storms occur
irregularly; in others they follow a seasonal pattern. Two types of storm are the initiating
agents for most rain caused floods: the violent thundershower, which is of short duration
and produces flash floods, and the prolonged, general rain which, through sheer quantity
of water, creates extensive flooding over entire watersheds. The flash flood is common
only in those regions, which experience heavy thunderstorms, that is, where unstable
maritime air masses are frequent visitors. However, the occasional thundershowers of
arid climates also lead to destructive floods. The thunderstorm may be of convective,
frontal, or orographic origin; so far as flooding is concerned, the important feature is its
intensity, not its origin.
The seasonal distribution of precipitation is the principal
determining factor in the regimes of rain-caused floods. Hence a quantitative estimate
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with a threshold value for rainfall may help to determine whether it is a significant rainy
day or not. The rainfall is determined with the threshold value of 2.54 cm or 1 inch and
any rainfall equal to or greater than 2.54 cm or 1 inch is taken as a significant rainy day.
The threshold value has been determined as per requirement of various user agencies as
said by the weather service personnel.
The principle climatic factors affecting crop production are the same as those
influencing all vegetation that is temperature, length of growing season, moisture
conditions, sunlight, and wind, but they must be considered in a different light with
respect to crops. In their influence on crops, the climatic factors are closely interrelated.
The others modify the effect of each. Daily climatic elements are of importance in
determining the efficiency of crop growth. In considering the conducing environment of
plants it is important to keep in mind that the microclimate immediately around the plant
is of vital significance. Similarly, moisture, light, and wind effects may be quite different
at and near ground level from conditions in the free air above. Human comfort (Rong et
al 2004) is affected seriously by extremes of temperatures. Summer maximum
temperature if it exceeds certain threshold value may cause discomfort. Similarly cold
wave conditions in winter also cause severe hardships. Hence a method which can give
spot specific forecast for a very hot day occurrence or not in summer as well as very cold
day occurrence in winter will also be useful in operational forecasting.
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1.6. SCOPE OF THE WORK
Decisions involving weather and climate affect every economic activity in some
way. The relationship between atmospheric conditions and representative problems in
transportation, communications, manufacturing, and construction fields of human
enterprise entails decisions at both the planning and the operational stages. Hence the
application of meteorology and climatology for improvement of efficiency in these fields
is most important. Whereas climate often is an important factor in long-range planning,
like in the selection of a factory site etc, weather has a dominant role in daily operations.
1.6.1. MOTIVATION OF THE STUDY
It is a common experience of operational forecasters that data for some
observatories are missing due to communication failures etc. and accuracy of synoptic
forecast depends on the data population. Hence a forecast based on ‘in situ’ data for any
station, if developed, will be helpful to the operational forecasters.
Again while climatic pattern forecast is available for the peninsula as a whole, no
specific seasonal forecast model has been developed for Cauvery delta.
The
agriculturists of Thanjavur district of Tamilnadu state need it very much. Hence these
two points are addressed in this work.
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1.6.2. OBJECTIVES OF THE STUDY
The main objectives of this work are
1) To develop some effective method for estimating the Northeast monsoon rainfall
over the Cauvery delta region within Tamilnadu which is the main rainy season.
2) To develop a potential method to forecast the dry and wet day occurrence at
specific spot or station during the Northeast monsoon season in real time basis;
also to forecast whether it is going to be a significant rain on the rainy day or not.
3) To develop a method to forecast the rainfall and wet / dry day in the summer
monsoon months of June to September over Tamilnadu similarly.
4) To attempt forecasting the summer maximum and winter minimum temperatures
over Tamilnadu with respect to threshold value.
1.7. ORGANIZATION OF THE THESIS
First chapter introduces the topics of weather forecast and Indian monsoon and
speaks about the objectives. Second chapter discusses the data mining techniques and its
applications. Third chapter deals with pattern visualization and the forecast of seasonal
rainfall for Cauvery delta region. Fourth and fifth chapters describe the spot specific
forecast for different types of stations during NE and SW monsoon seasons respectively.
Sixth chapter deals with prediction of extreme hot days during summer and extreme cold
days during winter. Seventh chapter states the qualitative and quantitative conclusions
and discussions.