Doppler Weather Radar analysis of short term cyclonic storm

On the east coast of India, during South-West monsoon period severe cyclonic storms are very rare and if they are short term cyclones then their prediction becomes very difficult due to rapid change in the intensity of the system. Though synoptic observations failed and satellite observations also cannot give decisive picture about such systems, in that case timely warning can not be issued by the weather agencies. Such a system was formed on 19 September, 2006 at about 250 km South-East of Kolkata (India). Very heavy rainfall associated with the system caused several human casualties and extensive damage to the property. According to news agencies, more than 100 people died and a million people became homeless due to heavy rainfall and strong winds associated with the cyclone during 19 September -21, 2006. At 0600 UTC, Doppler Weather radar (DWR) at Kolkata observed initial signatures of the system like a depression. Subsequently at 0900 UTC the observations indicated that the intensification of the system has taken place to a higher stage of deep depression and at about 1200 UTC clear spiral bands with a circular eye recorded by DWR confirmed for a fully developed severe cyclonic storm. The system weakened in to a deep depression at 1630 UTC after the landfall but again became a cyclonic storm at 2100 UTC of 19 September, 2006. Present study establishes that DWR is very useful for prediction of this short term cyclonic storm, its direction of movement and heavy rainfall associated. The maximum radial winds of the magnitude 32 m/s (64 knots/115 km/h) were also recorded by DWR at an altitude of 2.5 km in the eye wall region of the system. The high wind speed and the well defined structure of the cyclone observed by DWR confirmed that the system was a Severe Cyclonic Storm of T number 3.5. Records are available with surface observatories in the region for strong winds of the order of 110 km/h. This study also revealed that an early warning for strong winds and heavy rainfall could have been issued for development of such a short duration tropical cyclone using DWR data well in advance.

An insight into the severe floods in India during 2005, 2006 & 2007

In this paper severe flood during South West monsoon seasons of 2005, 2006 & 2007 have been identified with the related synoptic features. A total of 22 states reeled under severe floods during 2005, 2006 & 2007. In the south peninsula only Karnataka and Kerala were affected by severe floods. During 2006 & 2007 severe floods continued for a longer period and affected many states where as during 2005 severe floods affected most of the states except peninsular India but they continued for short periods comparatively.Formation of excess No’s of low pressure areas and depressions in the Bay of Bengal during July, Aug and Sept 2006, in succession , and their WNW movements upto M.P. and east Rajasthan caused severe floods in Gujarat, Maharashtra, Rajasthan, Madhya Pradesh, Chattisgarh, Orissa and Andhra Pradesh for a longer period i.e. from 28 July to 12 September 2006. During 2007, Shifting of monsoon trough, presence of secondary monsoon trough and trough in westerly, caused severe floods in most of the North eastern states from 12 July to 10 October 2007.

Spatial and Temporal Assessment of Rainfall Variability using GIS Approach in Trichy District of Tamil Nadu

Precipitation is one of the transportation components in hydrological cycle. The magnitude of precipitation swings with time and space. Majorly India receives precipitation in the form of rainfall. Precipitation plays a key role in the rainfed agriculture. The present study deals with the rainfall characteristics of Tiruchirappalli district, Tamil Nadu. Seasonal rainfall data from eighteen rain gauge stations (1971-2012) have been taken for analysis of seasonal and annual rainfall pattern of Tiruchirappalli district. Mean rainfall of the district is about 696 mm. The highest rainfall of 1247 mm recorded in the year 2005 and the lowest precipitation of 227 mm recorded in the year 1976. About 48 percent and 35 percent of the rainfall received in North East and South West Monsoon, respectively. Spatial rainfall distribution was studied with the help of Kriging interpolation technique and respective maps were prepared with Geographical Information System. The percentage departure of annual rainfall is classified under the category of excess, normal and large excess category. South East and central part of Tiruchirappalli receives moderate to low rainfall. North East parts of Tiruchirappalli district such as pullambadi, Lalgudi and nearby areas received maximum rainfall during North East Monsoon and South West Monsoon. In winter season Manapparai and Vaiyampatti region received more rainfall while in summer season Thottiam and Mayanur area received more rainfall. The two major highlighted crops in Trichy district are Banana and Onion. Tiruchirappalli district is one of the Banana growing belts in Tamil Nadu. Spatial distribution of rainfall maps will be helpful to form a crop plan for different crops to increase the agricultural productivity of Tiruchirappalli district and to ensure the food security.

Climatology of dry and wet spell over Vidarbha region during monsoon months

In this investigation 10-day period-wise simple probability, 10-day period-wise probability of consecutive dry and wet spells of different lengths, and month-wise different parameters, and properties of Markov Chain Model over Vidarbha region during south-west monsoon months have been studied. For this purpose, daily rainfall data (1 June – 30 September) of 11 stations covering all the districts of Vidarbha for the period 1960-90 have been utilized. The study reveals that over Vidarbha during monsoon season (June - September) probability of a day being wet and probability of consecutive wet spell of different lengths are by and large high during the last and first 10-day periods of July and August respectively when the monsoon is at its peak. During the first two 10-day periods in June and last two 10-day periods in September, the probabilities of a dry day and that of consecutive dry spell of different lengths and quite high. During July and August a maximum of 12-14 wet days are expected and wet spell, on an average, lasts for 2 days. Stationary probability of the occurrence of wet day (pi2) is found to be maximum during July making it the most humid month in the monsoon season.

Variability of south west monsoon rainfall in West Bengal : An application of principal component analysis

The principal component analysis is utilized to understand the spatial and temporal variability of monsoonal rainfall. The southwest monsoon rainfall data of West Bengal, situated over 21 stations widely spread over the state, has been analyzed for a period of 60 years for inter-annual variations. A coherent subset of 8 north and 13 south stations has been studied separately to produce statistically significant inter-annual signals. It is observed that the above/below transition is quite significant both for station rainfalls and principal components for state-wise and coherent zone analysis.

Wind steadiness up to 35 km and its variability before the southwest monsoon onset and the withdrawal

This paper brings out mainly on the quantitative approach to delineate wind direction variability through Wind Steadiness Factor (WSF) - a single parameter which depends on height, wind speed and wind direction. This can be used as a prognostic parameter for the onset and withdrawal of south west monsoon (SW Monsoon) over Kerala. A brief sketch on wind climatology up to 35 km over TERLS (8° 32' N / 76° 52' E) is also discussed to have a background knowledge. From the derived WSF climatology, it is seen that the region between 12.5 km to 18 km is of highest WSF during the SW Monsoon due to the Tropical Easterly Jet (TEJ). Vertical variation of annual WSF has shown well demarcated four layered structure and the wind rose constructed for each layer provides the contribution of WSF attributed by the dominant direction for that particular layer. A WSF value was estimated for the region between 12.5 km to 18 km over Thiruvananthapuram for each available rawin profile [0530 and 1730 hrs (IST)] and inferred that an early incidence and maintenance of WSF well above 80% prior the SW Monsoon supports an early SW Monsoon onset and reverse for a late onset. A late/early WSF decrease from a value of 80% followed by systematic further decrease is associated with late/early withdrawal of the SW Monsoon.

Present operational long range forecasting system for southwest monsoon rainfall over India and its performance during 2010

At present, India Meteorological Department (IMD) issues various monthly and seasonal operational forecasts for the south-west monsoon season using models based on latest statistical techniques with useful skill. Operational models are reviewed regularly and improved through in house research activities. For the forecasting of the south-west monsoon season (June – September) rainfall over the country as a whole, a newly introduced statistical ensemble forecasting system is used. In addition, models have been developed for the forecast of the monsoon season rainfall over four geographical regions (NW India, NE India, Central India and South Peninsula) of the country and forecast for the rainfall over the second half of the monsoon season over the country as a whole. Models have also been developed for issuing operational forecast for the monthly rainfall for the months of July, August & September over the country as a whole. Operational forecasts issued by IMD for 2010 south-west monsoon rainfall have been discussed and verified. In addition, the experimental forecasts for the season rainfall over the country as a whole based on bothstatistical and dynamical models received from various climate research institutes within the country other than IMD arealso discussed. The operational monthly and seasonal long range forecasts issued for the 2010 southwest monsoon season for the country as a whole were accurate. However, forecasts for the season rainfall over the 4 geographical regions (Northwest India, Central India, Northeast India and south Peninsular India) were not accurate as the forecast for South Peninsular India overestimated the actual rainfall and that for northeast India underestimated the actual rainfall. The experimental forecasts for the season rainfall over the country as whole from various climate research institutes within the country showed large variance (91 % - 112% of LPA).

QPF Model for lower Yamuna catchment, synoptic analogue method

The paper formulates a synoptic analogue model for issuing Quantitative Precipitation Forecast (QPF) for Lower Yamuna Catchment (LYC) based upon eleven years data (1998-2008) during southwest monsoon season. The results so derived were verified with realized Average Areal Precipitation (AAP) for the corresponding synoptic situation during 2009 southwest monsoon season. The performance of the model was observed Percentage Correct (PC) up to 86 % and for extreme events showed 100% correct with Heidke Skill Score (HSS) value 0.9. The experience during south west monsoon 2009 has shown that Synoptic analogue model can produce 24 hours advance QPF with accuracy and greater skill to facilitate the flood forecasters of Central Water Commission.