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The paper presents the impact of premonsoon meridional sea surface temperature (SST) gradient of a part of north Indian Ocean (84°E–92°E, 0°N–18°N) and frequency of north-west Pacific weather systems (mainly typhoons and super-typhoons) on the formation of monsoon depressions over Bay of Bengal. Analysis shows that decreasing frequency of monsoon depression over BOB may occur as a result of decreasing trend of premonsoon meridional SST gradients over the region 84°E–92°E and 0°N–18°N. Higher frequency of north-west Pacific weather systems (i.e. higher the total number of typhoons and super-typhoons) during the monsoon month (June, July, August, September-JJAS) may be considered as one of the important aspect for the reduction in monsoon depression frequency over BOB (mostly, during last decade). Tropical easterly jet (TEJ) strength (60°E–100°E, 5°N–20°N) is also analysed to check the relationship between convection over BOB and monsoon circulation pattern which is dependent on SST gradient as well as north-west Pacific weather system frequency. Possible explanation is also presented.

The Indian summer monsoon rainfall forecast and its verification has a direct impact on various sectors of public interest besides economy of the country. The present study highlights the verification of distribution forecast of synoptic method issued daily for six met subdivisions, comprising of five states of eastern India namely West Bengal, Sikkim, Bihar, Jharkhand and Odisha. Three years monsoon season rainfall data from 2011 through 2013 are used for the study area. The distribution-oriented verification is done for different rainfall classes like dry, isolated, scattered, fairly widespread and widespread to understand the usefulness of the synoptic method. Statistics are presented for both combined classes of Percentage Correct (PC) and Heidke Skill Score (HSS) of the met subdivision wise forecast and PC, POD and CSI for individual classes. It has been observed that among the met subdivision the efficiency of the method is highest in Sub Himalayan West Bengal (SHWB) & Sikkim followed by Gangetic West Bengal (GWB), Odisha, Jharkhand and Bihar.

In this paper, a simple relationship is employed to investigate the relative impacts on the movement and landfall of tropical cyclone in the Bay of Bengal region when wet component of atmospheric refractivity of different tropospheric levels is used as an input. Four tropical cyclones during pre-monsoon and post-monsoon season over the Bay of Bengal region were selected for the study. The radiosonde/radiowind data of coastal stations, namely Kolkata (Dumdum), Agartala, Bhubaneswar, Visakhapatnam, Machlipatnam, Chennai and Karaikal, were collected for the period of the cyclones under study. The wet component of atmospheric refractivity of different standard levels was plotted against time for the stations for every cyclone. The study suggests that the cyclone moves towards and crosses near the station having a relatively steeper increase in the wet component of atmospheric refractivity up to mid-tropical level. A possible explanation is also presented.

Tropical cyclones (TCs) are synoptic scale intense low pressure systems which form over the warm tropical oceans characterised by strong cyclonic winds and organised convection with heavy rainfall. The TC causes enormous damage to life and property at the time of crossing the coast and subsequent movement over the land. The TCs can impact over a wide area with its strong winds, heavy rains and storm surges. Gray (Hurricanes: their formation structure and likely role in the tropical circulation. In: Shaw DB (ed) Meteorology over the Tropical Oceans. Royal Meteorological Society, Bracknell, pp 155–218,1979) prepared a detailed climatology of the TCs in the global ocean basins. He observed that annually about 80 TCs form globally of which half to two thirds reach hurricane strength (maximum sustained winds greater than 33 m/s). East coast of India is frequently affected by TCs that form over Bay of Bengal (Tyagi et al., Inter-annual variation of frequency of cyclonic disturbances landfalling over WMO/ESCAP Panel Member Countries. WMO/TD-No. 1541 on 1st WMO International conference on Indian Ocean tropical cyclones and climate change, Muscat, Sultanate of Oman, 8–11 March 2009, WWRP-2010/2, 2010). During the period 1891–2010, about 134 severe TCs crossed the east coast of India (IMD, Cyclone e-Atlas-IMD tracks of cyclones and depressions over North Indian Ocean. www.rmcchennaieatlas.tn.nic.in, 2011; Atlas of storm track). Anthes (Tropical cyclones: their evolution, structure and effects. Meteorological Monographs, American Meteorological Society, Boston, 1982) has extensively studied the three dimensional structure of TC. He observed that the surface pressure is lowest at the centre of the TC and increases outward. The wind speed reaches its maximum value at nearly 40–80 km from the centre beyond which it decreases. The central parts are warmer than the surroundings and the temperature anomaly could be more than 10° K at the upper troposphere. The TC grows upto a height of 10–15 km. Intense TCs frequently develop an eye, which is a cloud free region at the centre of the storm characterised by the presence of subsidence.

In this paper an attempt has been made to study climatological characteristics and forecasting aspects of heavy rainfall over Kolkata for data of 34 years of period from 1974 to 2007. Total 184 events has been found out and the data set has been subjected to various types of analysis along with favourable synoptic system and critical index for occurrence of heavy rainfall over Kolkata. Average occurrence is found as 5.4 events per year. Monthly distribution shows maximum of 26% events in July followed by September 20%, August17% and June as 14%. Seasonal distribution naturally indicates maximum of 77% occurrence during monsoon followed by post-monsoon with 14% and pre-monsoon with 09 %. Synoptic analysis revealed that majority of heavy rainfall events occurred due to low pressure system (LPS). Study of 167 cases (during June to October) suggests that when any one of the favourable synoptic condition prevailed over the region and DPD-Wind-PW-WS index reaches a critical value, heavy to very heavy rain occurred over Kolkata and suburban areas.

In this paper a simple relationship is employed to investigate relative impacts on the movement and landfall of tropical cyclone in the Bay of Bengal region when geopotential height of different troposphere levels is used as an input. Five tropical cyclone during pre-monsoon and post-monsoon season over the Bay of Bengal region has been selected for the study. The RS/RW data of coastal stations namely Kolkata (Dumdum), Dhaka, Agartala, Bhubaneswar, Visakhapatnam, Machlipatnam, Chennai and Karaikal has been collected for the period of the cyclones under study. The geopotential height of different standard levels has been plotted against the time for the stations for every cyclone. The study suggests that the cyclone moves towards and cross near the station having relatively steeper decrease in geopotential height upto mid tropical level followed by increased in geopotential height.

Climatologically thunderstorm activity is quite common features over Guwahati Airport and its vicinity during premonsoon season. But a severe hailstorm with large size of hailstones and long duration is a rare event. Such a hailstorm with hailstone size of about 25mm occurred over Guwahati Airport (26.1°N, 91.6°E) on 2nd April, 2006 during midnight. In this paper an attempt has been made to investigate prerequisite synoptic and thermodynamic conditions of the atmosphere associated with this hailstorm. Under favourable synoptic and instability condition of thunderstorm occurrence, veering of winds over Guwahati between freezing level and 500 hPa level with vertical wind shear of horizontal winds exceeding 6.2 ms-1km-1 more appears to be conductive for the development of a hailstorm. Stability Indices viz., Showalter, K, Total-Total and Severe Weather Threat (SWEAT) index have shown predictability for this event.

Premonsoon season in north-east India is known for thunderstorm activity with moderate to heavy rain, gusty wind, squall and sometimes with the occurrences of hail fall. In this paper an attempt has been made to find out the climatological and synoptical aspects of hailstorms as well as squall over Guwahati Airport (26.18° N, 91.75° E) during Premonsoon season. 20 years data from 1987 to 2006 have been analyzed. The frequency of hail and squall during premonsoon season is observed to be 1.35 and 3.65 respectively. The most favourable time of occurrence of hail is during afternoon to late evening and that for the occurrence of squall is during evening to early morning over Guwahati Airport. The significant synoptic situations associated with occurrence of hail are Sea level trough from East Uttar Pradesh/Bihar to North-East India and low level cycir over Bihar and neighbourhood.

A Pilot project on ‘Severe Thunderstorm Observation and Regional Modeling (STORM)’ was undertaken by the Department of Science and Technology during April and May 2006. Regional Meteorological Centre, Guwahati too participated in this project and attempted to monitor each and every severe thunderstorm event. A total of 62 events have been identified on 30 days during 1st April to 25th May. The experiment was restricted upto 25th May in view of onset of monsoon on 27th May. Significant features of all these events are discussed in detail. Satellite imagery and radar echoes of selected events are included to describe the developments of these events. Thermodynamic state of atmosphere along with prevailing synoptic situation associated with the events has been discussed broadly. Results of the study confirm that the ‘Assam valley’ along Brahmaputra river experiences maximum number of events. Lower Assam is more prone to these events as compared to upper and south Assam. High thermodynamic instability, inhomogeneity in horizontal distribution of air temperatures and moisture near ground level and Convective Available Potential Energy (CAPE) value at four stations namely Guwahati, Agartala, Siliguri and Dibrugarh are the key factors in presence of the favourable lower level convergence exhibited by cyclonic circulation over sub-Himalayan West Bengal and adjoining west Assam moving eastwards along Brahmaputra valley with divergence aloft for occurrence of severe thunderstorm over this region. Also In some cases severe thunderstorm outbursts happen in the environment of very low CAPE value also due to proximity of complex terrain feature of this region.