North Bay Of Bengal Research Articles

Meteorological features associated with unprecedented precipitation over India during 1st week of March 2015

Unprecedented precipitation along with heavy falls occurred over many parts of India from 28th February to 2nd March 2015. Many of the stations of northwest and central India received an all time high 24 hr cumulative precipitation of March during this period. Even the national capital, New Delhi, broke all the previous historical 24 hr rainfall records of the last 100 years to the rainfall record in March 2015. Due to this event, huge loss to agricultural and horticultural crops occurred in several parts of India. In the present study, an attempt is made to understand the various meteorological features associated with this unprecedented precipitation event over India. It occurred due to the presence of an intense western disturbance (WD) over Afghanistan and neighbouring areas in the form of north–south oriented deep trough in westerlies in middle and upper tropospheric levels with its southern end deep in the Arabian Sea, which pumped huge moisture feed over Indian region. Also, there was a jet stream with core wind speed up to 160 knots that generated high positive divergence at upper tropospheric level over Indian region; along with this there was high magnitude of negative vertical velocity and velocity convergence were there at middle tropospheric level. It caused intense upward motion and forced lower levels air to rise and strengthen the lower levels cyclonic circulations (CCs)/Lows. Moreover, the induced CCs/Lows at lower tropospheric levels associated with WD were more towards south of its normal position. Additionally, there was wind confluence over central parts of India due to westerlies in association with WD and easterlies from anticyclone over north Bay of Bengal. Thus, intense WD along with wind confluence between westerlies and easterlies caused unprecedented precipitation over India during the 1st week of March 2015.

On increasing monsoon rainstorms over India

Using a high-resolution daily gridded rainfall data set for the period 1951–2015, new objective criteria were developed to determine rainstorms over the Indian region during the south-west monsoon season (June to September). The rainstorms thus identified have rainfall of 125 mm/day or more at the centre, cover minimum 50,000 km2 in area with rainfall of 25 mm or more and sustain for at least two consecutive days. These rainstorms have potential to cause large-scale floods and associated risk over the region in which they are present. The analysis identified 395 rainstorms during the period, 1951–2015, on an average 6 storms per season. About 40% of the rainstorms are associated with the low-pressure systems which form over the North Bay of Bengal and move north-westwards along the seasonal monsoon trough. The present analysis suggests that over northern parts of India, where majority of rainstorms form, frequency and duration of rainstorms have shown statistically significant increasing trends. During the period of 1951–2015, there was an increase from 4 to 8 rainstorms per year and from 12 to 27 rainstorm days per year. This increase has significant repercussions in terms of added risk of large-scale floods and associated causalities. Further analysis suggests that rainstorm activity over northern parts of India is strongly influenced by the colder sea surface temperature anomalies over the east equatorial Indian Ocean and associated moisture divergent flow and strong moisture convergence over the Indian landmass and the Bay of Bengal.

Impact of an upgraded model in the NCEP Global Ocean Data Assimilation System: The tropical Indian Ocean

The National Centers for Environmental Prediction (NCEP) and the Indian National Centre for Ocean Information Services (INCOIS) produces global ocean analysis based on the Global Ocean Data Assimilation System (GODAS). This study shows how upgrades to the forward model simulations from MOM4p0d to MOM4p1 impact ocean analyses over the tropical Indian Ocean in GODAS. Three experiments were performed with same atmospheric forcing fields: (i) using MOM4p0d (GODAS_p0), (ii) using MOM4p1 (GODAS_p1), both using observed temperature and synthetic salinity, and (iii) using MOM4p1 (GODAS_p1S) assimilating both observed temperature and observed salinity. Validation with independent observations shows significant improvement of subsurface temperature and salinity in the new analysis using MOM4p1 versus MOM4p0d. There is also improvement in the upper ocean current of the equatorial Indian Ocean. The impact of observed salinity on the upper ocean surface current is marginal, but there is significant improvement in the subsurface current. The seasonal and inter-annual variability of the Wyrtki jet and the equatorial undercurrent is improved in GODAS_p1 versus GODAS_p0. All analyses reproduced the Indian Ocean dipole, with the GODAS_p1S simulated sea surface temperature (SST) the most accurate. The temperature inversion over the north Bay of Bengal (BoB) is reproduced only in GODAS_p1S. The mean sea level over BoB and equatorial Indian Ocean improved in GODAS_p1S as compared with AVISO observation. The combined model upgrade and assimilation of observed salinity led to reduced root-mean-square deviation and higher correlation coefficient values in the sea level anomaly (SLA) when compared with satellite observations.

Near‐surface salinity and stratification in the north Bay of Bengal from moored observations

AbstractA thin layer of fresh water from summer monsoon rain and river runoff in the Bay of Bengal (BoB) has profound influence on air‐sea interaction across the south Asian region, but the mechanisms that sustain the low‐salinity layer are as yet unknown. Using the first long time series of high‐frequency observations from a mooring in the north BoB and satellite salinity data, we show that fresh water from major rivers is transported by large‐scale flow and eddies, and shallow salinity stratification persists from summer through the following winter. The moored observations show frequent 0.2–1.2 psu salinity jumps with time scales of 10 min to days, due to O(1–10) km submesoscale salinity fronts moving past the mooring. In winter, satellite sea surface temperature shows 10 km wide filaments of cool water, in line with moored data. Rapid salinity and temperature changes at the mooring are highly coherent, suggesting slumping of salinity‐dominated fronts. Based on these observations, we propose that submesoscale fronts may be one of the important drivers for the persistent fresh layer in the north BoB.

Trends in aerosol optical properties over South Asia

Spatial variation of trends in MODerate resolution Imaging Spectroradiometer (MODIS) remote sensing-derived aerosol optical properties, namely, aerosol optical depth (AOD) at 0.55 µm and Ångström exponent (α) from 2001 to 2014, is estimated on annual and seasonal basis over South Asia. Positive trends in annual mean AOD are found over 56% area of continental regions of South Asia where remote sensing data could retrieve, while ∼40% of Indian region show increasing trends in AOD. During winter and post-monsoon, >70% of the Indian subcontinent show positive trends in AOD due to increase in biomass burning emissions and owing to shallow boundary layer height during the seasons. The increasing trends in both AOD and α over central, north and some part of south, east and northeast India are found, which corroborate that fine-mode aerosols are increasing over the regions. The AOD exhibits negative trends over northwest, north, central, west and east region of India; and Pakistan and Sri Lanka during pre-monsoon. The negative trends in AODs are found over maximum part of the South Asian continent during monsoon. The significant and strong positive trends in AOD (2–3% per year) and α are found in the Bay of Bengal (BoB) and the Arabian Sea, moreover post-monsoon and monsoon seasons AODs exhibit significant and strong (>6% per annum) increasing trends in north BoB. Such positive trends reveal the enhancement in the abundant of fine-mode aerosols in the oceanic regions. Spatial variabilities in the AOD and α trends give rise to significant differential radiative impact in the regions which affect the atmospheric dynamics and also influence the extreme events such as tropical cyclones in South Asian regions. The trend informations will be useful in improving the emission inventories and our understanding on aerosols in regional and global climate.

In situ measured seasonal characteristics of near‐surface aerosols over Bay of Bengal and MODIS‐retrieved columnar properties: A multicampaign analysis

AbstractSeasonal changes in near‐surface aerosol mass loading and their chemical characteristics over the marine environment of Bay of Bengal (BoB) have been investigated, based on the ship‐based experiments carried out as part of three major field campaigns in premonsoon, winter, and monsoon seasons. These spatiotemporal properties of in situ measured near‐surface aerosols were compared with the columnar aerosol optical depth (AOD), Angstrom exponent, effective radius, mass concentration, fine‐mode AOD, and large‐mode AOD retrieved by Moderate Resolution Imaging Spectroradiometer (MODIS). The spatial heterogeneity in the chemical nature of aerosols for the three contrasting seasons has been addressed. In all the seasons, BoB is found to be polluted by anthropogenic aerosols as revealed by their chemical composition. North BoB exhibits highest near‐surface aerosol loading, columnar AOD, and aerosol mass concentration irrespective of the season. While the high AOD in winter is due to fine‐mode anthropogenic aerosols, that in monsoon is due to large‐sized sea‐salt aerosols. The fine‐mode AOD over BoB contributes 45% in monsoon, 69% in premonsoon, and 73% in winter. The column AOD in winter and premonsoon is ~50% of its monsoonal value. The e‐fold scale distances of total aerosol mass loading and those of various chemical species over BoB are also investigated. The column mass concentration has been estimated from the in situ measured near‐surface aerosol mass loading and compared with that retrieved from MODIS. Regions where low near‐surface aerosol mass loading was observed; the satellite‐retrieved columnar loading was found to be overestimated suggesting seasonal and regional differences in scale height.

Winter time chemical characteristics of aerosols over the Bay of Bengal: continental influence.

As part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) conducted under the Geosphere Biosphere Programme of Indian Space Research Organisation, ship-based aerosol sampling was carried out over the marine environment of Bay of Bengal (BoB) during the northern winter months of December 2008 to January 2009. About 101 aerosol samples were collected, covering the region from 3.4° to 21° N latitude and 76° to 98° E longitude-the largest area covered-including the south east (SE) BoB for the first time. These samples were subjected to gravimetric and chemical analysis and the total aerosol loading as well the mass concentration of the ionic species namely F(-), Cl(-), Br(-), NO2 (-), NO3 (-), PO4 (2-), SO4 (2-), NH4 (+), etc. and the metallic species, Na, Mg, Ca, K, Al, Fe, Mn, Zn, and Pb were estimated for each sample. Based on the spatial distribution of individual chemical species, the air flow pattern, and airmass back trajectory analysis, the source characteristics of aerosols for different regions of BoB were identified. Significant level of continental pollution was noticed over BoB during winter. While transport of pollution from Indo-Gangetic Plain (IGP) contributed to aerosols over north BoB, those over SE BoB were influenced by SE Asia. A quantitative study on the wind-induced production of sea salt aerosols and a case study on the species dependent effect of rainfall are also presented in this paper.

Development of a regional model for the North Indian Ocean

We have developed a one-way nested Indian Ocean regional model. The model combines the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory’s (GFDL) Modular Ocean Model (MOM4p1) at global climate model resolution (nominally one degree), and a regional Indian Ocean MOM4p1 configuration with 25km horizontal resolution and 1m vertical resolution near the surface. Inter-annual global simulations with Coordinated Ocean-Ice Reference Experiments (CORE-II) surface forcing over years 1992–2005 provide surface boundary conditions. We show that relative to the global simulation, (i) biases in upper ocean temperature, salinity and mixed layer depth are reduced, (ii) sea surface height and upper ocean circulation are closer to observations, and (iii) improvements in model simulation can be attributed to refined resolution, more realistic topography and inclusion of seasonal river runoff. Notably, the surface salinity bias is reduced to less than 0.1psu over the Bay of Bengal using relatively weak restoring to observations, and the model simulates the strong, shallow halocline often observed in the North Bay of Bengal. There is marked improvement in subsurface salinity and temperature, as well as mixed layer depth in the Bay of Bengal. Major seasonal signatures in observed sea surface height anomaly in the tropical Indian Ocean, including the coastal waveguide around the Indian peninsula, are simulated with great fidelity. The use of realistic topography and seasonal river runoff brings the three dimensional structure of the East India Coastal Current and West India Coastal Current much closer to observations. As a result, the incursion of low salinity Bay of Bengal water into the south-eastern Arabian Sea is more realistic.

Environmental assessment of marine sediments off Poompuhar, Southeast Coast of India

The environmental conditions Off Poompuhar was inferred with the aid of 38 sediment samples collected off Bay of Bengal north of Nagappattinam. Sediments were analysed to understand the geochemistry (textural analysis, TOC, CaCO 3 , major and trace elements) along the coastal tracts and their pollution rate using various geochemical indices. Metals like Cu, Cr and Zr shows enrichment more than 2, whereas Zr, Cr, Cu and Th show CF values > 1. The Igeo showed values less than one for all except Zr, Cu and Cr of which Cu showed a higher value of 9. The PLI value is < 1 in the study area. At the outset the study reveals that most of the elements are of lithogenic origin, except Cu, Cr and Zn which are of anthropogenic nature. As the area is pristine and no polluting industries nearby, the enrichment of elements might have been received from the upper reaches of River Cauvery.

Diurnal and spatial variation of Indian summer monsoon rainfall using tropical rainfall measuring mission rain rate

Summary This paper deals with the diurnal and spatial variation of summer monsoon (JJAS) rainfall, extremes and its frequency for a period of 12 years using the TRMM (Tropical Rainfall Measuring Mission) rain rate data set. Rainfall and frequency of rain events are more observed over the western coastal belts, northeastern regions, foothills of Himalayas and Bay of Bengal regions. Average seasonal rainfall is about 400 mm from 150 rainy hours. The rainfall amount and frequency of rain events show a peak at 1730 IST over Central India, 0230 IST over the foothills of Himalayas and 0830 IST over the north Bay of Bengal. Rainfall over the west coast is more evenly distributed throughout the day. Diurnal pattern of rain intensity is different than that of the rainfall, it is more over the Gujarat regions and peak is observed at 1730 IST and 0230 IST. The relative contribution of rainfall by each octet towards the seasonal total is different. Contribution of rainfall towards its seasonal is maximum at 1730 IST over the main land and is different over other regions. In addition to this, the diurnal pattern of high intensity (90th percentile and above) and low intensity (10th percentile and below) rainfall during the season were also analyzed to find out their spatial distribution. We found that the diurnal pattern is more prominent for the high intensity rainfall than that for the low rainfall intensity. Maximum rainfall hours show systematic southward propagation from early morning to late night.

Variability of the TRMM-PR total and convective and stratiform rain fractions over the Indian region during the summer monsoon

Level 3 (3A25) TRMM Precipitation Radar (PR) data are used for 13 years period (1998–2010) to prepare climatology of TRMM PR derived near surface rain (Total rain) and rain fractions for the 4-months duration of Indian Summer Monsoon season (June–September) as well as for individual months. It is found that the total rain is contributed mostly (99 %) by two rain fractions i.e. stratiform and convective rain fractions for the season as well as on the monthly basis. It is also found that total rain estimates by PR are about 65 % of the gauge measured rain over continental India as well as on sub-regional basis. Inter-annual variability of TRMM-PR rain estimates for India mainland and its sub-regions as well as over the neighboring oceanic regions, in terms of coefficient of variability (CV) is discussed. The heaviest rain region over north Bay of Bengal (BoB) is found to have the lowest CV. Another sub-region of low CV lies over the eastern equatorial Indian ocean (EEIO). The CVs of total rain as well as its two major constituents are found to be higher on monthly basis compared to seasonal basis. Existence of a well known dipole between the EEIO and the north BoB is well recognized in PR data also. Significant variation in PR rainfall is found over continental India between excess and deficit monsoon seasons as well as between excess and deficit rainfall months of July and August. Examination of rainfall fractions between the BoB and Central India on year to year basis shows that compensation in rainfall fractions exists on monthly scale on both the regions. Also on the seasonal and monthly scales, compensation is observed in extreme monsoon seasons between the two regions. However, much less compensation is observed between the north BoB and EEIO belts in extreme rain months. This leads to speculation that the deficit and excess seasons over India may result from slight shift of the rainfall from Central India to the neighboring oceanic regions of north BoB. Contribution of stratiform and convective rain fractions have been also examined and the two fractions are found to contribute almost equally to the total rain. Results are further discussed in terms of the possible impact of the two rain fractions on circulation based on possible difference is vertical profiles of latent heat of two types of rain. Substantial differences in the lower and upper tropospheric circulation regimes are noticed in both deficit and excess monsoon months/seasons, emphasizing the interaction between rainfall (latent heat) and circulation.

A study of vertical cloud structure of the Indian summer monsoon using CloudSat data

Precise specification of the vertical distribution of cloud optical properties is important to reduce the uncertainty in quantifying the radiative impacts of clouds. The new global observations of vertical profiles of clouds from the CloudSat mission provide opportunities to describe cloud structures and to improve parameterization of clouds in the weather and climate prediction models. In this study, four years (2007–2010) of observations of vertical structure of clouds from the CloudSat cloud profiling radar have been used to document the mean vertical structure of clouds associated with the Indian summer monsoon (ISM) and its intra-seasonal variability. Active and break monsoon spells associated with the intra-seasonal variability of ISM have been identified by an objective criterion. For the present analysis, we considered CloudSat derived column integrated cloud liquid and ice water, and vertically profiles of cloud liquid and ice water content. Over the South Asian monsoon region, deep convective clouds with large vertical extent (up to 14 km) and large values of cloud water and ice content are observed over the north Bay of Bengal. Deep clouds with large ice water content are also observed over north Arabian Sea and adjoining northwest India, along the west coast of India and the south equatorial Indian Ocean. The active monsoon spells are characterized by enhanced deep convection over the Bay of Bengal, west coast of India and northeast Arabian Sea and suppressed convection over the equatorial Indian Ocean. Over the Bay of Bengal, cloud liquid water content and ice water content is enhanced by ~90 and ~200 % respectively during the active spells. An interesting feature associated with the active spell is the vertical tilting structure of positive CLWC and CIWC anomalies over the Arabian Sea and the Bay of Bengal, which suggests a pre-conditioning process for the northward propagation of the boreal summer intra-seasonal variability. It is also observed that during the break spells, clouds are not completely suppressed over central India. Instead, clouds with smaller vertical extent (3–5 km) are observed due to the presence of a heat low type of circulation. The present results will be useful for validating the vertical structure of clouds in weather and climate prediction models.

Modulation of cyclonic disturbances over the north Indian Ocean by Madden - Julian oscillation

The relationship of genesis and intensity of cyclonic disturbances (CDs) over the north Indian Ocean with the Madden – Julian Oscillation (MJO) has been examined using 33 years (1975 - 2007) data of MJO index and best track of (CDs) developed by India Meteorological Department (IMD). The MJO index based on outgoing long wave radiation (OLR) and zonal wind in upper (200 hPa) and lower (850 hPa) troposphere (Wheeler and Hendon, 2004) has been used for this purpose.&#x0D; The MJO strongly modulates the genesis and intensity of CDs over the north Indian Ocean. However there are other factors contributing to cyclogenesis over the north Indian Ocean, as about 60% of cyclogenesis during monsoon and post-monsoon seasons are not significantly related with MJO. While the probability of cyclogenesis during monsoon season is higher with MJO in phase 4 and 5 (Maritime Continent), that during post-monsoon season is higher with MJO in phase 3 and 4 (east Indian Ocean and adjoining Maritime Continent). It indicates that while possibility of genesis during monsoon season is significantly suppressed with active MJO at phase 1, 7 and 8 (Africa, western Hemisphere and adjoining Pacific Ocean), there is no significant relationship between genesis and active MJO at phase 1, 7 and 8 during post-monsoon season. The anomalous cyclonic circulation at lower levels over central and north Bay of Bengal in association with MJO at phase 4 and 5 favours enhanced probability of cyclogenesis over the Bay of Bengal during monsoon season. The anomalous easterlies in association with MJO at phase 1 and development of anomalous ridge over south India in association with MJO at phase 7 and 8 which are weak monsoon features lead to suppressed cyclogenesis over north Indian Ocean during this season. The anomalous north-south trough in easterlies embedded with cyclonic circulation over the south west/west central Bay of Bengal in association with southerly surge over the region during active MJO in phase 3 and 4 most favourably influences the convection and enhances the probability of cyclogenesis over the north Indian Ocean during post-monsoon season.&#x0D; The genesis of CDs is more sensitive to phase than the amplitude while the intensification of CDs is more dependent on the amplitude of MJO. Comparing monsoon and post-monsoon seasons, the modulation of genesis, intensification and duration of CDs by the MJO is more during the monsoon season than the post-monsoon season.

Intensification of Aila (May 2009) due to a warm core eddy in the north Bay of Bengal

A very severe cyclonic storm “Aila” hit West Bengal on 26 May 2009. The storm intensified when it encountered with a warm core (SST = 31°C) anti-cyclonic eddy (ACE4) in the north Bay of Bengal. The storm intensity increased by 43% due to this eddy, which is comparable with that (34%) obtained from a best fit line (derived from several numerical experiments over north-west Pacific Ocean). The shallow mixed layer of the large-scale ocean and deep mixed layer inside the eddy appear to be crucial parameters besides translation speed of the storm (Uh), ambient relative humidity and thermal stratification below mixed layer, in the storm intensification. From the eddy size and Uh, the eddy feedback factor is found to be about 0.4 (i.e. 40%), which is close to the above. Since there exists an inverse relationship between Uh and UOHC (upper ocean heat content), slow (fast) moving storms require high (low) UOHC. The warm ACE4 with a high UOHC of 149 kj/cm2 (300% higher than the climatological value) and deep warm layer (D26 = 126 m) opposes the cooling induced by the storm and helps for the intensification of the storm through the supply of large enthalpy (latent + sensible) flux.

Study of Energy Fluxes over the Indian Ocean Prior and During the Summer Monsoon

There is an overall reduction in the incoming shortwave radiation and latent heat flux during surplus monsoon years for the month of May and monsoon season both for NCEPR (energy fluxes taken directly from reanalysis data of NCEP/NCAR) and NCEPC (energy fluxes computed with semi-empirical relations using basic fields of the reanalysis data) as compared to deficient monsoon years. The distribution of net heat flux is predominantly negative over the eastern Arabian Sea, Bay of Bengal, and Indian Ocean. The comparison of shortwave radiation flux and latent heat flux for NCEPR and NCEPC shows that NCEPC overestimates in the North Arabian Sea and North Bay of Bengal and underestimates in the Indian Ocean as compared to NCEPR. However, the pattern of energy fluxes is almost similar both for NCEPR and NCEPC.

Space borne SAR observations of oceanic internal waves in North Bay of Bengal

Sea surface manifestations of internal waves (IW) in the shallow continental shelf waters of North Bay of Bengal have been observed in almost all seasons imaged by Synthetic Aperture Radar (SAR) images of ERS1/2 and Envisat ASAR missions during the period 1993–2004. Shoreward propagating short-period IW packets are observed particularly in summer stratified coastal waters Off Chilka region. In summer waters, prominent short-period shoreward propagating internal waves of consecutive imageries of ERS-1/2 SAR (12th and 13th April 1996) are studied. On 4th October 2003, Envisat ASAR imaged strong surface manifestations of huge internal wave group wavelengths and those propagations were discussed in detail. In the research, we also made an attempt for temporal distribution of IW signatures on SAR in North Bay of Bengal.

Impact of internal waves on the acoustic field at a coastal station off Paradeep, east coast of India

Internal Wave (IW) characteristics and the impact of IW on acoustic field have been studied utilizing the hourly time series of temperature and salinity data collected at a coastal site off Paradeep (North Bay of Bengal) during 24 - 25 October 2008. The IW characteristics, viz; period ) ( per t , velocity ) ( vel C , wavelength ) (L and wave numbers ) (k are found to be 2.133 hrs – 34.72 hrs, 0.135 km hr -1 , 0.37 km - 6.2 km and 2.70 - 0.16 cycles km -1 respectively. The semi-diurnal tidal forces are predominant than diurnal as well as at other frequencies and its contribution is about 64% towards the total potential energy ( 34 . 3 0 = E J m -2 ). Sound velocity perturbations with space and time in the presence of IW field are examined from Garrettt-Munk (GM) model. Transmission loss anomaly for optimized source-receiver configuration at the depth of 53 m and range of 9 km has been computed from acoustic modeling. The loss in the acoustic transmission is found to be 38.4 dB in the presence of Low frequency IW field.

Evaluation of Indian summer monsoon rainfall features using TRMM and KALPANA-1 satellite derived precipitation and rain gauge observation

The study provides a concise and synthesized documentation of the current level of skill of the satellite (3B42RT, 3B42V-6, KALPANA-1) products over Indian regions based on the data gathered during the summer monsoon seasons of 2006, 2007 and 2008. The inter-comparison of satellite products with the rain gauge observations suggests that the TRMM 3B42V6 product could distinctly capture characteristic features of the summer monsoon, such as north–south oriented belt of heavy rainfall along the Western Ghats with sharp gradient of rainfall between the west coast heavy rain region and the rain shadow region to the east, pockets of heavy rainfall along the location of monsoon trough, over the east central parts of the country, over north-east India, along the foothills of Himalayas and over the north Bay of Bengal. The KALPANA-1 and 3B42RT products reproduce only the broadest features of mean monsoon seasonal rainfall. The near real-time products 3B42RT and KALPANA-1 underestimate the orographic heavy rainfall along the Western Ghats of India. The precipitation estimates from TRMM 3B42V6 product, when accumulated over the whole season, could reproduce the observed pattern. However, the TRMM 3B42RT and KALPANA-1 estimates are comparatively lower than the observed rainfall over most parts of the country during the season. Inter comparison reveals that the TRMM 3B42V6 product showed better skill in estimating the daily and seasonal mean rainfall over all India and also over four homogeneous regions of India.&#x0D;

Variability of convective activity over the Bay of Bengal and the Arabian sea

The present study is an attempt to examine seasonal and interannual variability of convective activity over the north Indian Ocean. The monthly mean Outgoing Long-wave Radiation (OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this study for a period of 26 year (1979 to 2004). The period has been divided into two groups of 13 years each with period (i) from 1979 to 1991 and period (ii) from 1992 to 2004. The monthly and seasonal variation of convective activity along with its linear trends analysis during four seasons [winter, Jan-Feb (JF); pre-monsoon, Mar-May (MAM); monsoon, Jun-Sep (JJAS); &amp; post-monsoon, Oct-Dec (OND)] are performed. &#x0D; &#x0D; &#x0D; The difference of seasonal OLR anomalies in winter season between period (ii) &amp; period (i) indicate increase in convective activity during recent period [period (ii)] compared to that of period (i) over the south Arabian Sea &amp; south Bay of Bengal and just the opposite occurred over the rest regions associated with a significant decreasing trend of convective activity over the north and central Arabian Sea (at 99.9% level and 99% level respectively). During MAM the Bay of Bengal and the eastern part of Arabian Sea are relatively more convective during the recent period compared to the former period with significant increasing trend of convective activity over central Bay of Bengal (99% level) and northern (95% level) Bay of Bengal. The convective activity increases mainly over the southern parts of the Arabian Sea (dominated in the month of July) and some parts of southern and eastern Bay of Bengal in recent period compared to that of former period during the monsoon season and is associated with significant increasing trend (at 95 % level) of convective activity over the north Bay of Bengal. The post monsoon season also shows significant (at 95% level) increasing trend in convective activity over south Arabian Sea.

Periodicity in intraseasonal variation of summer monsoon rainfall over Orissa, India in relation to synoptic disturbances

The summer monsoon rainfall over Orissa, a state of eastern India, shows characteristic intraseasonal and interannual variability, due to interaction of basic westerly flow with orography and the synoptic scale monsoon disturbances including low-pressure systems and cyclonic circulations extending upto mid-tropospheric level (LPSC). These systems normally develop over the north Bay of Bengal and move west-northwestwards along the monsoon trough. The essence of this study is to find out the main features of the intraseasonal variability of daily monsoon rainfall over Orissa in relation to synoptic systems like LPSC and its implication on the interannual variation of rainfall. For this purpose, the actual and mean daily rainfall data of 31 uniformly distributed stations, six homogeneous regions and Orissa as a whole during monsoon season (June–September) over a period of 20 years (1980–1999) are subjected to auto-correlation and power spectrum analyses. The actual and average daily scores of significant EOFs and actual daily occurrence along with daily probability of occurrence of the LPSC influencing rainfall over Orissa during the same period are also subjected to auto-correlation and power spectrum analyses.