A análise de uma população em longo prazo permite a identificação da ausência ou presença de variações nos padrões de ciclo de vida, de distribuição espaço-temporal e dinâmica populacional. O ciclo de vida do camarão sete-barbas Xiphopenaeus kroyeri foi investigado na Enseada de Ubatuba, litoral norte do Estado de São Paulo (23º25\' - 23º27\'S / 45º00\' - 45º03\' W) ao longo de um intervalo de treze anos. Amostras de material biológico, de água de fundo e de sedimento foram coletadas em quatro pontos de amostragem em três períodos de 12 meses cada: janeiro a dezembro de 1998, julho de 2006 a junho de 2007 e setembro de 2010 a agosto de 2011. Dados obtidos em 1999 foram incluídos para as análises de crescimento individual, de longevidade e de idade de maturação. Altas abundâncias de fêmeas maduras e de jovens foram respectivamente associadas a valores de temperatura da água de fundo acima e abaixo da média da região. Indivíduos adultos predominaram em pontos com variação temporal da composição granulométrica do sedimento enquanto que os jovens se estabeleceram naqueles em que o sedimento permaneceu fino entre os períodos. Em 1998, a abundância de jovens, o peso individual de machos e fêmeas de todas as categorias demográficas, a idade de maturação morfológica e fisiológica e a longevidade foram maiores do que nos outros períodos. Os parâmetros da função de crescimento de machos e fêmeas variaram entre os períodos, sem apresentarem uma tendência temporal clara. Os padrões anuais de reprodução e recrutamento foram bastante semelhantes entre os períodos. As alterações nos fatores ambientais provocadas por variações na dinâmica de massas de água, pela ocorrência de El Niño e alterações na intensidade de pesca foram consideradas como possíveis agentes relacionadas às variações nos padrões do ciclo de vida da espécie na região / Long-term studies on a population provide detection of the presence or absence of interannual variation in patterns of its life cycle, spatio-temporal distribution, and dynamics. This study investigate the population of the seabob shrimp Xiphopenaeus kroyeri within a period of 13 years in Ubatuba Bay, northern coast of São Paulo state, Brazil (23º25\' - 23º27\'S / 45º00\' - 45º03\' W). Shrimp, bottom water and sediment samples were collected from four sampling sites during three 12-month periods: January-December 1998, July 2006-June 2007, and September 2010-August 2011. Monthly data collected in 1999 were included in growth, longevity and maturation age analyses. High abundance of mature females and juveniles were associated to the occurrence of temperature values below and above the region average, respectively. Adults predominated in sites that showed temporal variation of sediment texture while juveniles occurred in sites where sediment remained fine between the study periods. In the 1998 period values of juveniles were more abundant, individual weight of all demographic categories was higher, both morphological and physiological maturity was attained later and lifespan was longer than on the following periods. Growth function parameters of males and females varied between periods with no clear temporal trend. Monthly patterns of reproduction and recruitment were similar between the study periods. Changes in the environmental conditions caused by variations in the dynamics of the local water masses, occurrence of El Niño and fishery intensity were considered as possible factors related to the interannual variations in the life cycle pattern of X. kroyeri in the study region

Le phytoplancton est un élément primordial dans les réseaux trophiques marins et il est un acteur principal dans les cycles biogéochimiques de la planète. Cependant, des incertitudes subsistent autour des facteurs environnementaux influençant sa saisonnalité ainsi que sa capacité à se développer. L’objectif majeur de cette thèse est d’étudier la réponse du phytoplancton à la variabilité interannuelle des facteurs environnementaux en Mer Méditerranée. Plus précisément, il s’agit de déterminer l’influence de ces derniers sur la saisonnalité du phytoplancton.Dans un premier temps, la variabilité interannuelle des cycles annuels de biomasses phytoplanctoniques observables en Méditerranée a été analysée. Certaines régions, tel que les zones de formation d’eau dense, présentent une variabilité interannuelle importante. L’une des régions les plus variables est la zone de formation d’eau dense en Méditerranée Nord-Occidentale. Une approche multi-outils basée sur des observations a été mise en place pour l’étude des variations spatiale et temporelle de la saisonnalité du phytoplancton dans cette région. Le rôle crucial du mélange vertical et de la disponibilité en lumière sur la saisonnalité du phytoplancton a été évalué. Il est démontré qu’une couche de mélange profonde pendant l’hiver augmente l’intensité du bloom phytoplanctonique printanier, due à une présence plus importante dans la communauté phytoplanctonique de micro-phytoplancton. En conséquence, le taux de production primaire printanier augmente. Enfin, ces modifications de la communauté phytoplanctonique et de la production provoquent une augmentation du stock de carbone organique produit au printemps. / The phytoplankton are essential for the oceanic trophic webs and for biogeochemical cycles on Earth. However, uncertainties remain about the environmental factors influencing its seasonality, and its growing efficiency. The main objective of this thesis is to characterize the responses of the phytoplankton to the interannual variability of the environmental factors, in the Mediterranean Sea. More precisely, we aim to assess the influence of the environmental factors on phytoplankton seasonality. The interannual variability of the phytoplankton annual cycles are analyzed in the Mediterranean Sea, thus highlighting the regions associated with annual cycle variability, like the ones where deep-water formation events occur recurrently. One of these regions is the North-Western Mediterranean Sea. A multiplatform approach based on in situ observations is implemented to analyze the spatial and temporal variability of the phytoplankton seasonality in this particular region. The influences of mixed layer depth and the light availability on phytoplankton seasonality are assessed. An intense deepening of the mixed layer (related to the deep convection) increases the magnitude of the phytoplankton spring bloom. Moreover, the strong deepening of mixed layer seems to induce favorable conditions for an important accumulation of micro-phytoplankton (composed of diatoms mainly). In turn, the phytoplankton production rate increases, mostly, the primary production rate of diatoms. Finally, at the scale of the North-Western Mediterranean Sea, the shift in the phytoplankton community structure and in production induces an increase of the organic carbon stock produced during spring.

No description available.

<p>Coastal plain ponds are an understudied and threatened wetland ecosystem with many unique environmental attributes. Research in these ponds can investigate species-environment relationships, while simultaneously providing ecosystem-specific information crucial to their continued conservation and management. This dissertation explores patterns in coastal plain pond vegetation composition and species-environment relationships across space, through time, and in the seed bank and standing vegetation.</p><p>In a two-year field study at 18 coastal plain ponds across the island of Martha's Vineyard, Massachusetts, I investigated species-environment relationships within and among ponds. I identified vegetation species presences and abundances within 1 m2 quadrats, which ran continuously along transects established perpendicular to the water's edge. Species data were analyzed against local and landscape-scale environmental data. I also conducted a one-year seed bank study in which sediments from four coastal plain ponds were incubated in growth chambers and composition was compared to the standing vegetation. One hundred and thirty-four plant species were identified during vegetation sampling and 38 species were identified from incubated sediments.</p><p>I found significant compositional change across space in response to environmental gradients, with patterns in species composition occurring at both local and landscape scales. Elevation was the only local factor strongly correlated with species composition. Significant landscape-scale environmental factors included surficial geology and pond water salinity. Species composition was significantly correlated with hydrologic regime in 2005 but not in 2006. Overall patterns in vegetation species composition and abundance were more closely related to landscape-scale environmental variables than to local environmental variables. </p><p>I also found that coastal plain ponds undergo significant compositional change from one year to the next. Interannual variability disproportionately affected certain ponds and quadrats more than others, highlighting patterns in the relationships between compositional change and environmental attributes. Specifically, ephemeral ponds, ponds located on the moraine, ponds with high specific conductance values, and quadrats located closer to the waterline exhibited greater compositional change from 2005 to 2006 than permanent ponds, ponds located on the outwash plain, ponds with low specific conductance values, and quadrats located further from the waterline. </p><p>Finally, I found that coastal plain ponds exhibit a low degree of similarity between composition in sediments and standing vegetation. More species were identified in the standing vegetation than in the seed bank, and in most cases average species richness per quadrat was higher in the standing vegetation than in the seed bank. Seed bank and standing vegetation samples from ponds with different surficial geology were compositionally distinct. Seed bank samples from permanent and ephemeral ponds were compositionally distinct whereas standing vegetation samples were not.</p> / Dissertation

In this thesis, the role of soil water storage change on the annual water balance is evaluated based on observations at a large number of watersheds located in a spectrum of climate regions, and an annual water balance model is developed at the seasonal scale based on Budyko hypthesis. The annual water storage change is quantified based on water balance closure given the available data of precipitation, runoff, and evaporation estimated from remote sensing data and meteorology reanalysis. The responses of annual runoff, evaporation, and storage change to the interannual variability of precipitation and potential evaporation are then analyzed. Both runoff and evaporation sensitivities to potential evaporation are higher under energy-limited conditions, but storage change seems to be more sensitive to potential evaporation under the conditions in which water and energy are balanced. Runoff sensitivity to precipitation is higher under energylimited conditions; but both evaporation and storage change sensitivities to precipitation are higher under water-limited conditions. Therefore, under energy-limited conditions, most of precipitation variability is transferred to runoff variability; but under waterlimited conditions, most of precipitation variability is transferred to storage change and some of precipitation variability is transferred to evaporation variability. The main finding of this part is that evaporation variability will be overestimated by assuming negligible storage change in annual water balance, particularly under water-limited conditions. Budyko framework which expresses partitioning of water supply at the mean annual scale, is adapted to be applicable in modeling water cycle in short terms i.e., iv seasonal and interannual scales. Seasonal aridity index is defined as the ratio of seasonal potential evaporation and the difference between precipitation and storage change. The seasonal water balance is modeled by using a Budyko-type curve with horizontal shifts which leads prediction of seasonal and annual storage changes and evaporation if precipitation, potential evaporation, and runoff data are available.

In the Tropical Atlantic Ocean, the main mode of SST variability is the Atlantic Equatorial Mode or Atlantic Niño, which is strongly associated with rainfall patterns in northeastern Brazil and the West Africa Monsoon. The region of largest interannual variability, where the Atlantic Cold Tongue forms, is also a region of consistent biases in climate models. In this study, we investigate the interannual variability of the Tropical Atlantic and its changes in the recent decades in terms of the Bjerknes Feedback Index in a set of seven ocean reanalyses for the periods 1980-1999 and 2000-2010 and for an XX century ocean reanalysis for 1950-2010. Warming trends are observed in SSTs in the cold tongue region, as well as a decrease interannual variability. These in turn are associated with a weakening in the Bjerknes Feedback in the early XXI century, resulting from a stronger thermal damping and weaker thermocline feedback, associated with a weaker response of equatorial zonal thermocline slope to equatorial zonal wind stress. However, the spread among the reanalysis products is large, which makes necessary the use of multiple products and an ensemble analysis to minimize errors and obtain more robust results. This is further reinforced as no significant shifts in the Bjerknes Feedback Index were found for the period previous to 1980, since only one reanalysis product covers this period and its individual errors are large. / No Atlântico Tropical, o principal modo de variabilidade da temperatura da superfície do mar é o modo equatorial, ou El Niño do Atlântico, que está fortemente associado aos padrões de precipitação no Nordeste do Brasil e à Monção Oeste-Africana. A região de maior variabilidade interanual, onde se forma a Língua Fria do Atlântico, é também uma região de consistente discordância entre modelos climáticos. Neste estudo, são investigadas a variabilidade interanual do Atlântico Tropical e suas mudanças nas últimas décadas por meio do Índice do Feedback de Bjerknes considerando um grupo de sete reanálises oceânicas para os períodos de 1980-1999 e 2000-2010 e uma reanálise do século XX para 1950-2010. Um aquecimento é observado na região da língua fria, assim como uma diminuição na variabilidade interanual. Essas mudanças estão ligadas a um enfraquecimento do Feedback de Bjerknes no início do século XXI, como resultado de um amortecimento térmico mais intenso e um enfraquecimento do feedback da termoclina, associado a uma resposta mais fraca do gradiente zonal da termoclina equatorial à tensão de cisalhamento do vento. Contudo, a dispersão entre as reanálises é alta, o que torna necessária a análise comparativa de múltiplos produtos, visando obter resultados mais robustos. Da mesma forma, não foi possível tirar conclusões sobre mudanças no Feedback de Bjerknes no período anterior a 1980, uma vez que somente uma reanálise cobria este período e os erros individuais são grandes.

Le Golfe du Bengale (GdB), dans l'océan indien Nord, est sous l'influence d'intenses vents de mousson, qui se renversent saisonnièrement. Les fortes pluies et les apports fluviaux associés à la mousson de Sud-Ouest font du GdB l'une des régions les moins salées des océans tropicaux. La forte stratification haline proche de la surface qui en découle contribue à limiter le mélange vertical, ce qui maintient des températures de surface élevées et favorise la convection atmosphérique et les pluies. Cette stratification en sel a ainsi des implications profondes sur les échanges air-mer et sur le climat des pays riverains. L'objectif de ma thèse est d'améliorer la description de la variabilité de la salinité de surface (SSS) du GdB, et de comprendre ses mécanismes aux échelles de temps saisonnières à interannuelles. Les climatologies existantes ont permis de mettre en évidence un cycle saisonnier marqué de la SSS, avec un dessalement intense de la partie Nord du bassin pendant l'automne, suivi par une expansion de ces eaux dessalées le long du bord Ouest du bassin. Cette langue dessalée s'érode finalement pendant l'hiver, pour revenir à son extension minimale au printemps. Cependant, la rareté des observations in-situ de SSS ne permet d'observer les fluctuations interannuelles autour de ce cycle saisonnier que de manière parcellaire dans le GdB. Le développement récent de la télédétection spatiale de la SSS (missions SMOS et AQUARIUS) a ouvert de nouvelles opportunités à cet égard. Cette technologie reste toutefois délicate dans le cas d'un bassin de petite taille tel que le GdB, du fait des contaminations éventuelles du signal de SSS par les interférences radio et par les sources d'origine continentale. Une validation systématique des produits satellites par comparaison à un jeu de données in-situ exhaustif montre qu'Aquarius capture de façon réaliste les évolutions saisonnières et interannuelles de la SSS partout dans le GdB. A l'inverse, SMOS ne parvient pas à restituer une salinité meilleure que les climatologies existantes. / Located in the Northern Indian Ocean, the Bay of Bengal (BoB) is forced by intense seasonally reversing monsoon winds. Heavy rainfall and strong river runoffs associated with the southwest monsoon makes the bay one of the freshest regions in the tropical ocean. This surface fresh water flux induces strong near surface salinity stratification, which reduces vertical mixing and maintains high sea surface temperatures and deep atmospheric convection and rainfall. This intense near surface haline stratification has therefore profound implications on the air-sea exchanges, and on the climate of the neighboring countries. The goal of my thesis is to improve the description of the Sea surface salinity (SSS) variability in the BoB and to understand the oceanic and atmospheric processes driving this variability at seasonal and interannual timescales. Existing climatologies reveal a marked seasonal cycle of SSS with an intense freshening of the northern part of the basin during fall that subsequently spreads along the western boundary. This fresh pool finally erodes during winter, to reach its minimal extent in spring. The paucity of in-situ SSS observations however prevented to monitor the interannual fluctuations around this seasonal picture with a good spatial coverage. The recent development of SSS remote-sensing capabilities (with SMOS and AQUARIUS satellites) may help with that regard. However this is particularly challenging for a small semi-enclosed basin such as the Bay of Bengal, because of the potential contamination of the SSS signal by radio frequency interferences and land effects in the near coastal environment. A thorough validation of these satellite products to an exhaustive gridded in-situ dataset shows that Aquarius reasonably captures the large-scale observed seasonal and interannual SSS evolution everywhere in the BoB while SMOS does not perform better than existing climatologies, advocating for improvements of its SSS retrieval algorithm there.

L'upwelling du Sud Vietnam (SVU) joue un rôle clef dans la dynamique océanique et la productivité biologique en Mer de Chine du Sud. Cette thèse vise à quantifier la variabilité interannuelle du SVU et identifier les facteurs et mécanismes en jeu. Pour cela, un jeu de simulations numériques pluri-annuelles à haute résolution a été utilisé. Le réalisme du modèle a été évalué et optimisé par comparaison aux observations in-situ et satellites. Les résultats montrent que la grande variabilité du SVU est fortement pilotée par le rotationnel du vent estival, et liée à l'oscillation ENSO via son impact sur le vent. Cependant, cette influence du vent est significativement modulée par la variabilité intrinsèque océanique liée aux interactions entre la vorticité associée aux tourbillons océaniques et le vent, et dans une moindre mesure par la circulation océanique de grande échelle et les fleuves. Ces conclusions sont robustes aux choix effectués pour corriger la dérive de surface du modèle. / The summer South Vietnam Upwelling (SVU) is a major component of the South China Sea circulation that also influences the ecosystems. The objectives of this thesis are first to quantitatively assess the interannual variability of the SVU in terms of intensity and spatial extent, second to quantify the respective contributions from different factors (atmospheric, river and oceanic forcings; ocean intrinsic variability OIV; El-Niño Southern Oscillation ENSO) to the SVU interannual variability, and third to identify and examine the underlying physical mechanisms. To fulfill these goals we use a set of sensitivity eddy-resolving simulations of the SCS circulation performed with the ROMS_AGRIF ocean regional model at 1/12° resolution for the period 1991-2004. The ability of the model to realistically represent the water masses and dynamics of the circulation in the SCS and SVU regions was first evaluated by comparison with available satellite and in-situ observations. We then defined a group of sea-surface-temperature upwelling indices to quantify in detail the interannual variability of the SVU in terms of intensity, spatial distribution and duration. Our results reveal that strong SVU years are offshore-dominant with upwelling centers located in the area within 11-12oN and 110-112oE, whereas weak SVU years are coastal-dominant with upwelling centers located near the coast and over a larger latitude range (10-14oN). The first factor that triggers the strength and extent of the SVU is the summer wind curl associated with the summer monsoon. However, its effect is modulated by several factors including first the OIV, whose contribution reaches 50% of the total SVU variability, but also the river discharge and the remote ocean circulation. The coastal upwelling variability is strongly related to the variability of the eastward jet that develops from the coast. The offshore upwelling variability is impacted by the spatio-temporal interactions of the ocean cyclonic eddies with the wind stress curl, which are responsible for the impact of the OIV. The ocean and river forcing also modulate the SVU variability due to their contribution to the eddy field variability. ENSO has a strong influence on the SVU, mainly due to its direct influence on the summer wind. Those results regarding the interannual variability of the SVU are robust to the choice of the surface bias correction method used in the model. We finally present in Appendix-A2 preliminary results about the impacts of tides.

Monsoons and Intertropical Convergence Zones (ITCZ) exhibit variability at various temporal and spatial scales. The temporal scale of variability encompasses scales from the intraseasonal through interannual to interdecadal time scales. Anthropogenic climate change can also have an impact on ITCZ and monsoons. Thus it is necessary to assess the ability of coupled ocean atmospheric models (commonly known as AOGCM) to simulate these aspects of variability of tropical climate. This has been studied with simulations from 20 AOGCMs and their AGCM from IPCCAR4 archive. In addition, we have used our own 100 year simulation with CCSM2 and also simulations with its AGCM viz. CAM2. Our analysis shows that most model have significant bias in tropical rainfall and SST. Most models underestimate SST except over a few regions such as the Eastern boundaries of Atlantic and Pacific Oceans. The AGCMs which are forced with observed SSTs have much higher annual mean rainfall as compared to AOGCMs. There is a strong correlation between error in shortwave reflectance at the top of the atmosphere and error in SST. The ability of coupled ocean-atmosphere models and their atmosphere-alone counterparts to simulate the seasonal cycle of rainfall over major monsoon regions and also over oceanic ITCZ. It is found that over the Indian monsoon region, most AGCMs overestimate the seasonal cycle while AOGCMs have a more realistic seasonal cycle. This inspite of the fact that most AOGCMs underestimate the SST over the Indian region. It is shown that this is related to errors in precipitable water-rainfall relationship in most models i.e. for a given amount of precipitable water, most models overestimate the rainfall. Thus lower SST reduces the precipitable water and hence the amount of rainfall is reduced. Therefore, the mutual cancellation of errors leads to a more realistic seasonal cycle in AOGCMs. The seasonal cycle over Africa was analysed with the help of a diagnostic model. Over Southern Africa, most models show simulate a less stable atmosphere and hence the rainfall is overestimated. A technique based on Continous Wavelet Transform in Space and Time (CWTST) has been modified to seperate northward and southward propagating modes of BSISO over the Indian and West Pacific regions. It was seen that over the Indian region, northward propagating modes were more prominent in comparison to southward modes. It was also found that the predominant spatial scale (of about 30o) did not show much interannual variability but the associated temporal scale showed significant variation. Both AOGCMs and AGCMs simulations were analysed to investigate the impact of coupling on intraseasonal activity. Most AOGCMs were able to simulate the predominant spatial scale but were unable to simulate the associated temporal scale correctly. These problems persisted with AGCMs also. It was also found that for AGCMs, there were some variations between ensemble members of the AGCMs. Comparing BSISO in increased GHG scenarios with present day simulations we found that in general, power in the spectrum increases. This could be related to higher mean precipitation that has been simulated by most AOGCMs when GHG are increased. The interannual variability in the tropics with special reference to Tropical Biennial Oscillation (TBO) and ENSO has been studied. The changes in these modes of variability due to anthropogenic climate change has also been assessed. We found that in most models over the Nino3.4 region, the mode of variation shifts from a near-four period (in pre-industrial simulations) to that of TBO mode in increased GHG (green house gas) scenario. This suggests that with increasing GHGs, ENSO quasi-periodicity might shift to about two years. It is also interesting to note that for observed rainfall, OLR and 850 hPa winds, the TBO mode has higher variance over the Eastern Indian Ocean, indicating that the TBO mode might be related to Indian Ocean Dipole Mode and EQUINOO (Equatorial Indian Ocean Oscillation).

In the current study, we have presented a systematic analysis of the diurnal cycle of rainfall over the Indian region using satellite observations, and evaluated the ability of the Weather Research and Forecasting Model (WRF) to simulate some of the salient features of the observed diurnal characteristics of rainfall. Using high resolution simulations, we also investigate the underlying mechanisms of some of the observed diurnal signatures of rainfall. Using the Tropical Rain-fall Measuring Mission (TRMM) 3-hourly, 0.25 ×0.25 degree 3B42 rainfall product for nine years (1999-2007), we extract the finer spatial structure of the diurnal scale signature of Indian summer monsoon rainfall. Using harmonic analysis, we construct a signal corresponding to diurnal and sub-diurnal variability. Subsequently, the 3-hourly time-period or the octet of rain-fall peak for this filtered signal, referred to as the “peak octet,” is estimated with care taken to eliminate spurious peaks arising out of Gibbs oscillations. Our analysis suggests that over the Bay of Bengal, there are three distinct modes of the peak octet of diurnal rainfall corresponding to 1130, 1430 and 1730 IST, from north central to south Bay. This finding could be seen to be consistent with southward propagation of the diurnal rainfall pattern reported by earlier studies. Over the Arabian sea, there is a spatially coherent pattern in the mode of the peak octet (1430 IST), in a region where it rains for more than 30% of the time. In the equatorial Indian Ocean, while most of the western part shows a late night/early morning peak, the eastern part does not show a spatially coherent pattern in the mode of the peak octet, owing to the occurrence of a dual maxima (early morning and early/late afternoon). The Himalayan foothills were found to have a mode of peak octet corresponding to 0230 IST, whereas over the Burmese mountains and the Western Ghats (west coast of India) the rainfall peaks during late afternoon/early evening (1430-1730 IST). This implies that the phase of the diurnal cycle over inland orography (e.g., Himalayas) is significantly different from coastal orography (e.g., Western Ghats). We also find that over the Gangetic plains, the peak octet is around 1430 IST, a few hours earlier compared to the typical early evening maxima over land. The second part of our study involves evaluating the ability of the Weather Research and Fore-casting Model (WRF) to simulate the observed diurnal rainfall characteristics. It also includes conducting high resolution simulations to explore the underlying physical mechanisms of the observed diurnal signatures of rainfall. The model (at 54km resolution) is integrated for the month of July 2006 since this period was particularly favourable for the study of diurnal cycle. We first evaluate the sensitivity of the model to the prescribed sea surface temperature (SST) by using two different SST datasets, namely Final Analyses (FNL) and Real-time Global (RTG). The overall performance of RTG SST was found to be better than FNL, and hence it was used for further model simulations. Next, we investigated the impact of different parameterisations (convective, microphysical, boundary layer, radiation and land surface) on the simulation of diurnal cycle of rainfall. Following this sensitivity study, we identified the suite of physical parameterisations in the model that “best” reproduces the observed diurnal characteristics of Indian monsoon rainfall. The “best” model configuration was used to conduct two nested simulations with one-way, three-level nesting (54-18-6km) over central India and Bay of Bengal. While the 54km and 18km simulations were conducted for July 2006, the 6km simulation was carried out for the period 18-24 July 2006. This period was chosen for our study since it is composed of an active period (19-21 July 2006), followed by a break period (22-24 July 2006). At 6km grid-spacing the model is able to realistically simulate the active and break phases in rainfall. During the chosen active phase, we find that the observed rainfall over central India tends to reach a maximum in the late night/early morning hours. This is in contrast to the observed climatological diurnal maxima of late evening hours. Interestingly, the 6km simulation for the active phase is able to reproduce this late night/early morning maxima. Upon further analysis, we find that this is because of the strong moisture convergence at the mid-troposphere during 2030-2330 IST, leading to the rainfall peak seen during 2330-0230 IST. Based on our analysis, we conclude that during both active and break phases of summer monsoon, mid-level moisture convergence seems to be one of the primary factors governing the phase of the diurnal cycle of rainfall. Over the Bay of Bengal, the 6km model simulation is in very good agreement with observations, particularly during the active phase. The southward propagation observed during 19-20 July 2006, which was not captured by the coarse resolution simulation (54km), is exceedingly well captured by the 6km simulation. The positive anomalies in specific humidity attain a maxima during 2030-0230 IST in the north and during 0830-1430 IST in the south. This confirms the role of moisture convergence in the southward propagation of rainfall. Equally importantly we find that while low level moisture convergence is dominant in the north Bay, it is the mid-level moisture convergence that is predominant in the south Bay.