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Sources of errors in the measurements of underwater profiling radiometer
Get PDFThere are various sources of errors from the measurements of optical parameters using a radiometer, which can be classified as mode of deployment, instrument and environment. The errors from the deployment are primarily from the ship and superstructure shadows. Instrument could be a source of error arising from its self-shadow, drift in the calibration and temperature effects. There could be large errors, which at times may be unavoidable to environment factors such as wave focusing at the surface layers, sea state conditions which may affect the tilt of the instrument, atmospheric conditions such as cloud cover, solar elevation, wind and rain. Radiometric optical data in water could also get affected due to Raman scattering and fluorescence effects. Here we discuss the above sources of errors and how they could be minimized. From the measurements carried out in the coastal waters off Goa and Arabian Sea using the hypespectral radiometer, we propose simple protocol to measure the data and also screen the erroneous data measured from the radiometer
Modelling December 2004 Indian Ocean tsunami: A coastal study
No full textDecember 2004 tsunami in the Indian Ocean region has been simulated using MIKE-21 HD model. The vertical displacement of the seabed is incorporated into the numerical simulation by using time-varying bathymetry data. In the open ocean, sea surface height from altimeter observation has been used to validate the model results. To the west of the rupture zone, the crest is observed to precede the trough of the tsunami waves while to the east, trough preceded the crest. The model performance along the coastal region has been validated using de-tided sea levels from tide gauge measurements at Tuticorin, Chennai, Vishakapattanam, and Paradip ports along the east coast of India. Unique coastal characteristics of the tsunami waves, wave height, and wave celerity are reasonably simulated by the numerical model. Spectral analysis of tide gauge observations and corresponding model results has been done, and the distribution of frequency peaks from the analysis of gauge observations and the model results is observed to have a reasonable comparison. Low-frequency waves, contributed from the coastally trapped edge waves, are found to dominate both the tide gauge observations and the model results. The subsequent increase in the tsunami wave height observed at Chennai, Vishakapattanam, and Paradip has been explained on the basis of coastally trapped edge waves. From the validation studies using altimeter data and tide gauge data, it is observed that the model can be used effectively to simulate the tsunami wave height in the offshore as well as in the coastal region with satisfying performance
Mixing to monsoons: Air-sea interactions in the bay of Bengal
Get PDFMore than 1 billion people depend on rainfall from the South Asian monsoon for their livelihoods. Summertime monsoonal precipitation is highly variable on intraseasonal time scales, with alternating "active" and "break" periods. These intraseasonal oscillations in large-scale atmospheric convection and winds are closely tied to 1°C-2°C variations of sea surface temperature in the Bay of Bengal
Anomalous features of mid-tropospheric CO2 during Indian summer monsoon drought years
No full textIn this study, we have examined the impact of droughts on the atmospheric CO2 spatial variability and changes its emission scenario over India. Surface monitoring over India is very sparse and have started recently. Satellite retrievals from AIRS/Aqua during 2004-2011 are used here to understand CO2 variability over India. During recent decade, Indian region witnessed two drought years 2004 and 2009 and no flood. Year 2009 was mega drought for India ever seen, when actual rainfall was approx-23 below the mean rainfall over most part of India. Satellite retrieved mid-tropospheric CO2 indicates increase of about 3ppm during summer monsoon drought. Enhanced impact of drought conditions is also seen on subsequent seasons, winter (JF) and hot pre-monsoon season (MAM). Post-monsoon season (OND) does not show any clear impact, as rainfall during this season has its own variability quite different than summer monsoon (JJAS) may be compensating changes in CO2 values. Decrease in vegetation and weak circulation patterns during drought conditions may have influences on distribution of CO2 over Indian region
Sources and elemental composition of summer aerosols in the Larsemann Hills (Antarctica)
No full textAtmospheric aerosols play a major role in the global climate change. A better physical characterization of the chemical composition of atmospheric aerosols, especially in remote atmosphere, is an important step to reduce the current uncertainty in their effect on the radiative forcing of the climate. In the present work, surface aerosols have been studied over the Southern Ocean and over Bharati, Indian Research Station at Larsemann Hills at the Antarctic coast during the summer season of 2009-2010. Aerosol samples were collected using optical particle counter (OPC) and high-volume air sampler. PM10 and PM2.5 aerosol samples were analyzed for various water-soluble and acid-soluble ionic constituents. The Hysplit model was used to compute the history of the air masses for their possible origin. Supplementary measurements of meteorological parameters were also used. The average mass concentration for PM10 over the Southern Ocean was found to be 13.4 μg m3. Over coastal Antarctica, the mass of PM10 was 5.13 μg m-3, whereas that of PM2.5 was 4.3 μg m-3. Contribution of marine components, i.e., Na, Cl and Mg was dominant over the Southern Ocean (79 ) than over the coastal Antarctica where they were dominant in coarse mode (67 ) than in fine mode (53 ) aerosols. The NH4/nss-SO4 ratio of 1.12 in PM2.5 indicates that the NH4 and SO4 ions were in the form of NH4HSO4. Computation of enrichment factors indicate that elements of anthropogenic origin, e.g., Zn, Cu, Pb, etc., were highly enriched with respect to crustal composition
Influence of monsoons on atmospheric CO2 spatial variability and ground-based monitoring over India
No full textThis study examines the role of Asian monsoons on transport and spatial variability of atmospheric CO2 over the Indian subcontinent, using transport modeling tools and available surface observations from two atmospheric CO2 monitoring sites Sinhagad (SNG) and Cape Rama (CRI) in the western part of peninsular India. The regional source contributions to these sites arise from the horizontal flow in conduits within the planetary boundary layer. Greater CO2 variability, greater than 15ppm, is observed during winter, while it is reduced nearly by half during summer. The SNG air sampling site is more susceptible to narrow regional terrestrial fluxes transported from the Indo-Gangetic Plains in January, and to wider upwind marine source regions from the Arabian Sea in July. The Western Ghats mountains appear to play a role in the seasonal variability at SNG by trapping polluted air masses associated with weak monsoonal winds. A Lagrangian back-trajectory analysis further suggests that the horizontal extent of regional sensitivity increases from north to south over the Indian subcontinent in January (Boreal winter). © 2014 Elsevier B.V
Subseasonal variations of indian summer monsoon with special emphasis on drought and excess rainfall years
No full textIndian Summer Monsoon Rainfall (ISMR) experiences quasi-periodic drought and excess rainfall. In this study we examined the contribution of individual month's (June–September) rainfall in the seasonal excess (deficit). Analysis of 110 years (1901–2010) of observed precipitation and 20th century reanalysis data highlights the importance of subseasonal variations of rainfall in modulating Indian summer monsoon. A month which contributes to seasonal excess (deficit) is primarily controlled by large-scale La Niña (El Niño) forcing but with favourable local conditions from the tropical Indian Ocean. On the other hand excess (deficit) rainfall of an individual month which is not contributing to seasonal excess (deficit) is controlled mainly by the local forcing. Such local forcing however is short lived and does not persist more than a month. Our analysis reveals that June is not contributing considerably to the seasonal excess rainfall in the recent years. On the other hand contribution of September rainfall to the seasonal extremes is more frequent in the recent years. During September enhanced El Niño (La Niña) conditions and the local forcing contribute to the seasonal ISMR deficit (excess). It is important to note that none of the above 110 years experienced excess (deficit) rainfall during all the four monsoon months except 1972. This study advocates the need of subseasonal (or monthly) rainfall prediction for better socio-economic benefit
In search of influence of stratospheric Quasi-Biennial Oscillation on tropical cyclones tracks over the Bay of Bengal region
No full textThe ability of the stratospheric Quasi-Biennial Oscillation (QBO) to modulate tropospheric circulation allows it to modulate tracks of tropical cyclones (TCs). This possibility is examined using data on the cyclone tracks over the Bay of Bengal (BOB) during the period 1948-2010. TC tracks (excluding the El Niño-Southern Oscillation years) when stratified as per the phases of QBO show the evidence of influence of stratospheric QBO on tracks. In pre-monsoon and post-monsoon seasons, during easterly phase of QBO, cyclones move westward/northwestward while during westerly phase they move northward/northeastward. The evidence of influence of QBO on modulation of TC tracks is reported for the first time during pre-satellite period (1948-1978) and satellite period (1979-2010) over the BOB. The QBO influence is observed to be significantly stronger during easterly phase as compared to that in the westerly phase. Numbers of cyclones are more during easterly phase (during pre-monsoon and post-monsoon) than during the westerly phase. A mechanism is proposed whereby QBO of the zonal winds in the stratosphere influences the movement of cyclone tracks in the BOB by redistributing deep convection associated with cyclones by modulating steering winds. The possible interaction between QBO and cyclone activity is investigated through the variation of tropopause height, geo-potential height and Outgoing Longwave Radiations
On the anomalous precipitation enhancement over the Himalayan foothills during monsoon breaks
No full textAn intriguing feature associated with 'breaks' in the Indian summer monsoon is the occurrence of intense/flood-producing precipitation confined to central-eastern parts of the Himalayan (CEH) foothills and north-eastern parts of India. Past studies have documented various large-scale circulation aspects associated with monsoon-breaks, however the dynamical mechanisms responsible for anomalous precipitation enhancement over CEH foothills remain unclear. This problem is investigated using diagnostic analyses of observed and reanalysis products and high-resolution model simulations. The present findings show that the anomalous precipitation enhancement over the CEH foothills during monsoon-breaks emerges as a consequence of interactions between southward intruding mid-latitude westerly troughs and the South Asian monsoon circulation in its weak phase. These interactions facilitate intensification of mid-tropospheric cyclonic vorticity and strong ascending motion over the CEH foothills, so as to promote deep convection and concentrated rainfall activity over the region during monsoon-breaks. Mesoscale orographic effects additionally tend to amplify the vertical motions and precipitation over the CEH foothills as evidenced from the high-resolution model simulations. It is further noted from the model simulations that the coupling between precipitation and circulation during monsoon-breaks can produce nearly a threefold increase of total precipitation over the CEH foothills and neighborhood as opposed to active-monsoon conditions
Impact of satellite-retrieved atmospheric temperature profiles assimilation on Asian summer monsoon 2010 simulation
No full textThe impact of assimilation of satellite-retrieved atmospheric temperature profiles data in simulating mean monsoon circulation and rainfall of summer monsoon 2010 is examined in this study using the regional climate model, Weather Research and Forecasting model (WRF). Two experiments are performed; the first one is the control experiment (WRF-CTL) where no assimilation is carried out, and the second one is similar to the first one, but satellite-retrieved atmospheric temperature profiles are assimilated using a four-dimensional data assimilation method (WRF-AIRS). Mean monsoon features such as the low level jet, monsoon trough, tropical easterly jet, meridional pressure gradient and the spatial distribution of rainfall are better simulated in WRF-AIRS. Correlation coefficients between the observed and WRF-AIRS (WRF-CTL) daily zonal wind shear, meridional wind shear and rainfall indices over the Indian summer monsoon region are 0.98, 0.96 and 0.67 (0.32, 0.35 and 0.23), respectively. The zonal and meridional wind indices over the western Pacific and East Asia are 0.9 and 0.8 (0.6 and 0.5), respectively. Spatial distribution of rainfall displays double ITCZ like rainfall pattern over the tropical Indian Ocean in WRF-CTL, whereas WRF-AIRS display a single ITCZ pattern, which is similar to the observed one. The temporal evolution of the vertical structure of temperature (associated with rainfall activity over the monsoon core region) shows warming in the midtroposphere to upper troposphere (by 0.5 to 1.5 °C) and cooling in the midtroposphere to lower troposphere (by 0.5 to 1.0 °C). This atmospheric temperature distribution associated with rainfall is well simulated by WRF-AIRS. Under heavy rainfall conditions, WRF-AIRS produces strong vertical motion consistent with the observations but is absent (or weak) in WRF-CTL. This study deduces that the assimilation of temperature profiles in the regional climate model can significantly improve the dynamical and thermodynamical features of monsoon by representing the vertical distribution of temperature more realistically. Our analysis reiterates that the Asian summer monsoon circulation is mainly controlled by thermal forcing. Our study suggests that it is essential to improve the existing parameterization schemes for better simulation of summer monsoon. © 2013 Springer-Verlag Wien