109 research outputs found

    Geoinformatic and Hydrologic Analysis using Open Source Data for Floods Management in Pakistan

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    There is being observed high variability in the spatial and temporal rainfall patterns under changing climate, enhancing both the intensity and frequency of the natural disasters like floods. Pakistan, a country which is highly prone to climate change, is recently facing the challenges of both flooding and severe water shortage as the surface water storage capacity is too limited to cope with heavy flows during rainy months. Thus, an effective and timely predication and management of high flows is a dire need to address both flooding and long term water shortage issues. The work of this thesis was aimed at developing and evaluating different open source data based methodologies for floods detection and analysis in Pakistan. Specifically, the research work was conducted for developing and evaluating a hydrologic model being able to run in real time based on satellite rainfall data, as well as to perform flood hazard mapping by analyzing seasonality of flooded areas using MODIS classification approach. In the first phase, TRMM monthly rainfall data (TMPA 3B43) was evaluated for Pakistan by comparison with rain gauge data, as well as by further focusing on its analysis and evaluation for different time periods and climatic zones of Pakistan. In the next phase, TRMM rainfall data and other open source datasets like digital soil map and global land cover map were utilized to develop and evaluate an event-based hydrologic model using HEC-HMS, which may be able to be run in real time for predicting peak flows due to any extreme rainfall event. Finally, to broaden the study canvas from a river catchment to the whole country scale, MODIS automated water bodies classification approach with MODIS daily surface reflectance products was utilized to develop a historical archive of reference water bodies and perform seasonal analysis of flooded areas for Pakistan. The approach was found well capable for its application for floods detection in plain areas of Pakistan. The open source data based hydrologic modeling approach devised in this study can be helpful for conducting similar rainfall-runoff modeling studies for the other river catchments and predicting peak flows at a river catchment scale, particularly in mountainous topography. Similarly, the outcomes of MODIS classification analysis regarding reference and seasonal water and flood hazard maps may be helpful for planning any management interventions in the flood prone areas of Pakistan

    The effect of the Madden-Julian Oscillation on station rainfall and river level in the Fly River system, Papua New Guinea

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    The Madden-Julian oscillation (MJO) is the dominant mode of intraseasonal variability in tropical rainfall on the large scale, but its signal is often obscured in individual station data, where effects are most directly felt at the local level. The Fly River system, Papua New Guinea, is one of the wettest regions on Earth and is at the heart of the MJO envelope. A 16 year time series of daily precipitation at 15 stations along the river system exhibits strong MJO modulation in rainfall. At each station, the difference in rainfall rate between active and suppressed MJO conditions is typically 40% of the station mean. The spread of rainfall between individual MJO events was small enough such that the rainfall distributions between wet and dry phases of the MJO were clearly separated at the catchment level. This implies that successful prediction of the large-scale MJO envelope will have a practical use for forecasting local rainfall. In the steep topography of the New Guinea Highlands, the mean and MJO signal in station precipitation is twice that in the satellite Tropical Rainfall Measuring Mission 3B42HQ product, emphasizing the need for ground-truthing satellite-based precipitation measurements. A clear MJO signal is also present in the river level, which peaks simultaneously with MJO precipitation input in its upper reaches but lags the precipitation by approximately 18 days on the flood plains

    Remote Sensing of Precipitation: Part II

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    Precipitation is a well-recognized pillar in the global water and energy balances. The accurate and timely understanding of its characteristics at the global, regional and local scales is indispensable for a clearer insight on the mechanisms underlying the Earth’s atmosphere-ocean complex system. Precipitation is one of the elements that is documented to be greatly affected by climate change. In its various forms, precipitation comprises the primary source of freshwater, which is vital for the sustainability of almost all human activities. Its socio-economic significance is fundamental in managing this natural resource effectively, in applications ranging from irrigation to industrial and household usage. Remote sensing of precipitation is pursued through a broad spectrum of continuously enriched and upgraded instrumentation, embracing sensors which can be ground-based (e.g., weather radars), satellite-borne (e.g., passive or active space-borne sensors), underwater (e.g., hydrophones), aerial, or ship-borne. This volume hosts original research contributions on several aspects of remote sensing of precipitation, including applications which embrace the use of remote sensing in tackling issues such as precipitation estimation, seasonal characteristics of precipitation and frequency analysis, assessment of satellite precipitation products, storm prediction, rain microphysics and microstructure, and the comparison of satellite and numerical weather prediction precipitation products

    The relationship between Indian monsoon rainfall and low-pressure systems

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    Indian summer monsoon precipitation is significantly modulated by synoptic-scale tropical low pressure areas (LPAs), the strongest of which are known as monsoon depressions (MDs). Despite their apparent importance, previous studies attempting to constrain the fraction of monsoon precipitation for which such systems are responsible have yielded an unsatisfyingly wide range of estimates. Here, a variant of the DBSCAN algorithm is implemented to identify nontrivial, coherent rainfall structures in TRMM-3B42 precipitation data. Using theoretical considerations and an idealised model, an effective capture radius is computed to be 200 km, providing upper-bound attribution fractions of 57% (17%) for LPAs (MDs) over the monsoon core zone and 44% (12%) over all India. These results are also placed in the context of simpler attribution techniques. A climatology of these clusters suggests that the central Bay of Bengal (BoB) is the region of strongest synoptic organisation. A k-means clustering technique is used to identify four distinct partitions of LPA (and two of MD) track, and their regional contributions to monsoon precipitation are assessed. Most synoptic rainfall over India is attributable to short-lived LPAs originating at the head of the BoB, though longer-lived systems are required to bring rain to west India and east Pakistan. Secondary contributions from systems originating in the Arabian Sea and south BoB are shown to be important for west Pakistan and Sri Lanka respectively. Finally, a database of precipitating-event types is used to show that small-scale deep convection happens independently of MDs, whereas the density of larger-scale convective and stratiform events are sensitive to their presence - justifying the use of a noise-rejecting algorithm

    Cloudbursts in Indian Himalayas: a review

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    Cloudbursts in and around the southern rim of the Indian Himalayas are elusive in terms of their position and time of occurrences. Most of the reported cloudbursts are in the interior of the Himalayas and hence their observation itself is limited. Most of these events are reported once their affect in terms of loss to life and property is experienced in the downstream habitats. In addition, they are mostly associated with flash floods as an impact of the torrential precipitation. The principal understanding of the cloudburst is associated with sudden heavy deluge of precipitation in very less time interval over a very small area. Except this understanding and India Meteorology Department (IMD) definition of > 100 mm/h precipitation over a geographical region of approximately 20–30 km2, nothing much else is known about these events. There are a very few studies carried out on understanding of these events. Present paper synthesizes the available information and research on cloudburst events and tries to define it based on associated dynamics, thermodynamics and physical processes leading to a cloudburst event. Thus in the present work, characterizations and impacts of cloudburst leading from precipitation to dynamical to thermodynamical to large scale forcings to orographical forcings to followed geomorphology to impacts are intertwined to present comprehensive portray of it. Most of the cloudburst events are seen occurring in the elevation range of 1000 m to 2500 m within the valley folds of the southern rim of the Indian Himalayas. Apart from some of the large scale flow shown by few of the studies, it is found that cloudburst events are convectively triggered followed by orographically locked systems. These intertwined mechanisms lead cloudburst events to form. Amiss of any one of these mechanisms will not lead the cloudburst mechanism to form. These interactions in the present paper established the vagaries associated with the cloudburst events

    Central Asia’s Changing Climate: How Temperature and Precipitation Have Changed across Time, Space, and Altitude

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    Changes in climate can be favorable as well as detrimental for natural and anthropogenic systems. Temperatures in Central Asia have risen significantly within the last decades whereas mean precipitation remains almost unchanged. However, climatic trends can vary greatly between different subregions, across altitudinal levels, and within seasons. Investigating in the seasonally and spatially differentiated trend characteristics amplifies the knowledge of regional climate change and fosters the understanding of potential impacts on social, ecological, and natural systems. Considering the known limitations of available climate data in this region, this study combines both high-resolution and long-term records to achieve the best possible results. Temperature and precipitation data were analyzed using Climatic Research Unit (CRU) TS 4.01 and NASA’s Tropical Rainfall Measuring Mission (TRMM) 3B43. To study long-term trends and low-frequency variations, we performed a linear trend analysis and compiled anomaly time series and regional grid-based trend maps. The results show a strong increase in temperature, almost uniform across the topographically complex study site, with particular maxima in winter and spring. Precipitation depicts minor positive trends, except for spring when precipitation is decreasing. Expected differences in the development of temperature and precipitation between mountain areas and plains could not be detected

    Extreme Precipitation in Nepal. Trends and Key Processes

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    Nepal is located at the rim of the Himalayas and houses the highest mountains of the world. Owing to the complex topography and a seasonal monsoon climate, Nepal experiences precipitation events of considerable intensity. Large amounts of rain lead to natural hazards like landslides, floods, infrastructure damage, agricultural losses, and human casualties. It is therefore important to understand whether there are changes in extreme precipitation in Nepal, and which physical processes lead to an extreme event while taking into account the spatial variability of rainfall. To approach these questions the setup of this thesis is threefold. First, a measurement based climatology of precipitation was established and trends in extreme precipitation were detected. Second, synoptic scale conditions associated with extreme precipitation in Nepal were revealed, and third, a case study was used to proceed to the process level and obtain a better understanding of how involved processes interact and finally end in an extreme event. The first manuscript (Paper I) aims to assess the rainfall climatology and trends in extreme precipitation based on rain gauge data in Nepal from 1971-2010. Rain gauge data show that most of the annual precipitation is recorded during the Indian summer monsoon with considerable variability in time and space. Upper quantiles and annual maxima occur mainly during the Indian summer monsoon. The seasonal precipitation varies with the El Niño-Southern Oscillation (ENSO). This correlation vanishes with increasing quantiles. Trends in precipitation extremes were assessed using linear regression, quantile regression, and non-stationary extreme value theory. Moreover, parameter estimation for the non-stationary extreme value distribution was performed applying a maximum likelihood and a Bayesian approach. Multiple approaches add information regarding the method sensitivity of the trends. The study concludes that despite high spatial variability in the trends of extreme precipitation, Far-West Nepal shows a robust positive trend in extreme precipitation across the different methods. The significant changes in extreme precipitation found in Paper I urge a better understanding of the involved physical processes, which motivates the second and third manuscript (Paper II and Paper III). Paper II investigates atmospheric synoptic scale conditions and moisture sources related to extreme precipitation events in Nepal. The high spatial variability in daily rainfall was taken into account by clustering daily precipitation from rain gauges using K-means clustering. As a result, spatial patterns of daily rainfall were established dividing Nepal into West, Central, and East Nepal. The study focuses on extreme precipitation events during which the 99.5 percentile was exceeded at least at five stations at the same time in one cluster. Based on the resulting set of extreme precipitation events, a composite study was conducted for each cluster using meteorological fields from Era-Interim reanalysis. The study shows that large scale atmospheric flow was angled toward the Himalayas at the cluster location during an extreme event following mid- and upper-tropospheric trough structures in geopotential height. Tracking of low pressure systems indicates that the large scale flow conditions guided the low pressure systems toward the Himalayas where they rain out. These results show that the large scale flow conditions mainly determined the location of the extreme event. A Lagrangian moisture source diagnostic reveals anomalously abundant moisture sources over land, particularly over the Indo-Gangetic plain, along the path of the low level flow. The moisture was likely provided by foregone precipitation events over this region preconditioning the soil moisture for additional uptake. It was further found that monsoon break conditions were prevailing during 25 %-43% of all extreme events during July and August. To go deeper into the responsible physical processes and their interplay, Paper III focuses on one case, the extreme precipitation event on 19 July 2007 in Nepal. This extreme event was part of a sequence of precipitation events contributing to the South Asia flood 2007 affecting 20 million people. The study is based on rain gauge data, TRMM 3B42, Era-Interim reanalysis, Lagrangian trajectories, and a high resolution numerical simulation. The combination of these different datasets allows a multiscale analysis of the considered extreme precipitation event. The evolution of the extreme event started with individual convective cells forming over Nepal that were invigorated by moist low-level inflow with high convective available energy. The individual cells organized upscale into an intense wide convective system and resulted in torrential rain with over 250mm within 24 hours. The synoptic scale conditions were similar to Paper II, permitting and orchestrating the development of this extreme event. The following conditions were identified: prior to the extreme event precipitation events preconditioned the soil moisture along the Indo-Gangetic plain, anomalously high moisture sources were available along the path of the low level flow which was characteristic for monsoon break periods, abundant moisture sources enabled the formation of moist airmasses fueling the convective system, and the airmasses were destabilized by topographic and quasi-geostrophic forcing where the final trigger mechanism was probably the upslope flow. Besides investigating an interesting extreme precipitation event, this study shows how synoptic conditions can co-exist and interact to form a system of unusual intensity. Together, the three studies provide the basis for a comprehensive understanding of extreme precipitation events in Nepal. The interplay between atmospheric circulation and moisture sources are of particular importance. The conditions, as described in Paper II and III, have to be just right to provoke an extreme event and should therefore be usefull to increase the ability of forecasting an extreme event. Challenges resulting from the pronounced changes in extreme precipitation (Paper I) can be approached supported by the conditions found in Paper II and III. The involved processes can be persued in future studies to gain further insights which will hopefully foster new research and useful findings for Nepal

    Uncertainties in the Hydrological Modelling Using Remote Sensing Data over the Himalayan Region

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    Himalayas the “roof of the world” are the source of water supply for major South Asian Rivers and fulfill the demand of almost one sixth of world’s humanity. Hydrological modeling poses a big challenge for Himalayan River Basins due to complex topography, climatology and lack of quality input data. In this study, hydrological uncertainties arising due to remotely sensed inputs, input resolution and model structure has been highlighted for a Himalayan Gandak River Basin. Firstly, spatial input DEM (Digital Elevation Model) from two sources SRTM (Shuttle Radar Topography Mission) and ASTER (Advanced Space borne Thermal Emission and Reflection Radiometer) with resolutions 30m, 90m and 30m respectively has been evaluated for their delineation accuracy. The result reveals that SRTM 90m has best performance in terms of least area delineation error (13239.28 km2) and least stream network delineation error. The daily satellite precipitation estimates TRMM 3B42 V7 (Tropical Rainfall Monitoring Mission) and CMORPH (Climate Prediction Center MORPHing Technique) are evaluated for their feasibly over these terrains. Evaluation based on various scores related to visual verification method, Yes/no dichotomous, and continuous variable verification method reveal that TRMM 3B42 V7 has better scores than CMORPH. The effect of DEM resolution on the SWAT (Soil Water Assessment Tool) model outputs has been demonstrated using sixteen DEM grid sizes (40m-1000m). The analysis reveals that sediment and flow are greatly affected by the DEM resolutions (for DEMs>300m). The amount of total nitrogen (TN) and total phosphorous (TP) are found affected via slope and volume of flow for DEM grid size ≥150m. The T-test results are significant for SWAT outputs for grid size >500m at a yearly time step. The SWAT model is accessed for uncertainty during various hydrological processes modeling with different setups/structure. The results reflects that the use of elevation band modeling routine (with six to eight elevation bands) improves the streamflow statistics and water budgets from upstream to downstream gauging sites. Also, the SWAT model represents a consistent pattern of spatiotemporal snow cover dynamics when compared with MODIS data. At the end, the uncertainty in the stream flow simulation for TRMM 3B42 V7 for various rainfall intensity has been accessed with the statistics Percentage Bias (PBIAS) and RSR (RMSE-observations Standard Deviation Ratio). The results found that TRMM simulated streamflow is suitable for moderate (7.5 to 35.4 mm/day) to heavy rainfall intensities (35.5 to 124.4 mm/day). The finding of the present work can be useful for TRMM based studies for water resources management over the similar parts of the world

    The impacts of climate change on the winter water cycle of the western Himalaya

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    Some 180 million people depend on the Indus River as a key water resource, fed largely by precipitation falling over the western Himalaya. However, the projected response of western Himalayan precipitation to climate change is currently not well constrained: CMIP5 GCMs project a reduced frequency and vorticity of synoptic-scale systems impacting the area, but such systems would exist in a considerably moister atmosphere. In this study, a convection-permitting (4 km horizontal resolution) setup of the Weather Research and Forecasting (WRF) model is used to examine 40 cases of these synoptic-scale systems, known as western disturbances (WDs), as they interact with the western Himalaya. In addition to a present-day control run, three experiments are performed by perturbing the boundary and initial conditions to reflect pre-industrial, RCP4.5 and RCP8.5 background climates respectively. It is found that in spite of the weakening intensity of WDs, net precipitation associated with them in future climate scenarios increases significantly; conversely there is no net change in precipitation between the pre-industrial and control experiments despite a significant conversion of snowfall in the pre-industrial experiment to rainfall in the control experiment, consistent with the changes seen in historical observations. This shift from snowfall to rainfall has profound consequences on water resource management in the Indus Valley, where irrigation is dependent on spring meltwater. Flux decomposition shows that the increase in future precipitation follows directly from the projected moistening of the tropical atmosphere (which increases the moisture flux incident on the western Himalaya by 28%) overpowering the weakened dynamics (which decreases it by 20%). Changes to extreme rainfall events are also examined: it is found that such events may increase significantly in frequency in both future scenarios examined. Two-hour maxima rainfall events that currently occur in 1-in-8 WDs are projected to increase tenfold in frequency in the RCP8.5 scenario; more prolonged (one-week maxima) events are projected to increase fiftyfold

    Moist convection within the Indian summer monsoon

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