Climatic trends in most parts of the world show a significant increase in rainfall amount, intensity and its frequency. Similarly, these trends are likely to continue in future as well. The major catastrophe caused by these rainfall trends comes as flooding, which is getting harder to predict, and mainly over the mountainous regions. Modelling these extreme rainfall events is crucial, which needs better forecasting skills and more understanding of existing Numerical Weather Prediction (NWP) modelling setup. Although, recent developments in NWP models increase the capabilities to simulate rainfall more precisely but the strengths and weaknesses of model need to be evaluated based on climatic conditions, terrain characteristics -- which include landuse, topography, new physical schemes and static datasets. Therefore, we have conducted comprehensive sensitivity and qualitative analysis with a numerous model setup, physical schemes and terrain datasets. / We contemplate various physical parameterizations and updated terrain datasets to simulate the rainfall over the complex topography using WRFV3-ARW modelling system. Additionally, the impact of topography and landuse on rainfall are discussed in detail along with a several combinations of newly available land surface, planetary boundary layer (PBL), cumulus, and cloud microphysics (MP) schemes. As a case study, we select the north region of Pakistan, which includes Khyber-Pakhtunkhwa (KPK) province and part of Hindukush-Karakoram-Himalaya (HKH), and this region have diversified the landscape and complex topographic features. For the sake of better understanding and comparative discussions, we study three extreme rainfall events; two of them occurred during monsoon period (i.e., July), while one in post monsoon period (i.e., September). / WRF-ARW 3.5.1 model is tuned and tested with GFS0.5 and CFSR/CFSv2 as forcing and lateral boundary conditions with a number of parameterization schemes. Similarly, to minimize the errors induced by terrain features, we apply wind correction and drag parameterizations. Furthermore, 3-arc-second hydrologically corrected SRTM digital elevation model (DEM), modified MODIS IGBP 30-arc-second, MODIS 15-arc-second and GLCNMO2008 landuse datasets were also integrated to WRF along with default datasets in WRF modelling system. / We verify the simulated rainfall by using observed, the Tropical Rainfall Measuring Mission (TRMM) and the Climate Prediction Centre morphing method (CMORPH) rainfall datasets. The GIS-based verification technique, called fisher-net is also introduced which is more compatible and flexible with other tools as well. / 隨著全球氣候變化的加劇，強暴雨極端天氣事件呈現突發、多發、併發的特點，其頻次、強度、持續時間、籠罩範圍近年來均呈現急速上升的趨勢。如何構建高時效性、高精度和高可用的極端天氣事件模擬工具，已經成為災害應急管理與回應等領域迫切需要解決的關鍵科學問題。作為極端天氣事件類比的核心，數值氣象模型對極端暴雨事件的模擬能力日益完善；然而，到目前為止，氣象模型的可靠性和有效性評價仍是其推廣應用的關鍵，特別是如何顧及土地利用類型、地形、新型物理機制和多源靜態資料庫前提下的模型可靠性評價仍面臨很多挑戰。因此，本文提出了顧及不同物理機制、地形特徵的模型構建、模型敏感性評價和定量分析方法。 / 首先，本文在顧及不同的物理參數和地形特徵的基礎上，利用WRFV3-ARW 建模系統實現了對複雜地形特徵下降雨過程的類比與分析。在此基礎上，本文充分考慮和利用新的土地下墊面、行星邊界層、積雲以及雲微物理機制，以詳細分析了地形和土地利用類型對降雨影響。實驗選用具有複雜地形結構和特徵的巴基斯坦北部的Khyber-Pakhtunkhwa (KPK)省和部分Hindukush-Karakoram-Himalaya (HKH) 區域；為了獲取更充分的分析結果，本位對該實驗區域內三次極端降雨事件進行了模擬和分析，包括季風期（例如七月）的兩次降雨事件和季風期過後（例如九月）的一次降雨事件。 / 其次，本文利用GFS0.5 和 CFSR/CFSv2 作為強迫和側邊界條件，設置多參數方案對WRF-ARW 3.5.1 模型進行了優化和測試。與此同時，為了降低由地形特徵導致的類比誤差，本文引入了風向糾正參數和風阻參數。除此之外，本文充分利用了水文糾正過後的3 弧秒精度的SRTM DEM 資料、30 弧秒精度的MODIS IGBP 資料、15 弧秒精度的MODIS 資料、GLCNMO2008 格式的土地利用資料、以及WRF 建模系統的預設資料，支撐WRF 的建模過程。 / 最後，為了驗證本文實驗結果的可靠性，本文利用TRMM 獲取的實測降雨量資料以及TRMM 提供的降雨資料庫驗證，基於GIS 的漁網驗證法，對上述模擬結果進行了詳細的分析 / Sultan, Shahzad. / Thesis Ph.D. Chinese University of Hong Kong 2015. / Includes bibliographical references (leaves 111-125). / Abstracts also in Chinese. / Title from PDF title page (viewed on 09, September, 2016). / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.

The West African Monsoon (WAM) is a significant component of the global monsoon system and plays a key role in the annual cycle of precipitation in the Sahel region of Africa (10°N to 20°N) during the summer months (July to September). Rainfall in the Sahel varies on timescales ranging from seasons to millennia as a result of changes in the WAM. In the last century, the Sahel experienced a relatively wet period (prior to the 1960s) followed by a period of severe drought (1970s-1980s) with higher-frequency variability superimposed on this low-frequency background signal. Understanding precipitation variability like that which occurred over the 20th Century and its impact on Sahel precipitation is critically important for skillful hydroclimate predictions and disaster preparedness in the region. Previous work has shown that the WAM responds to both internal atmospheric variability and external oceanic forcing. A large fraction of 20th Century Sahel rainfall variability has been linked to nearby and remote oceanic forcing from the Atlantic, Pacific, and Indian Oceans, suggesting that the ocean is the primary driver of variability. However, the mechanisms underlying the influence of sea surface temperature (SST) forcing to land based precipitation and the relative importance of the roles of different basins are not as well understood. To this end, the work completed in this thesis examines the physical mechanisms linking oceanic forcing to recent precipitation variability in the Sahel and identifies them alongside large-scale environmental conditions. A series of moisture budget decomposition studies are performed for the Sahel in order to understand the processes that govern regional hydroclimate variability on decadal and interannual time scales. The results show that the oceanic forcing of atmospheric mass convergence and divergence explains the moisture balance patterns in the region to first order on the timescales considered. On decadal timescales, forcing by the Indian and Atlantic Oceans correlate strongly with precipitation variability. The combination of a warm Indian Ocean and negative gradient across the Atlantic forces anomalous circulation patterns that result in net moisture divergence by the mean and transient flow. Together with negative moisture advection, these processes result in a strong drying of the Sahel during the later part of the 20th Century. Diagnosis of moisture budget and circulation components within the main rainbelt and along the monsoon margins show that changes to the mass convergence are related to the magnitude of precipitation that falls in the region, while the advection of dry air is associated with the maximum latitudinal extent of precipitation. On interannual timescales, results show that warm conditions in the Eastern Tropical Pacific remotely force anomalously dry conditions primarily through affecting the low-troposphere mass divergence field. This behavior is related to increased subsidence over the tropical Atlantic and into the Sahel and an anomalous westward flow of moisture from the continent, both resulting in a coherent drying pattern. The interannual signal is then further explored, particularly in light of the expected link between the El Niño Southern Oscillation and dry conditions in the Sahel, notably unseen during the historic El Niño event of 2015. Motivated by this, recent El Niño years and their precipitation signature in the Sahel along with the associated large-scale environmental conditions are examined. Two different outcomes for Sahel summer season are defined; an anomalously wet or an anomalously dry season coincident with El Niño conditions. The different precipitation patterns are distinguished by increased moisture supply for the wet years, which can be driven by both regional oceanic conditions that favor increased moisture convergence over the continent as well as weaker El Niño forcing. Finally, a series of new idealized SST-forced experiments that explore the causal link between oceanic forcing and the response of convection in the region on daily time resolution are discussed and preliminary results shown. These experiments aim to understand how convection in the Sahel responds to SST forcing using transient model simulations that track the evolving response of the WAM through time, day-by-day, under different oceanic conditions. Preliminary results show the stark differences in seasonal precipitation that occur when anomalies of opposite sign are applied in parts of the Atlantic and Pacific basin. There is also a suggestion of a difference in the timing of the rainy season when the model is run with different SST configurations.

An extreme precipitation categorization scheme, developed to temporally and spatially visualize and track the multi-scale variability of extreme precipitation climatology, is introduced over the continental United States and used as the basis for an observational dataset intercomparison. The categorization scheme groups three-day precipitation totals exceeding 100 mm into five precipitation categories, or "P-Cats". To assess observational uncertainty across a range of precipitation measurement approaches, we compare in situ station data from the Global Historical Climatology Network-Daily (GHCN-D), satellite derived data from the Tropical Rainfall Measuring Mission (TRMM), gridded station data from the Parameter-elevation Regression on Independent Slopes Model (PRISM), global reanalysis from the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA 2), and regional reanalysis from the North American Regional Reanalysis (NARR). While all datasets capture the principal spatial patterns of extreme precipitation climatology, results show considerable variability across the five-platform suite in P-Cat frequency, spatial extent, and magnitude. Higher resolution datasets, PRISM and TRMM, most closely resemble GHCN-D and capture a greater frequency of high-end totals relative to lower resolution products, NARR and MERRA-2. When all datasets are regridded to a common coarser grid, differences persist with datasets originally constructed at a high resolution maintaining the highest frequency and magnitude of P-Cats. Potential future applications of this scheme include tracking change in P-Cats over space and time, climate model evaluation, and assessment of model projected change.

This thesis presents an approach for generating long synthetic sequences of single-site daily rainfall which can incorporate low-frequency features such as drought, while still accurately representing the day-to-day variations in rainfall. The approach is implemented in a two-stage process. The first stage is to generate the entire sequence of rainfall occurrence (i.e. whether each day is dry or wet). The second stage is to generate the rainfall amount on all wet days in the sequence. The models used in both stages are nonparametric (they make minimal general assumptions rather than specific assumptions about the distributional and dependence characteristics of the variables involved), and ensure an appropriate representation of the seasonal variations in rainfall. A key aspect in formulation of the models is selection of the predictor variables used to represent the historical features of the rainfall record. Methods for selection of the predictors are presented here. The approach is applied to daily rainfall from Sydney and Melbourne. The models that are developed use daily-level, seasonal-level, annual-level, and multi-year predictors for rainfall occurrence, and daily-level and annual-level predictors for rainfall amount. The resulting generated sequences provide a better representation of the variability associated with droughts and sustained wet periods than was previously possible. These sequences will be useful in catchment water management studies as a tool for exploring the potential response of catchments to possible future rainfall.

Thesis (doctoral) - Delft University of Technology, Delft, 2002. / "Dissertation submitted in fulfillment of the requirements of the Board for Doctorates of Delft University of Technology and of the Academic Board of the International Institute for Infrastructural, Hydraulic and Environmental Engineering for the Degree of Doctor to be defended in public on Monday, 29 April 2002 at 13:30 hours in Delft, The Netherlands." Includes bibliographical references (p. [191]-199).

Thesis (Ph. D.)--University of Hong Kong, 1983.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 97-106).

published_or_final_version / Civil and Structural Engineering / Master / Master of Philosophy

published_or_final_version / Civil Engineering / Master / Master of Philosophy

No description available.