University of Twente Student Theses


Assessing diurnal variability of rainfall: A remote sensing based approach

Fenta, Ayele Almaw (2010) Assessing diurnal variability of rainfall: A remote sensing based approach.

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Abstract:Rainfall variability studies have wide range of applications in meteorology, climatology and hydrological processes. The assessment of the diurnal cycle of rainfall requires rainfall data at relatively high temporal resolution with large spatial coverage using evenly distributed rain gauges. In most tropical regions, however, observations are commonly available only at daily time steps from rain gauges that are sparsely and unevenly located. The objectives of this study are: to assess the diurnal cycle of rainfall using Tropical Rainfall Measuring Mission (TRMM) satellite observations, validate TRMM rainfall diurnal cycle estimates using ground observations and assess the spatial pattern of the rainfall diurnal cycle. One year (2005) of TRMM Microwave Imager (TMI) data for French Guiana and seven years (2002-2008) of Precipitation Radar (PR) and TMI data for the Upper Blue Nile basin were used for the rainy season. These two areas are selected since the areas are characterized by different latitudinal locations, different climatic and topographic settings. The analysis was based on GIS operations, statistical techniques such as correlation matrix and harmonic analysis. Moreover, since the Upper Blue Nile basin is characterized by high topographic variation, the spatial variability and the relation between rainfall and elevation was assessed taking transects across the basin. Over the two study areas the diurnal cycles of rainfall occurrence from TRMM estimates show significant correlation with the ground observations at 95 % confidence level. In the Upper Blue Nile basin, the PR conditional mean rain rate estimates are closer to the ground observations than the TMI. Over most parts of French Guiana, the maximum rainfall occurrence and conditional mean rain rate are observed from mid to late afternoon between 1500-1800 local standard time (LST). Observations from PR and TMI reveal that over most areas of the Upper Blue Nile basin, the rainfall occurrence and conditional mean rain rate are maximum between mid to late afternoon (1500-1800LST). However, exceptions to this are Lake Tana and the southwest and southeast parts of the basin where midnight and early morning maximum are observed. Along the Blue Nile River gorge the rainfall occurrence and conditional mean rain rate are maximum during the night (2000- 2300LST). In general, the mean of the diurnal cycle, the amplitude and the time of amplitude show spatially varying diurnal cycle owing to the high topographic variability of the basin. Along transects that are taken across the basin from north to south and east to west, it is observed that areas on the windward side of the high mountain ranges receive higher amount of rainfall than areas on the leeward side. As such, mountain orientation in relation to general pattern of air circulation influences the rainfall distribution resulting in rain shadow effect in the northeast parts of Choke mountain and the ridges in the northeast of the basin. Moreover, a direct relation between rainfall occurrence and elevation is observed especially for 1700-1800LST. The seasonal mean rainfall depth is highest in the southwest areas and central highlands of the basin while the areas in the north, northeast and along the Blue Nile gorge receive the least amount of rainfall. This study provides relevant information to understand the effect of terrain on diurnal variability of rainfall and variation of rainfall with elevation and mountain orientation using satellite based rainfall observations. As such, it is of high use for meteorological, climate and hydrological studies, water resources planning and management and hydropower development. Key words: French Guiana, Upper Blue Nile basin, TRMM, PR, TMI, rainfall variability, diurnal cycle
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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