Browsing by Author "Gebrehiwot, Niguse"

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    Effect of Topography in Satellite Rainfall Estimation Errors: Observational Evidence across Contrasting Elevation in the Blue Nile Basin
    (Addis Ababa University, 2013-05) Gebrehiwot, Niguse; Mekonnen, Gebremichael (PhD); Menberu, Meles (PhD)
    In this study, the effect of topography on spatial and temporal variability of rainfall on local (25 km by 25 km grid) scale was analyzed across two contrasting elevation locations. We deployed 10 in high elevation grid (average elevation 2097m) and 9 tipping rain gauges in low elevation grid (average elevation= 695 m). Based on high quality data from the dense rain gauges, we evaluated the spatio-temporal properties of rainfall and evaluated the errors in widely used high resolution satellite rainfall estimates. Three satellite rainfall estimates; TMPA 3B42RT, TMPA 3B42V7 and CMORPH were evaluated using statistical and categorical verification measures. Our results showed that there is significantly large spatio-temporal variability of rainfall at local scale. Diurnal cycle also indicated that, low elevation receives more rainfall at early morning and the high elevation gets more rainfall between the afternoon and mid-night. Compared to the high elevation grid, low elevation grid receives less seasonal accumulation which is less frequent but intense rainfall. High elevation area has more rain events (varying 91 to 105) than low elevation (range; 66 to 7 5) using 6hour minimum inter-event time. In high elevation maximum event rain rate is 67mm/hour but low elevation is l 22mm/hour. Our results also demonstrated clear difference between the two grid cells not only in terms of rainfall magnitude and property but also on the accuracy of satellite rainfall estimates which could be attributed to topography. 3B42RT, 3B42V7 and CMORPH overestimated total 3hour mean rainfall in low elevation (with bias 1.2, I.land 1.5 and ME 0.2, 0.1 and 0.5 respectively). All underestimated in high elevation by 0.9, 0.8 and 0.7, and ME ­ 0.2, -0.2 and -0.5 for 3B42RT, 3B42V7 and CMORPH respectively. The categorical statistics indicated that CMORPH performed better in its probability of detection (POD), but it is influenced by non rainy clouds. All products highly affected by miss rainy events on high elevation area and by more false alarms at low elevation area. From our results 3B42V7 is best in the low elevation and 3B42RT performed better in the high elevation area.