Browsing by Author "Terefe, Tadesse"

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    Assessment of spatial and temporal drought variability and its mechanisms over Ethiopia using observational data analysis and regional climate model experiments
    (Addis Ababa University, 2013-06) Terefe, Tadesse; Mengistu, Gizaw (PhD)
    Drought occurrences in Ethiopia and underlying physical mechanisms are investigated during the 1982 to 2009 period using the Standardized Precipitation Index (SPI) and Palmer Drought Severity Index (PDSI) derived from a station observation datasets and a regional climate model (RegCM4) simulation driven by ERA-Interim reanalysis. The RegCM4 model is evaluated for its performance over Ethiopia. The RegCM4 rainfall captured the observed spatial and temporal variability of rainfall over Ethiopia during the spring (FMA) and summer (JJAS) seasons during this period. The performance of the model is also evaluated against other observational atmospheric variables such as temperature, wind field and surface pressure. The RegCM4 shows a good performance in simulating the spatial and temporal variability of these atmospheric variables over Ethiopia. Trend empirical orthogonal function (TEOF), regression, wavelet and composite analyses based on observation and model simulation are then used to investigate the drought trend, frequency and intra-annual variability along with the associated large-scale circulation patterns. All methods of analysis applied to annual data show that the North and Northwest regions of Ethiopia had experienced frequent and more severe drought conditions during the 1980s that attained peak in 1983/1984 and showed a recovery phase afterwards. Drought becomes more frequent and intense in recent decades over southern and southwestern regions in tandem with apparent shift from northern Ethiopia. Analysis at the seasonal scale shows that drought events are more intense and frequent during the spring season than the summer season over the southern regions. Drought conditions during spring are significantly associated with the negative phase of ENSO, the weakening of the Arabian High and the subtropical westerly jet streams. On the other hand, drought conditions during summer are linked to the positive ENSO phase, a weakening of the upper level jet streams and a lower level moisture influx from the Atlantic and Indian oceans. An uneven heating of equatorial Pacific, II Atlantic and Indian Oceans is the main driver for both phases of ENSO, and corresponding anomalies in the moisture and circulation patterns during the two seasons
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    Further Development of Polar wind Model
    (Addis Ababa University, 2007-03) Terefe, Tadesse; Damtie, Baylie (PhD)
    This thesis deals with modeling of major ions outflow in the polar cap at altitudes higher than 400km. Different ion outflow models are numerical investigated and the pros and cons of these models are discussed. Observations are crucial for model validations. In this work we have used Akebono Satellite observations, which showed that between 5000 and 9000km altitudes H+ and 0+ can have higher outflow velocities in the day-side region that on the night-side [Abe and et al,1993a, 1993b]. classical polar wind model cannot explain fully these observations [Schunk and etal,1981, 1988]. In this thesis, in order to address the observation we discuss hydrodynamic model by considering driven shock wave. Most of observed properties are well explained by using this model