Continuous mixing layer height determination using the LD-40 ceilometer: a feasibility study

Climate of Northeastern Africa varies from humid to semi-arid with both abundant and scarce moisture. Hence, flooding and drought are frequent phenomena which have a direct impact on the agriculture, health, water and other socio-economic sectors of the region. Analysis for the expected changes in the frequency and intensity of extreme rainfall induced by climate change in Ethiopia is the aim of this study.

The Ethiopian area has been divided into three homogeneous rainfall regimes (Zone A-C).The seasonal classification of the region, especially over Ethiopia, is from February to May, June to September and October to January, called Belg, Kiremt and Bega, respectively. Here, more emphasis is given to the Kiremt and Belg seasons.

Anomalous wet and dry decades were identified using ERA-40 data. The condition of the main rain bearing systems during these decades has been assessed and from this result systems which favor more for the extremes have been detected. Among the systems that control the weather activity in the region are: Inter-tropical Convergence Zone (ITCZ), Tropical Easterly Jet (TEJ), East African Low Level Jet (EALLJ), El Niño-Southern Oscillation (ENSO) as well as Mascarena, St. Helena, Azores and Arabian High pressure systems. Then simulations from GFDL and CCCma were analyzed.

There is large decadal variability in the occurrence of extreme rainfall events. During the Kiremt season, 90-96% wet anomalies were mostly occurring in the 1958-1979 time range. The dry anomalous decades were mostly found in 1980-2001 for both zone-A and -B. During Belg, all the dry anomalies are in the third decade of March for zone-B. During April and May, most of the dry anomalies are in the first decade. For zone-C, most of the dry anomalies are in the third decade of March.

The sources of moisture, the means of transport and the dynamic conditions should be satisfied to have a wet anomaly. During the driest decades these conditions are not fulfilled. The future scenario runs from GFDL and CCCma indicate that both models have a consistent trend. However, as most models have limitations, even the seasonality is not well represented in both models