The African Sahel is one of the most vulnerable regions to climate variability at different time scales. It is an arid to semi-arid region with limited water resources. The summer rainfall is one of these sources, but it exhibits pronounced interannual variability. This thesis presents several aspects of Sahelian Sudan rainfall. Sudan is located at the eastern fringe of the Sahel and its least studied part. We have examined the impact of tropical deforestation on the rainfall, the moisture sources of the region and the temporal characteristics of the observed and modeled rainfall. In a sensitivity study we performed three simulations, one control simulation and then setting the surface condition of South Sudan to either grass or desert conditions. The rainfall was reduced by 0.1 − 0.9 in the grass scenario and by 0.1 − 2.1 mm day−1 (hereafter mm d−1) in the desert scenario. These changes also propagated northward into Sahelian Sudan, indicating a remote impact. The total moisture convergence into Sahelian Sudan was reduced by 11.5% and 21.9% for grass and desert conditions, respectively. The change in moisture convergence into the region motivated a comprehensive analysis of the moisture sources for the region. Two different modeling approaches, Lagrangian and Eulerian, were applied to identify the moisture sources and quantify their contributions to the total annual rainfall budget. The analysis shows that atmospheric flows associated with the Inter-Tropical Convergence Zone (ITCZ), e.g. from Guinea Coast, Central African and Western Sahel, brings about 40% − 50% of the annual moisture supply, while local evaporation adds about 20%. The rest of the moisture comes from the Mediterranean, Arabian Peninsula and the Southern part of the Indian Ocean. While there were differences in the details between the results from the two modeling approaches, they agree on the larger scale results. In an attempt to characterize the temporal character of the rainfall, observed and modeled daily rainfall from different regional climate models was classified into five categories: weak (0.1 −1.0), moderate (&gt;1.0 − 10.0), moderately strong (&gt;10.0 − 20.0), strong (&gt;20.0 − 30.0), and very strong (&gt;30.0) mm d−1. We found that most rain-days were in the weak to moderate rainfall categories, accounting for 60% − 75%. Days that have strong rainfall represent about 6% of the total rain-days, yet they represent about 28% − 48% of the total amount of the annual rainfall. Regional climate models fail to produce the strong rainfall, instead most of the modeled rain-days are in the moderate category and consequently the models overestimated the number of rain-days per year. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>

Das Tianshan-Gebirge, als „Wasserturm“ Zentralasiens, hat entscheidenden Einfluss auf die Wasserressourcen der Region. Untersuchungen von 1950 bis 2016 zeigen, dass der Jahresniederschlag in den meisten Teilen des Gebirges zunahm, außer im westlichen Tianshan, wo er abnahm. Es gibt hoch- und niedrigfrequente Schwankungen im Niederschlag mit 3-, 6-, 12- und 27-jährigen Quasiperioden. Auf Dekadenskala gab es zwei Trockenperioden (1950–1962, 1973–1984) und zwei Feuchtperioden (1962–1972, 1985–2016). Seit 2004 ist eine kontinuierliche Feuchtezunahme zu verzeichnen. Zusammenhänge wurden zwischen Zirkulationsmustern und dem Niederschlag identifiziert. Das East Atlantic-West Russia (EATL/WRUS)-Muster korreliert positiv mit dem Winter-Niederschlag. Das Scandinavia (SCAND)-Muster beeinflusst den Sommerniederschlag. Das Silk Road-Muster (SRP) war im Zeitraum 1964-1984 relevant. Die Feuchtigkeitsquellen für den Tianshan-Niederschlag stammen zu 93,2% von kontinentalen Quellen und nur begrenzt aus dem Ozean. Zentralasien ist die Hauptfeuchtequelle für das Gebirge. Im westlichen Tianshan kommt die Feuchtigkeit hauptsächlich von Zentralasien von April bis Oktober und von Westasien von November bis März. Im östlichen Tianshan tragen Ost- und Südasien sowie Sibirien konstant zur Feuchtigkeit im Sommer bei. Der Beitrag der Feuchtigkeit aus dem Nordatlantik zum Sommerniederschlag im nördlichen, zentralen und östlichen Tianshan zeigt einen abnehmenden Trend, obwohl dieser Beitrag ohnehin begrenzt ist. In Monaten mit extremem Winterniederschlag stammt die größte Zunahme der Feuchtigkeit im westlichen Tianshan aus Westasien, während Europa einen wichtigen Beitrag zu den extremen Winterniederschlägen im nördlichen Tianshan leistet. Im östlichen Tianshan ist die Feuchtigkeitszufuhr aus Ost- und Südasien sowie aus Sibirien während der extremen Niederschlagsmonate sowohl im Winter als auch im Sommer erhöht. / The Tianshan Mountains, the "water tower" of Central Asia, are crucial water sources. Precipitation variability and water vapor transport impact water distribution. The study assessed 1950-2016 precipitation using Mann-Kendall tests and EEMD on GPCC data. Multi-timescale precipitation variations were analyzed with NCEP/NCAR reanalysis, and moisture sources during 1979–2017 with ERA–Interim data. Most of Tianshan had increasing annual precipitation, except Western Tianshan, which experienced a downtrend. Precipitation exhibited 3- and 6-year cycles and 12- and 27-year cycles. On the decadal scale, two dry and two wet periods occurred, with continuous humidification since 2004. A significant positive correlation was found between East Atlantic-West Russia EATL/WRUS circulation pattern and winter precipitation. SCAND influenced Tianshan's summer precipitation, with a wet period after 1988 due to enhanced water vapor flux. SCAND and EAP strengthened water vapor fluxes to Tianshan. SRP impacted Tianshan's summer precipitation during 1964–1984. About 93.2% of Tianshan's moisture comes from continental sources. Central Asia dominates moisture supply. Western Tianshan receives moisture mainly from Central Asia (April to October) and Western Asia (November to March). Almost 13.0% of Eastern Tianshan's summer moisture originates from East and South Asia and Siberia, with steady contributions. Moisture from the North Atlantic Ocean to summer precipitation in Northern, Central, and Eastern Tianshan shows a decreasing trend, but limited overall contribution. Extreme winter precipitation in Western Tianshan is linked to moisture from West Asia. Europe significantly contributes to extreme winter precipitation in Northern Tianshan. Eastern Tianshan sees enhanced moisture from East and South Asia and Siberia during extreme precipitation months in winter and summer.