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On Sahelian-Sudan rainfall and its moisture sources
Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). (dynamical meteorology)
2015 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

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 (>1.0 − 10.0), moderately strong (>10.0 − 20.0), strong (>20.0 − 30.0), and very strong (>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.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Meteorology, Stockholm University , 2015. , s. 36
Nyckelord [en]
Sudan, Sahel, rainfall, land use, deforestation, moisture sources, moisture transport, Lagrangian, moisture tagging, regional model, climate modeling
Nationell ämneskategori
Meteorologi och atmosfärforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
URN: urn:nbn:se:su:diva-122731ISBN: 978-91-7649-281-9 (tryckt)OAI: oai:DiVA.org:su-122731DiVA, id: diva2:868385
Disputation
2015-12-11, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (Engelska)
Opponent
Handledare
Anmärkning

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Tillgänglig från: 2015-11-19 Skapad: 2015-11-10 Senast uppdaterad: 2019-11-01Bibliografiskt granskad
Delarbeten
1. Climate impact of deforestation over South Sudan in a regional climate model
Öppna denna publikation i ny flik eller fönster >>Climate impact of deforestation over South Sudan in a regional climate model
2013 (Engelska)Ingår i: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 33, nr 10, s. 2362-2375Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

This study examines the sensitivity of climate to changes in vegetation cover and land use in South Sudan. The focus lies on the effect of deforestation on precipitation and surface temperature especially during the rainy season. Sensitivity experiments are performed with the third version of the Abdus Salam International Centre for Theoretical Physics Regional Climate Model (RegCM3) where the present forest and vegetation cover south of 10 degrees N in Sudan are replaced by either grass or, as an extreme case, desert. The model experiments were conducted for a time period of almost 21 years, from January 1989 to August 2009, and were preceded by a control experiment to ascertain the fidelity of the model simulations. The experiments indicate that the vegetation changes affect precipitation and surface temperature in both Southern and Central Sudan significantly although the land cover changes were imposed only in the south. The precipitation during the rainy season (June through September) was reduced in the perturbed region by about 0.1-2.1 mm d(-1) for the desert scenario and by 0.1-0.9 mm d(-)1 for the grass scenario. The surface temperature increases by about 1.2 and 2.4 degrees C in the grass and desert scenario, respectively. The precipitation reduction is thus not only local but also extends to Central Sudan and neighbouring regions. The study demonstrates significant dependency for Southern and Central Sudan precipitation on the land use in Southern Sudan and indicates that the deforestation has both local and non-local regional climatic effects.

Nyckelord
land use change, regional modeling, climate change, drought, Sudan rainfall, vegetation change, deforestation
Nationell ämneskategori
Meteorologi och atmosfärforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-96670 (URN)10.1002/joc.3586 (DOI)000326103100008 ()
Anmärkning

AuthorCount:3;

Tillgänglig från: 2013-11-26 Skapad: 2013-11-25 Senast uppdaterad: 2019-12-17Bibliografiskt granskad
2. Lagrangian tracing of Sahelian Sudan moisture sources
Öppna denna publikation i ny flik eller fönster >>Lagrangian tracing of Sahelian Sudan moisture sources
2015 (Engelska)Ingår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 120, nr 14, s. 6793-6808Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The Sahelian Sudan is an arid to semiarid region that depends on the seasonal rainfall as the main source of water, but its rainfall has large interannual variability. Such dry regions usually have their main moisture sources elsewhere; thus, the rainfall variability is directly related to the moisture transport. This study seeks to identify source regions of water vapor for Sahelian Sudan during the monsoon period, from July to September. We have used the Lagrangian trajectory model FLEXPART driven by ERA-Interim reanalysis for the time period 1998 to 2008. The results show that most of the air masses that reach this region during the monsoon period have their major origins over the Arabian Peninsula, Central Africa, or are associated with the tropical easterly jet. Flow associated with Intertropical Convergence Zone contributes almost half of the total precipitated water; most of it comes from Central Africa. This suggests that moisture recycling is the major contributor, compared to Oceanic sources. The flows from the northeast (Arabian Peninsula and north Asia) and east (Horn of Africa and north Indian Ocean) contribute about one third of the precipitated water. The rest of precipitated water comes from the Mediterranean, subtropical Atlantic, and western Sahel, all with smaller contribution. Our results also indicate that different subregions of Sahelian Sudan have different moisture sources. Such result needs to be taken into account in seasonal forecasting practices.

Nyckelord
moisture transport, moisture sources, rainfall, Sahelian Sudan, Lagrangian modeling
Nationell ämneskategori
Meteorologi och atmosfärforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-120707 (URN)10.1002/2015JD023238 (DOI)000359804900005 ()
Tillgänglig från: 2015-09-17 Skapad: 2015-09-15 Senast uppdaterad: 2019-12-17Bibliografiskt granskad
3. Sources of Sahelian-Sudan moisture: Insights from a moisture-tracing atmospheric model
Öppna denna publikation i ny flik eller fönster >>Sources of Sahelian-Sudan moisture: Insights from a moisture-tracing atmospheric model
2016 (Engelska)Ingår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 121, nr 13, s. 7819-7832Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The summer rainfall across Sahelian-Sudan is one of the main sources of water for agriculture, human, and animal needs. However, the rainfall is characterized by large interannual variability, which has attracted extensive scientific efforts to understand it. This study attempts to identify the source regions that contribute to the Sahelian-Sudan moisture budget during July through September. We have used an atmospheric general circulation model with an embedded moisture-tracing module (Community Atmosphere Model version 3), forced by observed (1979-2013) sea-surface temperatures. The result suggests that about 40% of the moisture comes with the moisture flow associated with the seasonal migration of the Intertropical Convergence Zone (ITCZ) and originates from Guinea Coast, central Africa, and the Western Sahel. The Mediterranean Sea, Arabian Peninsula, and South Indian Ocean regions account for 10.2%, 8.1%, and 6.4%, respectively. Local evaporation and the rest of the globe supply the region with 20.3% and 13.2%, respectively. We also compared the result from this study to a previous analysis that used the Lagrangian model FLEXPART forced by ERA-Interim. The two approaches differ when comparing individual regions, but are in better agreement when neighboring regions of similar atmospheric flow features are grouped together. Interannual variability with the rainfall over the region is highly correlated with contributions from regions that are associated with the ITCZ movement, which is in turn linked to the Atlantic Multidecadal Oscillation. Our result is expected to provide insights for the effort on seasonal forecasting of the rainy season over Sahelian Sudan.

Nyckelord
Sahel, Sudan, rainfall, moisture sources, moisture tagging, rainfall variability
Nationell ämneskategori
Meteorologi och atmosfärforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-134287 (URN)10.1002/2015JD024575 (DOI)000380730500019 ()
Tillgänglig från: 2016-10-05 Skapad: 2016-10-03 Senast uppdaterad: 2019-12-17Bibliografiskt granskad
4. Characterization of the Sahelian-Sudan rainfall based on observations and regional climate models
Öppna denna publikation i ny flik eller fönster >>Characterization of the Sahelian-Sudan rainfall based on observations and regional climate models
2018 (Engelska)Ingår i: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 202, s. 205-218Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The African Sahel region is known to be highly vulnerable to climate variability and change. We analyze rainfall in the Sahelian Sudan in terms of distribution of rain-days and amounts, and examine whether regional climate models can capture these rainfall features. Three regional models namely, Regional Model (REMO), Rossby Center Atmospheric Model (RCA) and Regional Climate Model (RegCM4), are evaluated against gridded observations (Climate Research Unit, Tropical Rainfall Measuring Mission, and ERA-interim reanalysis) and rain gauge data from six arid and semi-arid weather stations across Sahelian Sudan over the period 1989 to 2008. Most of the observed rain-days are characterized by weak (0.1-1.0 mm/day) to moderate ( > 1.0-10.0 mm/day) rainfall, with average frequencies of 18.5% and 48.0% of the total annual rain-days, respectively. Although very strong rainfall events ( > 30.0 mm/day) occur rarely, they account for a large fraction of the total annual rainfall (28-42% across the stations). The performance of the models varies both spatially and temporally. RegCM4 most closely reproduces the observed annual rainfall cycle, especially for the more arid locations, but all of the three models fail to capture the strong rainfall events and hence underestimate its contribution to the total annual number of rain days and rainfall amount. However, excessive moderate rainfall compensates this underestimation in the models in an annual average sense. The present study uncovers some of the models' limitations in skillfully reproducing the observed climate over dry regions, will aid model users in recognizing the uncertainties in the model output and will help climate and hydrological modeling communities in improving models.

Nyckelord
Regional modeling, Climate model evaluation, Precipitation, Sahel, Rain-day analysis
Nationell ämneskategori
Meteorologi och atmosfärforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-154700 (URN)10.1016/j.atmosres.2017.12.001 (DOI)000426226500021 ()
Tillgänglig från: 2018-04-16 Skapad: 2018-04-16 Senast uppdaterad: 2019-12-17Bibliografiskt granskad

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Salih, Abubakr A. M.
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