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On Sahelian-Sudan rainfall and its moisture sources
Stockholm University, Faculty of Science, Department of Meteorology . (dynamical meteorology)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2015. , 36 p.
Keyword [en]
Sudan, Sahel, rainfall, land use, deforestation, moisture sources, moisture transport, Lagrangian, moisture tagging, regional model, climate modeling
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-122731ISBN: 978-91-7649-281-9 (print)OAI: oai:DiVA.org:su-122731DiVA: diva2:868385
Public defence
2015-12-11, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript.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

Available from: 2015-11-19 Created: 2015-11-10 Last updated: 2015-11-25Bibliographically approved
List of papers
1. Climate impact of deforestation over South Sudan in a regional climate model
Open this publication in new window or tab >>Climate impact of deforestation over South Sudan in a regional climate model
2013 (English)In: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 33, no 10, 2362-2375 p.Article in journal (Refereed) 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.

Keyword
land use change, regional modeling, climate change, drought, Sudan rainfall, vegetation change, deforestation
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-96670 (URN)10.1002/joc.3586 (DOI)000326103100008 ()
Note

AuthorCount:3;

Available from: 2013-11-26 Created: 2013-11-25 Last updated: 2017-12-06Bibliographically approved
2. Lagrangian tracing of Sahelian Sudan moisture sources
Open this publication in new window or tab >>Lagrangian tracing of Sahelian Sudan moisture sources
2015 (English)In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 120, no 14, 6793-6808 p.Article in journal (Refereed) 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.

Keyword
moisture transport, moisture sources, rainfall, Sahelian Sudan, Lagrangian modeling
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-120707 (URN)10.1002/2015JD023238 (DOI)000359804900005 ()
Available from: 2015-09-17 Created: 2015-09-15 Last updated: 2017-12-04Bibliographically approved
3. Sources of Sahelian Sudan moisture: insights from a moisture-tracing atmospheric model
Open this publication in new window or tab >>Sources of Sahelian Sudan moisture: insights from a moisture-tracing atmospheric model
(English)Manuscript (preprint) (Other academic)
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 effort to understand it. This study attempts to identify the source regions that contribute to Sahelian Sudan moisture budget during the monsoon months – July through September. We have used an atmospheric general circulation model with embedded water-tracing module (CAM3), forced by observed (1979-2013) sea-surface temperatures (SST). The result suggests that about 38.6% of the moisture comes with the atmospheric flow associated with the seasonal migration of the Intertropical convergence zone (ITCZ), which originates from Guinea Coast, Central Africa and the Western Sahel. The Mediterranean, Arabian Peninsula, and South Indian Ocean regions accounts 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 previous analysis that used the Lagrangian model FLEXPART. 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 of the rainfall over the region is highly correlated with contribution from regions that are associated with the ITCZ movement, which is linked to the Atlantic Multidecadal Oscillation (AMO). A dry regime during the time period 1979-1994 seems to be associated with the cold phase of AMO, and relatively wet period 1995-2013 is associated with the warm mode phase of AMO. 

Keyword
Sahel, Sudan, rainfall, moisture sources, moisture tagging, moisture transport
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-122712 (URN)
Available from: 2015-11-10 Created: 2015-11-10 Last updated: 2015-11-13Bibliographically approved
4. Characterization of the Sahelian-Sudan rainfall based on observations and regional climate models
Open this publication in new window or tab >>Characterization of the Sahelian-Sudan rainfall based on observations and regional climate models
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Sudan is part of the African Sahel region, which is known to be highly vulnerable to climate variability and change. However, the characteristics of observed and modeled rainfall in the country are rarely available in literature. The focus of this paper is two-fold: to analyze the rainfall aspects of Sahelian Sudan in terms of distribution of rain-days and amount, and to examine whether regional climate models can capture these rainfall features. Outputs from three regional models, namely REMO, RCA and RegCM4, have been evaluated against gridded observations and rain-gauge data from six arid and semi-arid weather stations spread 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) rainfalls, with average frequencies of 18.5% and 48.0% out of the total annual rain-days, respectively. Although very strong rainfall events (>30.0 mm/day) occur rarely, they account for a large portion of the annual rainfall amount. The performance of the models varies both spatially and temporally. RegCM4 output is the closest to the observations in reproducing the annual rainfall cycle, especially for the more arid locations, but an unusual peak in June is present. All three models fail to capture the frequency of very strong rainfall events and, thus, underestimate the contribution of extreme rainfall events to the total annual number of rain-days and rainfall amount. Nevertheless, more moderate rainfall events from the models compensate this underestimated rainfall amount. REMO and RCA show a systematic tendency to overestimate the number of rain-days. Generally, the source of errors in rainfall modeling can be attributed to the convection parameterization in the models. This study suggests that rainfall occurring due to large-scale atmospheric circulation also contributes to the error. The present study uncovers some of the models’ limitations in skillfully reproducing the observed climate over dry regions.  It will help climate and hydrological modeling communities, i.e. developers, in improving the models and also users in recognizing the uncertainties in model outputs.

Keyword
regional modeling, climate model evaluation, precipitation, Sahel, TRMM, rain day analysis
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-122714 (URN)
Available from: 2015-11-10 Created: 2015-11-10 Last updated: 2015-11-13Bibliographically approved

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