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Characterization of the Sahelian-Sudan rainfall based on observations and regional climate models
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Physical Geography.
Stockholm University, Faculty of Science, Department of Meteorology .
(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 [en]
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: urn:nbn:se:su:diva-122714OAI: oai:DiVA.org:su-122714DiVA: diva2:868303
Available from: 2015-11-10 Created: 2015-11-10 Last updated: 2015-11-13Bibliographically approved
In thesis
1. On Sahelian-Sudan rainfall and its moisture sources
Open this publication in new window or tab >>On Sahelian-Sudan rainfall and its moisture sources
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
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:nbn:se:su:diva-122731 (URN)978-91-7649-281-9 (ISBN)
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

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