Monitoring Rainfall Trends and Extremes in South Sudan and Sudan Using Daily Station and Satellite Rainfall Data for 1983-2013

The African Sahel, the region between about 12-18°N stretching from the Atlantic to the Red Sea coast, is currently experiencing a rainfall recovery from the drought period in the 1970s and 1980s. This recovery is often associated with more intense daily rainfall totals and, at least in the western part of the Sahel, with a prolonged rainy season. The countries of South Sudan and Sudan stretch from about 4°N to 22°N. They encompass a variety of climate zones, ranging from dry and wet sub-humid, semi-arid Sahelian to hyper-arid Saharan desert climates in the north. Studies investigating spatio-temporal characteristics of trends and extremes in daily rainfall are hitherto lacking for this large area in eastern North Africa. Thus, the extent of rainfall recovery, if any, and trends in extremes are not well monitored. To fill this gap, rainfall trends from two latest generation, daily satellite-based rainfall products, i.e. Precipitation Estimation from Remote Sensing Information using Artificial Neural Network (PERSIANN) and Climate Hazards Group InfraRed Precipitation (CHIRPS ) with station data, will be compared to trends from 30 stations for the period 1983-2013 in the present contribution. During the period 1998-2013, this comparison will be extended to daily data from the Tropical Rainfall Measuring Mission (TRMM) 3b42 version 7 dataset. Trend analyses comprise absolute values, the Standardized Precipitation Index (SPI) and climate extremes indices defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) and monsoon onset and withdrawal dates. In addition, latest trends will be put into the context of multi-decadal variability of daily rainfall totals from long-term observations at the 30 stations. Preliminary results from the satellite products seem to corroborate the proposed recent rainfall recovery only in the western Sudanian Sahel. Especially since 1998, the eastern Sudanian Sahel, lying adjacent to the Ethiopian Highlands, shows a strong downward trend in rainfall. Thus, it appears that the region of Sahelian rainfall recovery from the central Sahel does not extend eastward towards the foothills of the Ethiopian highlands where a major agricultural region of Sudan is located. Moreover, the station data suggest that the present recovery, if any, has by far not led to the wetness of the 1950s and 1960s. While the CHIRPS and PERSIANN satellite data are consistent with these overall trends, some station trends do not agree with the satellite rainfall trends at all. This is especially true when it comes to number of rainfall days exceeding certain thresholds. Here, satellite products suffer from not reproducing the intensity distribution of daily rainfall adequately. Comparison with the TRMM product, one of the best available products, suggests that PERSIANN is superior to CHIRPS over the Sudan countries in this regard. The latter two products also show strikingly different trends in the vicinity of the Ethiopian Highlands. It is concluded here that ground truth data are irreplaceable in terms of monitoring rainfall characteristics on daily time scales that are especially relevant for rain-fed farming practices in the study region.