Prediction of Onset of Rainy Season in Various Climatic Zones of Sudan

Abstract

A mathematical model has been developed, the physical part of the model is based on the concept of the variation in equivalent potential temperature (), that occurs as a result of the seasonal, monthly and daily variation of moisture of the summer monsoon flow over Sudan. Equivalent potential temperature () is a conservative physical parameter with respect to both saturated and unsaturated (dry) atmospheric physical processes, which means the physical characteristics of () will not change temporarily or specially and could be used as a predictor of the onset and cessation of rainy season (predictant). could be calculated form the saturation vapor pressure and the saturation specific humidity. For the purpose of this study the onset of rainfall defined as the event, which signal the start of the rain spell in which the total precipitation fall after first became positive and continue positive for at least 15 days has a total amount of 20 mm. The data used for the mathematical model were the daily mean values of surface pressure (P). Daily rainfall data, daily maximum temperature, daily minimum temperature, daily mean values of relative humidity and dates of first precipitation of 20 mm. The Data extend for ten years for every selected station. Selection of the years depends on the continuity and consistency of the data needed. The eight selected stations were Khartoum, which represents the semi-desert climatic zone, Kassala that represents the dry zone, Sinnar and Nyala, which represent the semi-dry climatic zone, Gadarif and Malakal, which represent the semi-humid climatic zone, Wau and Juba, which represent the humid climatic zone. The data used in the mathematical model were collected from the data bank of Sudan Meteorological Authority. The global oceans SSTs data used to run the dynamical model are the National Center for Environmental Prediction (NCEP) reanalysis fields, also called the optimum interpolation (OI) SSTs data set. The regression equation to test the relation between the onset dates predicted by the model and the actual onset dates for all the selected stations during the selected years shows that R²= 0.95, which means that the model was able to explain 95% of the cases. Assessment of the model shows that values of onset days range between 60 to 90, which represent the model prediction for the two wet zones, the semi-humid zone and the humid zone are less scattered compared to the values range between 100 to 160, which represent the model prediction for the three dry zones, the semi-desert zone, the dry zone and the semi-dry zone are more scattered. It is clear that the model shows best agreement with the wet zones rainfall onset dates. The dynamical model results indicate that the general circulation patterns, either simulated from ECHAM 4.5 or reanalysis of NCEP are reasonably effective in monitoring climate of particular area. In this study, the circulation climatologic as well as annual patterns are investigated preferably at surface, 850mb, 700mb and 200mb levels with emphasis given to June to September (JJAS). Results of the dynamical model have shown that these levels play a great role in enhancing moisture and generate dynamical instabilities over various sectors of the region. The wind patterns generated from ECHAM 4.5 and NCEP are demonstrated for selected constant pressure levels.