Evaluation and adaptation of a regional climate model for the Horn of Africa: rainfall climatology and interannual variability

Abstract

This study evaluates the ability of the Abdus Salam International Center for Theoretical Physics (ICTP) version 3 Regional Climate Model (RegCM3) to reproduce the observed rainfall amounts and distribution over the topographically varied region of the Horn of Africa. Simulations are performed for the widespread very dry 1984 and locally very wet 1996 years using all of the alternative convective schemes available in the ICTP RegCM3. Extensive comparisons of simulations for 1984 and 1996 reveal that the Emanuel scheme best captures the rainfall patterns and interannual variations for the two extreme years over the Horn of Africa. This scheme therefore is selected for specific adaptation for the region, despite its excessive rainfall estimation there and its demonstrated deficiencies over other regions of Africa not of interest in this study.To reduce rainfall overestimation by the Emanuel scheme, several sensitivity experiments are performed by varying the key parameters that control the rate of convective mass flux, the fraction of condensed water converted to precipitation, and the heating and moistening characteristics of the environment. Results show that the amount of condensed water that ultimately falls out as rain crucially affects simulated rainfall amounts. With an appropriate adjustment to this key parameter, the excessive rainfall amount produced by the Emanuel scheme is reduced substantially. Furthermore, evaluation of RegCM3 simulations for [1982][1983][1984][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994][1995][1996][1997][1998][1999] shows that the modified Emanuel convective scheme not only reproduces the 18-year average rainfall realistically but also captures the interannual variability adequately over the Horn of Africa. The correlation between the modified Emanuel-simulated and Ethiopian station rainfall is quite strong (+0.66). This customized ICTP RegCM3 model now can be used with confidence for the Horn of Africa to study regional rainfall processes and variability and to dynamically downscale seasonal rainfall forecasts.

Large‐scale atmospheric circulation and global sea surface temperature associations with Horn of Africa June–September rainfall

“…Consequently, Eritrea averages only about 600 mm of rain in June-August (Kidanemariam, 2003) whereas the much wetter Ethiopian highlands have a mean monsoon season rainfall of more than 1400 mm (e.g. Figure 1 of Segele and Lamb, 2005; Figure 7 of Segele et al, 2008). The main rain zone is associated with the strongest low-level convergence (Figure 3(a) and (b)) and upper tropospheric divergence (Figure 3(d)), and coincides with areas of weaker but deeper ascending motion west of 40-42°E (west of the upper level subsiding current in Figure 4(a)) and south of 10-12°S (Figure 4(b)).…”

“…This area of large water vapour transport divergence includes western Ethiopia, which receives the highest summer rainfall totals (e.g. Figure 1 of Segele and Lamb, 2005; Figure 7 of Segele et al, 2008). This disparity may result from much of Ethiopian summer rainfall being produced more by westward propagating convective systems that intensify locally due to orographic ascent rather than from in situ convection (Segele and Lamb, 2005).…”

Assessment of the Performance of CORDEX Regional Climate Models in Simulating East African Rainfall

“…Globally, there has been a marked increase in the number of RCM simulations (Alley et al 2007); however, very few RCM studies have been performed over the East African region (Sun et al 1999a;Indeje et al 2000;Anyah, 2005;Anyah et al 2006;Anyah and Semazzi 2006;Anyah and Semazzi, 2007;Segele et al 2009a;Diro et al 2012), and these studies are largely based on the results from a single RCM. However, each model has its strengths and weaknesses.…”

Sensitivity of Horn of Africa Rainfall to Regional Sea Surface Temperature Forcing

“…Through sensitivity analysis, the authors showed that the parameter lo which determines the amount of condensed water that ultimately falls out as rain has the most direct impact on simulated rainfall amounts. Adjustment of this key parameter (see [9]) substantially improved simulated rainfall amounts using the Emanuel scheme. The temporal correlation between customized model-simulated and observed station rainfall (Figure 1) was +0.66 [9].…”

“…Based on such assessment, Segele et al [9] showed that RegCM3 overestimated observed rainfall in the Horn of Africa when using the Emanuel scheme. To improve model agreement, the authors varied model parameters that controlled the rate of convective mass flux adjustment (α), the fraction of condensed water that can be converted to precipitation, and the heating and moistening characteristics of the environment (σs, σd).…”