## 4 Summary and Conclusions  
In this work, the potential role of two vegetation-runoff systems (in simulating the SSI of Tropical Africa) was investigated within the framework of the RegCM4. The results showed that the CN-VIC performs better than CN-TOP scheme, particularly in the JJA season in agreement with the results reported by [17]. Quantitatively, the RegCM4 model performance varies with space and time. For instance, the CN-VIC shows a better agreement than the CN-TOP in area 1 during the summer/winter season. In addition, there is no considerable difference between the two simulations over area 3. Despite noted biases, the CN-VIC coupled system can be used in future studies concerning the SSI over tropical Africa. However, the four parameters of the VIC surface dataset need to be recalibrated in comparison with in-situ observations of Africa as reported by [3; 6; 14] and additional sensitivity studies are needed to ensure a good performance of the CN-VIC over tropical Africa. Additionally, the role of land-surface hydrology schemes needs to be incorporated in high resolution RCMs to better constrain the SSI with respect to reanalysis/satellite products and improve the simulated SSI in comparison with reanalysis or satellite products and in-situ observations. Possible effects of the simulated SSI on the surface energy balance, surface climate, terrestrial carbon fluxes (e.g., gross primary production; GPP) and potential evapotranspiration (PET) of tropical Africa were also discussed. For instance, ref. [6] reported that the CN-VIC simulates more downward short and long wave radiation fluxes than the CN-TOP against reanalysis product leading to high rate of heat emitted from the ground. Such  
Figure 5: The figure shows a Box-and-whisker plot to show the simulated surface solar irradiance (SSI; in W m\({}^{-2}\)) spread for the CN-TOP (in red) and CN-VIC (in green) with respect to the SRB reanalysis product (in blue) for the winter (right column) and summer (left column) seasons over three sub-area averages highlighted in Figure 1.  
behaviour can explain why the CN-VIC overestimates the sensible heat flux (SHF) more than the CN-TOP particularly over the Congo basin. Furthermore, the low bias of the SSI (as simulated by the CN-VIC) contributes to the high warm bias of the 2-m mean air temperature (T2M) in comparison with the Climate Research Unit gridded product (CRU; [18]). In addition, the SSI biases can be partially responsible for the positive/negative biases of the GPP as reported by [6; 14]. Also, ref. [6] reported that vegetation-runoff systems show significant changes in the simulated SSI and T2M. Therefore, the simulated boreal summer PET changes considerably between the two systems with respect to the CRU product as reported by [16]. Future study will address the following points:  
1. The role of aerosols and various clouds cover schemes which notably affect the simulated SSI. 2. Downscaling CMIP5 General Circulation Models (GCMs) participating in the CMIP5/ CMIP6 simulations (e.g., [19; 20]) using the regional coupled RegCM4-CLM45-CN-VIC model with enabling the dynamic vegetation (DV) module and LULCC to assess the possible future changes in the simulated SSI under the moderate scenario (RCP4.5) and extreme scenario (RCP8.5). 3. Examine the potential influence of the vegetation-runoff systems on the vertical radiation profile. This research received no external funding. Not applicable. This work was conducted as a part of the OFID-ICTP program in the ICTP institute. The climate group in the ICTP (Earth System Physics; ESP) team is acknowledged for providing the RegCM code, computational facilities and input data to run the model. SRB data were obtained from the NASA Langley Research Center Atmospheric Sciences Data Center NASA/GEWEX SRB Project. NCEP/NCAR2 Reanalysis data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at [[http://www.esrl.noaa.gov/psd](http://www.esrl.noaa.gov/psd)]([http://www.esrl.noaa.gov/psd](http://www.esrl.noaa.gov/psd)) (accessed on 1 May 2018).