the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Apr 2025
Optimizing Precipitation Parameterizations in Regional Climate Model (RegCM5): A Case Study of the Upper Blue Nile Basin (UBNB)
Abstract. Accurate simulation of precipitation over complex terrains such as the Upper Blue Nile Basin (UBNB) is essential for water resource management and climate impact assessments. The UBNB is characterized by complex terrain and convective precipitation systems that challenge the fine-scale climate simulation processes. This research aims to investigate the best precipitation parameterizations in the Regional Climate Model System (RegCM5) simulating different convective and large-scale schemes over the UBNB domain with 10 km resolution. The RegCM5 is driven by the fifth generation atmospheric reanalysis (ERA5) for a period of (2000–2009) using the hydrostatic dynamical core. The total precipitation simulations of the different calibration scenarios are assessed to select the most optimal RegCM5 configuration over the UBNB. Results show that the Emanuel scheme coupled with Nogherotto-Tompkins (NoTo) is the most effective parameterization to capture precipitation but reveals significant overestimation with an accepted wet bias of 66 %. The model highlights challenges in reproducing the UBNB's precipitation variability with a moderate to relatively good correlation of precipitation patterns from 0.46 to 0.77. Sensitivity analysis suggests that the interplay between the model's hydrostatic configuration and vertical domain extent significantly influences precipitation outputs, where deficiency in capturing the large-scale circulations. The research recommended to focus on dynamics advancement, and exploring parameterization schemes enhancing the precipitation representation such as the Planetary Boundary Layer (PBL) in Future.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-532', Anonymous Referee #1, 20 Aug 2025
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RC2: 'Comment on egusphere-2025-532', Anonymous Referee #2, 04 Sep 2025
This manuscript investigates the performance of the RegCM5 model with a hydrostatic dynamical core in simulating precipitation over the complex terrain of the Upper Blue Nile Basin (UBNB). The study leverages high-resolution ERA5 reanalysis data to evaluate various convective and microphysical parameterization schemes, ultimately identifying the Emanuel convection scheme coupled with the Nogherotto-Tompkins (NoTo) microphysics scheme as the optimal configuration. The following points should be addressed to improve the clarity, robustness, and overall quality of the manuscript:
Major points:
- The statement that even the best parameterization shows a 66% bias is significant. This level of error raises concerns about the model's credibility for the region. The authors should provide a more comprehensive discussion on how to interpret this result. Specifically, they should: a) Clearly state what level of bias is considered acceptable for the intended applications of the model in this study. b) Discuss whether the model, despite the bias, successfully captures the dominant spatial and temporal patterns of climate variability, which can sometimes be more important than the absolute values for certain applications.
- The methodology does not mention a threshold for defining a "wet day" (e.g., >0.1 mm/day). Please clarify if all precipitation values, including trace amounts, were used in the analysis. The low frequency of zero precipitation values shown in Figure 7 suggests the model (and possibly the observations) rarely simulates completely dry days. Confirming the wet-day threshold is important for interpreting intensity and frequency results.
- The manuscript compares different parameterizations but lacks a detailed discussion on the physical reasons why the optimal scheme (Emanuel + NoTo) performed best. The authors should elaborate on the key physical differences between the schemes (e.g., in their triggering of convection, closure assumptions, or ice-phase processes) and provide a hypothesis for why this specific combination is more suitable for representing the dominant atmospheric processes in the UBNB.
Minor points:
- The 10km resolution may be insufficient to capture local processes in the UBNB's complex terrain. Justification for this choice (e.g., computational limits or sensitivity tests) should be provided, along with discussion on how resolution impacts simulation accuracy.
- Figures 1 and 2 should be merged to clearly show the UBNB within the larger model domain, improving readability.
- L158 - Although monthly evaluation is common, adding daily-scale metrics would strengthen the assessment of rainfall distribution and intensity.
- The spatial discontinuities in simulated precipitation (Fig. 4), especially in the northeast, require physical explanation—g., topography representation, land-atmosphere feedbacks, or convective processes.
- The selection of Emanuel+NoTo needs clarification, as Kain-Fritsch+NoTo performed well in explained variance for key seasons (Fig. 6). The trade-offs and reasons for the final choice should be explicitly explained.
Citation: https://doi.org/10.5194/egusphere-2025-532-RC2
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- 1
Review: "Optimizing Precipitation Parameterizations in Regional Climate Model (RegCM5): A Case Study of the Upper Blue Nile Basin (UBNB)"
A set of 7 numerical experiments using Regional Climate Model version 5 (RegCM5) was conducted to understand the better precipitation parametrization to reproduce the monthly and daily precipitation over the upper blue Nile basin (UBNB). The numerical experiments combine different cumulus convection schemes (over the land the Kain-Fritsch, or Grell, or Emanuel, or Tiedke, or Emanuel are used, combined with Emanuel over the sea) and large-scale precipitation schemes (SUBEX and NoTo). Simulations were driven by ERA5 reanalysis and PP7 precipitation data was used as observational reference. For the comparison between simulated and observed precipitation it is used some statistical indices. The main conclusion is that predominates a general overestimation of the simulated rainfall in most seasons of the year, with an acceptable performance being obtained by the combination of Emanuel for cumulus convection and NoTo for large-scale precipitation schemes. The analysis of simulation performance in the region is a relevant topic with large potential to add new knowledge related to models' physical parameterizations behavior in the tropics. However, as highlighted in the major points, there are some statements/interpretations that need to be clarified/better founded before the acceptance of the manuscript.
Major points
“Comparison of precipitation parametrizations in Regional Climate Model (RegCM5): A case study for Upper Blue Nile Basin (UBNB)”
Starting with L24-277 and following with L37-45. After that, L27-37 would be followed by L46-58.
I noted that the main objective of the text is not clearly established at the end of the introduction. There are some indications of the objectives in L64-67. I suggest that the authors reorganize the objectives clearly at the end of introduction.
Minor points
In many parts of the text (L31; L46; L142-150; L195; L265), the citations are not correct. For example: L108 - in (Giorgi et al, 2023b) should be in Giorgi et al. (2023b); L148 should be “by Holslag et al. (1990)”; L149 should be “by Zeng et al. (1998)” and many others in the text. Please, check all the text.
L30-31 - should be “... interannual) controlled by the Global …. Abtew et al. (2019) found … ”
L37 - should be “ … rainfall since it is the main rain …"L39;
L109-111 - should be “are driven by atmospheric variables and SST from ERA5 reanalysis data from ECMWF (Hersbach et al., 2020) with 0.25o x 0.25o of horizontal resolution for the period 2000-2009. For evaluation, observed … for the period 2001-2009” since SST was already defined in L31.
L118 - should be “The domain has a 10 km horizontal … longitude (Fig.2 ), involving …””
L121 - The 18 vertical levels is a very small number for a 10km horizontal resolution. Why only 18 vertical levels?
L127 - should be “... in the different numerical experiments …” since the authors used the code as it is, i.e., they do not change any parameter or physical parametrization.
L139-140 - should be “Hence, a new set of simulations was conducted by using Nogherotto-Tompkins (NoTo; Nogherotto et al., 2016) microphysic scheme, which treats the mixed …. Over East Africa, Godoshava and Semazzi (2019) revealed …. In addition, Kalmár et al. (2021) …”
L150 - In many parts of the text, the authors used Emanuel_NoTo, Tiedke_Noto and Kain-Fritsch_NoTo. This information should be in Table 2.
L157 - remove “calibration”; it is more common to use “relative bias” instead “percent bias”.
L158 - What is the interpretation of the RSR index? What is the range of acceptance of RSR? It was calculated considering a time series or for the mean spatial pattern in UBNB? Please, clarify it in the text and also include a reference for the index.
L163 - remove “estimated”
L165 - remove “to check spatiotemporal distribution” since this information is already in the beginning of the phrase.
L180 - should be “ … for the period 2001-2009 since the year 2000 was considered as …”
L190 - What is the meaning of “reduces the significance between CCs”? Please, clarify in the text.
L194 - should be “ are presented in Fig. 3b”.
L183 - should be “ … UBNB the Fig. 3 and Table 3 show the evaluation of the simulated precipitation for the seven numerical experiments””
L202 - Suggestion “The performance of the experiments S5, S6 and S7, which use the NoTo microphysics scheme, is evaluated by analyzing the spatial pattern …”
L206 - should be “The model also overestimated rainfall in eastern …”
L218 - should be “need an improvement”
L221 - should be “exhibit a low correlation”
L242-243 - should be “ characteristics simulated in the experiments S5, S6 and S7 and observed by PP7 during the FMAM, JJAS and ONDJ (Fig. 7). “
L245 - should be “... (S6) slightly better represents the observed distribution of the daily precipitation during the FMAM (Fig. 7a)”
L278 - I think that should be better “non-convective precipitation and reduces the biases in the simulated total precipitation.”
L296 - What is the meaning of “refine the physical parametrizations”?
Figures: 1) The labels in most of the figures need to be improved since they are very small, which makes it difficult for readers to interpret the figures. 2) Please, highlight the UBNB basin (shown in Fig. 1) in panel (a) of Figure 2.