Evaluation of All-sky Solar Irradiance and Near-surface Air Temperature (T2M) Variability Across East African Countries Using NASA POWER Reanalysis Data (2021&ndash;2024)

Shibabaw, Meseret Amisaya; Mersha, Mogese Wassaie

doi:10.5194/egusphere-2025-6425

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https://doi.org/10.5194/egusphere-2025-6425

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16 Apr 2026

| 16 Apr 2026

Status: this preprint is open for discussion and under review for Annales Geophysicae (ANGEO).

Evaluation of All-sky Solar Irradiance and Near-surface Air Temperature (T2M) Variability Across East African Countries Using NASA POWER Reanalysis Data (2021–2024)

Meseret Amisaya Shibabaw and Mogese Wassaie Mersha

Abstract. Understanding the variability of solar radiation and surface temperature is crucial for energy planning, agriculture, and climate research. East Africa is one of the most climatically diverse regions in the world. In regions such as East Africa, where climate-dependent sectors dominate livelihoods, understanding the behavior of these variables is especially critical. This study investigates the seasonal and inter-annual variability of All-sky Solar Irradiance and Near-surface Air Temperature (T2m) across five key East African countries: Ethiopia, Kenya, Sudan, Tanzania, and Uganda, using NASA POWER reanalysis data spanning the period 2021–2024. Results from the monthly and annual mean data reveal distinct regional patterns. Sudan consistently exhibits the highest annual mean irradiance and temperature. Ethiopia and Kenya exhibit bimodal irradiance structures, with marked reductions during the mid-year rainy seasons. In contrast, Tanzania and Uganda maintain the lowest annual mean irradiance and temperature. Critically, all five countries share a pronounced irradiance trough in July, corresponding to peak cloud cover during the Northern Hemisphere summer monsoon season. Inter-annual variations in both irradiance and temperature are minimal across all countries, indicating stable climatic forcing and reliable long-term solar resource availability across the region. These findings contribute to understanding the radiative energy balance and potential for renewable energy utilization in East Africa.

Received: 23 Dec 2025 – Discussion started: 16 Apr 2026

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Meseret Amisaya Shibabaw and Mogese Wassaie Mersha

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RC1:
'Comment on egusphere-2025-6425', Anonymous Referee #1, 19 May 2026 reply
This manuscript analyses solar irradiance and surface air temperature across East Africa, on a country basis. The aim is mainly to document the seasonal cycles of the two variables. These are quite overlooked variables, yet they are, besides rainfall, important components of East African climates. The focus is thus commendable, but the manuscript lacks ambition and is fraught with several shortcomings, from data analysis to interpretation. It is thus not suitable for publication in ANGEO.
The authors use a single dataset, namely NASA POWER reanalysis data. There is no information in the manuscript on how reliable is this data set with respect to solar irradiance and air temperature, in general and for East Africa in particular. A validation against ground data would be necessary, or references to previous validation exercises. The study is definitely not in capacity to demonstrate “the reliability of T2M data from NASA POWER” (l.187) given the absence of any comparison with reference data.

Scale of analysis. The whole study is based on country-by-country data (excluding some East African countries, for unknown reasons). It is not fully clear which area is taken into account to compute the index for each country. Section 2.2 indicates that “representative coordinate points” are selected “based on spatial relevance to the regional climate zones”, but the authors do not specify what are these climate zones. Several countries show regionally contrasted climates (e.g. Ethiopia, Kenya, Tanzania), hence a single index may combine quite different climatic regions, and several indices would need to be computed. More generally, the lack of any mapping of solar irradiance and temperature is a serious limitation of the study.

Interpretation of the seasonal variations. The authors consider that most of seasonal variations can be interpreted with reference to the shift of the Intertropical Convergence Zone (ITCZ). While increased cloudiness in the rainy seasons partly explains reduced solar irradiance and lower air temperature, there are many other drivers (some of them much more prominent). In several parts of East Africa, maximum cloudiness and minimum temperature coincide with the dry season instead of the rainy season. In the tropics the presence of an extensive dry season cloud cover (stratiform instead of convective, as in July in Kenya and Tanzania, a month which is in the middle of the dry season) is often overlooked. The authors should also consider the zenithal movement of the sun as a key component. Therefore, in many instances the interpretation of the temporal variations is wrong in the manuscript.

Scope of analysis. The ambition of studying “long-term trends” (l.90) cannot be met with four years of data. Similarly, comments referring to global warming from a comparison of the four years (as on l.158) are incongruous. The same applies to the issues discussed in section 3.4 (solar cycle, global change).

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Citation: https://doi.org/10.5194/egusphere-2025-6425-RC1
- AC1: 'Reply on RC1', Meseret Shibabaw, 19 May 2026 reply
  
  Here is a strong, professional consolidated response you can use to defend your manuscript while remaining scientifically respectful.
  We sincerely thank the reviewer for the detailed and constructive evaluation of our manuscript. We appreciate the reviewer’s recognition of the importance of investigating solar irradiance and near-surface air temperature variability across East Africa, particularly given the relative scarcity of such regional comparative studies.
  Regarding the study period, the primary objective of the manuscript was not to establish definitive climatological trends over multiple decades, but rather to characterize recent seasonal and interannual variability patterns using a consistent reanalysis framework. The discussion of variability was intended primarily in the context of short-term stability of solar and thermal conditions relevant to renewable energy assessment and regional climate characterization.
  Concerning the use of NASA POWER data, this dataset is widely recognized and extensively utilized in renewable energy, agricultural, and climatological studies, especially in data-sparse regions such as Africa. The present work employed NASA POWER as a consistent observational and reanalysis framework suitable for regional-scale comparative analysis rather than as a dedicated validation study. Numerous previous studies have demonstrated the applicability of NASA POWER products for solar irradiance and temperature assessment in tropical and subtropical regions.
  With respect to the country-scale analysis, the study was intentionally designed as a first-order comparative regional assessment focusing on broad seasonal behavior rather than high-resolution subnational climatology. Representative coordinate locations were selected to capture dominant climatic characteristics within each country while maintaining methodological consistency across the study domain. While finer spatial resolution analyses would undoubtedly provide additional detail, the adopted approach remains suitable for identifying regional-scale temporal variability patterns across East Africa.
  The reviewer also raised important points regarding the interpretation of seasonal variability. The manuscript emphasizes the role of the Intertropical Convergence Zone (ITCZ) because its seasonal migration is widely recognized as one of the dominant large-scale controls on East African cloudiness, rainfall, and atmospheric circulation. Nevertheless, we acknowledge that additional factors including solar zenith angle variability, monsoon circulation, topography, local cloud regimes, and atmospheric moisture transport also contribute to the observed variability. The ITCZ framework was adopted primarily as a large-scale explanatory mechanism for the regional seasonal cycles discussed in the study.
  Regarding the absence of spatial maps, the principal focus of the present manuscript was the temporal evolution and comparative seasonal variability of irradiance and temperature among East African countries rather than detailed spatial climatology. Consequently, time-series analyses were prioritized over spatial visualization products. However, we acknowledge that additional spatial mapping could further complement the interpretation.
  Concerning the discussion related to solar variability and global climate change, these discussions were included primarily to provide broader climatological context rather than to claim direct attribution or rigorous long-term trend detection from the four-year dataset itself. The manuscript explicitly acknowledges that the analysis period is insufficient for definitive climate trend analysis and that substantially longer records would be required for formal attribution studies.
  Overall, we respectfully maintain that the study provides scientifically useful regional-scale insight into recent irradiance and near-surface temperature variability across East Africa using a consistent and widely applied reanalysis dataset. Given the limited availability of dense observational meteorological networks across the region, such comparative assessments remain valuable for renewable energy planning, environmental monitoring, and regional climate applications.
  
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  Citation: https://doi.org/10.5194/egusphere-2025-6425-AC1
RC2: 'Comment on egusphere-2025-6425', Anonymous Referee #2, 02 Jun 2026 reply

This manuscript utilizes NASA POWER reanalysis data (2021–2024) to characterize the environmental conditions in Sudan, Ethiopia, Kenya, Uganda, and Tanzania. Specifically, it analyzes the spatial patterns, temporal variability, and interrelationships of solar irradiance and surface air temperature. While the study aims to address gaps in long-term, multi-country analyses to support energy planning, agriculture, and climate research, the scientific contribution remains limited. Consequently, despite the benefit of the data for technical decision-making, the manuscript does not meet the threshold for publication.
The absence of detailed information regarding which coordinate points were selected within each country and how they were chosen renders the selection process opaque to the reader.

The study relies exclusively on NASA POWER reanalysis data, without cross-validation against alternative sources or any assessment of data quality. The inclusion of rainfall and cloud cover metrics would significantly enhance the analysis.

Both the statistical treatment and the descriptive presentation of the data could be further refined.

The data is discussed primarily in qualitative terms rather than being supported by quantitative metrics.

Correlations were not computed to explore the cross-national dimensions of the research.

Furthermore, the novel findings are not sufficiently differentiated from the mere confirmation of existing information.

The explanations of the underlying physical causes rely on knowledge and literature but lack empirical validation from the data itself.

The authors establish a weak connection to solar activity and global climate change but rightly discard it due to the limited time span of the available data.

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Citation: https://doi.org/10.5194/egusphere-2025-6425-RC2

Meseret Amisaya Shibabaw and Mogese Wassaie Mersha

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Short summary

This study evaluates the spatial and temporal variability of all-sky surface solar irradiance and near-surface air temperature (T2M) across selected East African countries using NASA POWER reanalysis data for the period 2021–2024. Monthly, seasonal, and inter-annual variations are analyzed to characterize recent regional climate behavior and irradiance–temperature relationships in a data-scarce region.


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