The Cloud Area Fraction (CAF) and Cloud Radiative Effect (CRE) are among the key metrics for understanding Earth’s climate, as they modulate shortwave and longwave radiation. This study, therefore, analyzes the vertical distribution of CAF and their associated CRE over East Africa (EA) using the Modern-Era Retrospective Analysis for Research and Applications Version (MERRA-2) reanalysis and Clouds and the Earth’s Radiant Energy System (CERES) observational data from 2000 to 2025. The vertical CAFs for low, middle, and high clouds are computed at pressure levels and spatially averaged over EA. These are used to derive shortwave CRE (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\:{\text{C}\text{R}\text{E}}_{\text{S}\text{W}}\)</EquationSource> </InlineEquation> in Wm− 2), longwave CRE (<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\:{\text{C}\text{R}\text{E}}_{\text{L}\text{W}}\)</EquationSource> </InlineEquation> in Wm− 2), and net CRE (in Wm− 2) from all-sky and clear-sky radiative fluxes. The validation of MERRA-2 CAF against CERES observations showed it to be reliable (r = 0.749), supporting its use for analyzing cloud-radiation patterns, including vertical cloud distribution. The study of vertical CAF and CRE over EA using MERRA-2 reveals a strongly stratified cloud climatology shaped by topography, lake impacts, and large-scale circulation. The vertical CAFs at Low, mid, and high clouds show distinct spatial and temporal patterns, with high clouds intensifying the most with more than 60% of the total CAF (<InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\:\sim\)</EquationSource> </InlineEquation>0.256), while the contribution of the low CAF against the total covers <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\:\sim\)</EquationSource> </InlineEquation>18% (<InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\:\sim\)</EquationSource> </InlineEquation>0. 076) is the least. Further, the spatiotemporal CRE is dominated by shortwave cooling, ranging from − 14.71 to -54.36 W m−2, which moderates regional temperatures. Cloud layers interact to influence rainfall: low clouds enhance local convection, mid clouds regulate moisture transport, and high clouds mark mature convective systems. This collectively reinforces EA’s bimodal rainfall pattern and highlights the critical role of clouds in regional climate dynamics. This study recommends future analyses that integrate long-term model outputs with satellite and ground-based observations to validate cloud patterns. Such emphasis should focus on the CREs of low, middle, and high clouds, their links to rainfall, and model uncertainties to improve understanding of the role of cloud dynamics over EA’s climate. Graphical Abstract The satellite map of East Africa is presented in the geographical abstract, with key features including Lake Victoria, the highlands of central Kenya and northern Tanzania, and the lowland coastal plains along the Indian Ocean. The graphical abstract highlights that the study focuses on the comprehensive use of MERRA-2 reanalysis and CERES datasets of Cloud Area Fraction (CAF) and Cloud Radiative Effect (CRE) to assess vertical cloud stratification and cloud-radiation interactions over East Africa. The analysis captures both the spatial and temporal evolution of cloud characteristics, recognizing their fundamental role in regulating the Earth’s radiation budget through vertical structure and radiative interactions. Spatial patterns reveal marked regional heterogeneity with enhanced cloud coverage over the study region. Further, distinct variations in radiative impacts are observed with cloud altitude, where low-level clouds predominantly contribute to shortwave radiative cooling. In contrast, high-level clouds exhibit a mixed radiative influence due to competing longwave warming and shortwave reflection effects. The analyses and further discussion on cloud-radiation interactions demonstrate pronounced vertical variability in both CAF and CRE, emphasizing the outsized role of high clouds in modulating the regional energy balance.

Vertical Distribution of Cloud Area Fraction and Associated Cloud Radiative Effects over East Africa Using MERRA-2 Reanalysis and CERES (2000–2025) Data

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发表日期：

2026年6月15日

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Geoffrey W. Khamala,
John W. Makokha,
Churchill Wanyera,
Kanike Raghavendra Kumar,
Mansour Almazroui,
Pelati Althaf,
Aqil Tariq

摘要

The Cloud Area Fraction (CAF) and Cloud Radiative Effect (CRE) are among the key metrics for understanding Earth’s climate, as they modulate shortwave and longwave radiation. This study, therefore, analyzes the vertical distribution of CAF and their associated CRE over East Africa (EA) using the Modern-Era Retrospective Analysis for Research and Applications Version (MERRA-2) reanalysis and Clouds and the Earth’s Radiant Energy System (CERES) observational data from 2000 to 2025. The vertical CAFs for low, middle, and high clouds are computed at pressure levels and spatially averaged over EA. These are used to derive shortwave CRE ( \(\:{\text{C}\text{R}\text{E}}_{\text{S}\text{W}}\) in Wm^− 2), longwave CRE ( \(\:{\text{C}\text{R}\text{E}}_{\text{L}\text{W}}\) in Wm^− 2), and net CRE (in Wm^− 2) from all-sky and clear-sky radiative fluxes. The validation of MERRA-2 CAF against CERES observations showed it to be reliable (r = 0.749), supporting its use for analyzing cloud-radiation patterns, including vertical cloud distribution. The study of vertical CAF and CRE over EA using MERRA-2 reveals a strongly stratified cloud climatology shaped by topography, lake impacts, and large-scale circulation. The vertical CAFs at Low, mid, and high clouds show distinct spatial and temporal patterns, with high clouds intensifying the most with more than 60% of the total CAF ( \(\:\sim\) 0.256), while the contribution of the low CAF against the total covers \(\:\sim\) 18% ( \(\:\sim\) 0. 076) is the least. Further, the spatiotemporal CRE is dominated by shortwave cooling, ranging from − 14.71 to -54.36 W m⁻², which moderates regional temperatures. Cloud layers interact to influence rainfall: low clouds enhance local convection, mid clouds regulate moisture transport, and high clouds mark mature convective systems. This collectively reinforces EA’s bimodal rainfall pattern and highlights the critical role of clouds in regional climate dynamics. This study recommends future analyses that integrate long-term model outputs with satellite and ground-based observations to validate cloud patterns. Such emphasis should focus on the CREs of low, middle, and high clouds, their links to rainfall, and model uncertainties to improve understanding of the role of cloud dynamics over EA’s climate.

Graphical Abstract

The satellite map of East Africa is presented in the geographical abstract, with key features including Lake Victoria, the highlands of central Kenya and northern Tanzania, and the lowland coastal plains along the Indian Ocean. The graphical abstract highlights that the study focuses on the comprehensive use of MERRA-2 reanalysis and CERES datasets of Cloud Area Fraction (CAF) and Cloud Radiative Effect (CRE) to assess vertical cloud stratification and cloud-radiation interactions over East Africa. The analysis captures both the spatial and temporal evolution of cloud characteristics, recognizing their fundamental role in regulating the Earth’s radiation budget through vertical structure and radiative interactions. Spatial patterns reveal marked regional heterogeneity with enhanced cloud coverage over the study region. Further, distinct variations in radiative impacts are observed with cloud altitude, where low-level clouds predominantly contribute to shortwave radiative cooling. In contrast, high-level clouds exhibit a mixed radiative influence due to competing longwave warming and shortwave reflection effects. The analyses and further discussion on cloud-radiation interactions demonstrate pronounced vertical variability in both CAF and CRE, emphasizing the outsized role of high clouds in modulating the regional energy balance.