Faculty of Science

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Browsing Faculty of Science by Author "Angeyo, H.K."

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    Aerosol optical depth and precipitation rate projections over east africa utilizing self organizing map
    (The International Journal Of Science & Technoledge, 2017-03-01) Makokha, John Wanjala; Angeyo, H.K.; Muthama, John Nzioka
    Assessment of future aerosols impacts on both regional and global climate change requires a comprehensive projection tool that reliably provides information on aerosol evolution characteristics with high fidelity. In the current study, we propose an algorithm based on Self-Organizing Map (SOM) and Community Atmosphere Model 4 (CAM4) for long term Aerosol Optical Depth (AOD) and Precipitation Rate (PR) projections over East Africa. To start with, AOD and PR retrievals from Moderate Resolution Imaging Spectroradiometer (MODIS) and Tropical Rainfall Measurement Mission (TRMM) respectively were cross validated with simulation from CAM4 so as to assess the uncertainty between the measured and simulated retrievals from 2000 to 2014.The error analysis between CAM4 simulations and MODIS measurements (from 2000 to 2014)shows a close match where R2 varies from 0.58 to 0.83 with a corresponding RMSE of between 0.014 and 0.065 (for AOD). Likewise, the uncertainty between simulate and measured PR from CAM4 and TRMM showed an estimated R2 to range between 0.40 and 0.78 while the RMSE varied from 0.021 to 0.091 in the same period and study sites. Based on proposed SOM algorithm and simulated CAM4 retrievals over each study site, an increase of between 1.34-2.43 % for AOD and a decrease of between 1.03-1.98 % in PR are projected over the region by 2030.
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    Estimation of radiative forcing due to aerosols over selected sites in Kenya
    (Kenya Meteorological Society : Journal of Meteorology and Related Sciences, 2013-01-01) Makokha, John Wanjala; Angeyo, H.K.
    Variations in the radiative characteristics of aerosols can be used to quantify their effects on climate. This study evaluated the temporal-spatial variability of aerosol radiative characteristics at λ = 440 nm, λ = 675 nm, λ = 870 nm and λ = 1020 nm over the Nairobi-1°S, 36°E, Mbita-0°S, 34°E and Malindi-2°S, 40°E sites of Kenya. Aerosol optical properties from AERONET were used as inputs in the Coupled Ocean Atmosphere Radiative Transfer (COART) code to model aerosol radiative effects. The results over Nairobi showed an increase in reflectance of 2.6%, 6.7%, 7.2% and 2.4% for 2006–2007 at the specified wavelengths, respectively. Drops of 2.7%, 12.2%, 50.6% and 25.6% were noted in the same wavelengths for the 2007–2008 period. The reflectance over Mbita (0.2284) was higher than that over Nairobi (0.1396) at λ = 675 nm for 2007, due to biomass burning at site. Maritime conditions and aerosols coupled with long range transport of monsoon winds explain the higher reflectance observed over Malindi when compared to Nairobi, except for λ = 440 nm in 2008. This is as a result of aerosols from vehicular and industrial emissions that dominate the λ = 440 nm over Nairobi. The variability of downward and upward spectral irradiance measured at the surface and 12 km levels depended on the wavelength of measurement, but was temporally invariant. Upward irradiance decreased with increasing Solar Zenith Angles (SZAs) due to strong Fresnel reflection at large angles. The equality in the upwelling irradiances at the two atmospheric levels at all sites for λ = 870 nm and λ = 1020 nm was due to the near IR absorption by aerosols. The radiant flux lost in the spectral range 440–1020 nm remained relatively constant over the study sites, and thus the influence of aerosols on radiative characteristics was independent of both site and period of study.
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    Investigation of radiative characteristics of the Kenyan atmosphere due to aerosols using sun spectrophotometry measurements and the COART model
    (Taiwan Association for Aerosol Research : Aerosol and Air Quality, 2013-01-01) Makokha, John Wanjala; Angeyo, H.K.
    Variations in the radiative characteristics of aerosols can be used to quantify their effects on climate. This study evaluated the temporal-spatial variability of aerosol radiative characteristics at λ = 440 nm, λ = 675 nm, λ = 870 nm and λ = 1020 nm over the Nairobi-1°S, 36°E, Mbita-0°S, 34°E and Malindi-2°S, 40°E sites of Kenya. Aerosol optical properties from AERONET were used as inputs in the Coupled Ocean Atmosphere Radiative Transfer (COART) code to model aerosol radiative effects. The results over Nairobi showed an increase in reflectance of 2.6%, 6.7%, 7.2% and 2.4% for 2006–2007 at the specified wavelengths, respectively. Drops of 2.7%, 12.2%, 50.6% and 25.6% were noted in the same wavelengths for the 2007–2008 period. The reflectance over Mbita (0.2284) was higher than that over Nairobi (0.1396) at λ = 675 nm for 2007, due to biomass burning at site. Maritime conditions and aerosols coupled with long range transport of monsoon winds explain the higher reflectance observed over Malindi when compared to Nairobi, except for λ = 440 nm in 2008. This is as a result of aerosols from vehicular and industrial emissions that dominate the λ = 440 nm over Nairobi. The variability of downward and upward spectral irradiance measured at the surface and 12 km levels depended on the wavelength of measurement, but was temporally invariant. Upward irradiance decreased with increasing Solar Zenith Angles (SZAs) due to strong Fresnel reflection at large angles. The equality in the upwelling irradiances at the two atmospheric levels at all sites for λ = 870 nm and λ = 1020 nm was due to the near IR absorption by aerosols. The radiant flux lost in the spectral range 440–1020 nm remained relatively constant over the study sites, and thus the influence of aerosols on radiative characteristics was independent of both site and period of study.
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    Sun-photometric study and multivariate analysis ofaerosol optical depth variability over some representative sites of the Kenyan atmosphere
    (International Journal of BioChemiPhysics,, 2015-12-01) Makokha, John Wanjala; Angeyo, H.K.; Muthama, John Nzioka
    The goal of this study was to explore the temporal-spatial characteristics of aerosol optical depth (τ)over the Kenyan urban (Nairobi-1°S, 36°E), rural (Mbita-0°S, 34°E) and maritime (Malindi-2°S, 40°E) atmospheres using sun spectrophotometric measurements obtained from Aerosol Robotic Network (AERONET).AERONET measurements have been taken in Kenya since 2006 and are aimed at assessing aerosol effects on climate and improving the aerosol data base in the region. The multivariate nature of environmental measurements however allows only a limited understanding of atmospheric aerosol characteristics when univariate analysis technique is used. Temporal-spatial characteristics of atmospheric aerosol optical depth can be understood comprehensively if it is appropriately retrieved from ground-based spectrophotometric measurements and then decoupled and analyzed using multivariate analysis techniques since they can explore groups of variables simultaneously, thus providing a more meaningful insight into the temporal-spatial variability of τ is inevitable. The influence of rain and dry spells and temperature on τ at wavelengths, λ = 440 nm and λ = 1020 nm as quantified by Principal Component Analysis (PCA) ranged between 76-83 % and 7-14 %and 4-7 % respectively for all the sites. It was found out that urban heat island (over Nairobi) and local air circulation effects (over Mbita and Malindi) modulate the characteristics of aerosol optical depth over the studied sites. Spatial variability in τ as shown by Hierarchical Cluster Analysis (HCA) is independent of measurement wavelength but dependent on aerosol burden in the atmosphere for each site. The individual and coupled influence of weather parameters on atmospheric aerosols has been\ isolated and quantified and found to be site dependent.