Abstract
The spatio-temporal characteristics of the clearness index (KT) were investigated using daily global solar irradiance measurements (290–2800 nm) for the period of 2000–2014 at 21 sites in Korea, a complex region in East Asia with a distinct monsoon season and heavy aerosol loading year-round. The annual mean KT value for all sites is 0.46, with values of 0.63 and 0.25 for clear and overcast skies, respectively. The seasonal variations in monthly average KT show a minimum of 0.37 in July at all sites except for Jeju, where the value was 0.29 in January. The maximum value (KT = 0.51) is observed in October, followed by a secondary peak (KT = 0.49) during February–April. The lowest KT value (KT = 0.42) was observed at both the Seoul and Jeju sites, and the highest (KT = 0.48) in the southeastern regions. Increases in average KT exceeding 4% per decade were observed in the middle and southeastern regions, with the maximum (+8% per decade) at the Daegu site. Decreasing trends (<−4% per decade) were observed in the southwestern regions, with the maximum (−7% per decade) at the Mokpo site. Cloud amount, relative humidity, and aerosol optical depth together explained 57% of the variance in daily mean KT values. The contributions of these three variables to variations in KT are 42%, 9% and 6%, respectively. Thus, the variations in KT in Korea can be primarily attributed to the presence of clouds and water vapor, with relatively weak aerosol effects.
Highlights
Solar radiation in the ultraviolet, visible, and near-infrared ranges is the major source of energy for the climate and ecosystems on Earth
The variations in KT in Korea can be primarily attributed to the presence of clouds and water vapor, with relatively weak aerosol effects
In this study we have developed multiple regression models to evaluate the individual contributions of clouds, water vapor, and aerosols to KT variations for the 15 years from 2000 to 2014, using the coefficients of partial correlation (r) and the beta coefficients (β) from the statistical equation, R2 = β1 r11 ` β2 r12 ` β3 r13 ` β4 r14
Summary
Solar radiation in the ultraviolet, visible, and near-infrared ranges is the major source of energy for the climate and ecosystems on Earth. Global solar radiation refers to the total amount of solar energy reaching the Earth’s surface, which varies dramatically over temporal and spatial scales due to varying atmospheric conditions above the surface. These variations can be attributed to the absorption and scattering of radiation by clouds, water vapor, aerosols, and other gases in the atmosphere, but are difficult to predict, primarily because of non-linear interactions in this complex system [1,2,3]. To quantify the extinction of solar radiation in the atmosphere, the combined contributions of various atmospheric parameters that affect incoming solar radiation need to be examined. The ratio of the global solar radiation measured at the surface to the extraterrestrial solar radiation is defined as the clearness index (KT ) [4,5,6,7,8]
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