Observations and implications of diurnal climatology and trends in direct and diffuse solar radiation over China

International audienceSolar radiation received at the Earth's surface ($R_s$) is comprised of two components, the direct radiation ($R_d$) and the diffuse radiation ($R_f$). $R_d$, the direct beam from the sun, is essential for concentrated solar power generation. $R_f$, scattered by atmospheric molecules, aerosols, or cloud droplets, has a fertilization effect on plant photosynthesis. But how $R_d$ and $R_f$ change diurnally is largely unknown owing to the lack of long-term measurements. Taking advantage of 22 years of homogeneous hourly surface observations over China, this study documents the climatological means and evolutions in the diurnal cycles of $R_d$ and $R_f$ since 1993, with an emphasis on their implications for solar power and agricultural production. Over the solar energy resource region, we observe a loss of $R_d$ which is relatively large near sunrise and sunset at low solar elevation angles when the sunrays pass through the atmosphere on a longer pathway. However, the concentrated $R_d$ energy covering an average 10-hr period around noon during a day is relatively unaffected. Over the agricultural crop resource region, the large amounts of clouds and aerosols scattering more of the incoming light result in $R_f$ taking the main proportion of $R_s$ during the whole day. $R_f$ resources and their fertilization effect in the main crop region of China further enhances since 1993 over almost all hours of the day