Estimating Shortwave Clear‐Sky Fluxes From Hourly Global Radiation Records by Quantile Regression

摘要

Estimates of radiative fluxes under cloud‐free conditions (“clear‐sky”) are required in many fields, from climatic analyses of solar transmission to estimates of solar energy potential for electricity generation. Ideally, these fluxes can be obtained directly from measurements of solar fluxes at the surface. However, common standard methods to identify clear‐sky conditions require observations of both the total and the diffuse radiative fluxes at very high temporal resolution of minutes, which restricts these methods to a few, well‐equipped sites. Here we propose a simple method to estimate clear‐sky fluxes only from typically available global radiation measurements (R sd ) at (half‐)hourly resolution. Plotting a monthly sample of observed R sd against the corresponding incoming solar radiation at the top of atmosphere (potential solar radiation) reveals a typical triangle shape with clear‐sky conditions forming a distinct, linear slope in the upper range of observations. This upper slope can be understood as the fractional transmission of solar radiation representative for cloud‐free conditions of the sample period. We estimate this upper slope through quantile regression. We employ data of 42 stations of the worldwide Baseline Surface Radiation Network to compare our monthly estimates with the standard clear‐sky identification method developed by Long and Ackerman (2000, https://doi.org/10.1029/2000JD900077 ). We find very good agreement of the derived fractional solar transmission ( R 2 = 0.73) across sites. These results thus provide confidence in applying the proposed method to the larger set of global radiation measurements to obtain further observational constraints on clear‐sky fluxes and cloud radiative effects. Plain Language Summary Even under cloud‐free condition we typically observe lower solar radiative fluxes at the surface than the incoming solar radiation at the top of atmosphere. A fraction of the solar radiation is being absorbed in the atmosphere and thereby reduces the amount received at the surface. Common standard methods to estimate the clear‐sky fluxes from ground‐based observations need a very high temporal resolution of 1 min and additional measurements of diffuse radiation to identify cloud‐free conditions. Since these data are not broadly measured, we propose an alternative method, which exploits the typically linear relationship of observed, cloud‐free solar radiation fluxes to the incoming solar radiation at the top of the atmosphere. This relationship can be estimated by the statistical method of quantile regression. We test this alternative approach with high‐quality surface radiation data and compare it with the standard approach. Results show that the alternative method yields good agreement of monthly fractional clear‐sky transmission and thus clear‐sky fluxes for the 42 sites evaluated. This provides confidence to utilize the commonly measured global radiation for the estimation of clear‐sky fluxes in applications ranging from solar energy planning to a climatology of the shortwave cloud radiative effect from direct observations.