Comparison of Pyranometric and Pyrheliometric Methods for the Determination of Sunshine Duration

Abstract

Two pyranometric methods for the determination of sunshine duration (SD) from global irradiance measurements are evaluated by means of summated sunshine seconds derived from pyrheliometric measurements in combination with the WMO threshold of 120 W m?2 for the direct solar irradiance. The evaluation is performed using direct and global radiation measurements made at the Cabauw Baseline Surface Radiation Network (BSRN) site in the Netherlands for the period March 2005?February 2006. The ?Slob algorithm? uses 10-min mean and extreme values of the measured global irradiance and parameterized estimates of the direct and diffuse irradiance. The ?correlation algorithm? directly relates SD to 10-min mean measurements of global irradiance. The cumulative pyrheliometric SD for the mentioned period is 1429 h. Relative to this value, the Slob algorithm and correlation algorithm give ?72 h (?5%) and +8 h (+0.6%). On a daily mean basis, the values are ?0.22 ± 0.05 h day?1 and 0.03 ± 0.03 h day?1, respectively. By means of tuning the irradiance parameterizations of the Slob algorithm, the yearly cumulative and daily mean differences can be reduced to +7 h (+0.5%) and 0.02 ± 0.04 h day?1, respectively. It is concluded that, by use of either algorithm, it is possible to estimate daily sums of SD from 10-min mean measurements of global irradiance with a typical uncertainty of 0.5?0.7 h day?1. For yearly sums, the uncertainty typically amounts to 0.5%.