Use of a clear-day solar radiation model to homogenize solar radiation measurements in Hawai‘i

The reliability of solar irradiance measurements is dependent on the calibration of the pyranometer used to make them. Internationally recognized standards for pyranometer calibrations require resources that may not be available to all observers. To fill this gap, we propose a method of homogenizing hourly solar radiation time series based on comparison of clear-sky radiation observations against estimates derived from a clear-sky radiation model. Inhomogeneities in the record are identified based on shifts in pyranometer response in relation to the model estimates, and corrected to produce a homogeneous time series. This method was applied to the ∼23 year record of hourly solar irradiance measurements recorded at three climate stations along a 870 m elevation gradient (2120–2990 m) on the slopes of Haleakalā Volcano, Maui, Hawai‘i. Corrections resulted in changes in monthly solar radiation of as much as 6.8–7.4% among the three stations.

► Abrupt shifts in pyranometer response from sensor recalibration and replacement.
► Relationship between modeled and measured clear sky solar radiation.
► Identify and correct inhomogeneities in the observed solar radiation time series.
► Corrections changed monthly mean solar radiation by as much as 6.8–7.4%.