The Engerer2 separation model estimates the diffuse fraction Kd from inputs of global horizontal irradiance, UTC time, latitude, and longitude. The model was initially parameterized and validated on 1-min resolution data for Australia and performed best out of the 140 models in global validation studies. This research reparameterizes Engerer2 on a global training dataset and at many common temporal resolutions (1-min, 5-min, 10-min, 15-min, 30-min, 1-h, and 1-day), so that it may be more easily implemented in the future; the need for the user to perform prerequisite calculations of solar angles and clear-sky irradiance has also been removed for ease of use. Comparing the results of the new 1-min parameterization against the original Engerer2 parameterization on a global testing dataset, the root mean squared error (RMSE) improves from 0.168 to 0.138, the relative RMSE from 30.4% to 25.1%, the mean bias error from 8.01% to –0.30%, and the coefficient of determination (R2) from 0.80 to 0.86; hence, there is a significant improvement to the model. Engerer2 was unsuited to 1-day averages; however, it performed remarkably well at all other averaging periods. A climate specific analysis found poor suitability of Engerer2 in polar climates; however, improvement and suitability were found for all other climates and temporal averaging periods. Code for the model are provided as supplementary material in languages R, Python, and Matlab®—selected for their wide-adoption in academia and industry—and they can also be found in the Github repository: Engerer2-separation-model.
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