VISTA: A spatial forecast verification method for solar irradiance using thin-plate spline deformation

Solar irradiance forecast verification almost always relies on measurements from ground-based radiometric stations, which are only sparsely and unevenly distributed, thus limiting the understanding of the spatial error characteristics of high-resolution gridded forecasts. To address that, this study introduces the VISTA (Verification of Irradiance Spatial fields via Thin-plate Alignment), a deformation-based spatial verification method that leverages a pair of thin-plate splines to align the forecast field with the observation field. VISTA is a two-step procedure: (1) a set of control points is first identified using Shi–Tomasi corner detection, and (2) the deformation is then performed based on aligning the control points with a penalty. A comprehensive performance measure is designed to quantify displacement, amplitude, and structural errors, thereby enabling a systematic performance ranking of different forecasting methods. The empirical part of the work revolves around several sets of nowcasts (i.e., 0–4-h ahead) issued using irradiance retrieved from the Fengyun-4A satellite. Nevertheless, VISTA is general and can be applied to other spatial forecast fields of interest.