To obtain accurate forecasts of photovoltaic power generation, the use of forecast datasets of meteorological elements from numerical prediction models, specifically global horizontal irradiance (GHI), is necessary. This study seeks to validate, and therefore improve GHI forecasts. Ground-based data from Japan Meteorological Agency (JMA) stations are used in a JMA mesoscale model (MSM) during the time period from 2008 to 2012 and temporal and spatial characteristics of forecast errors are analyzed. Statistical monthly evaluations show that associated errors vary between seasons, with monthly GHI mean bias error values ranging from -60 to +45 W/m(2) and root mean square errors (RMSEs) ranging from 95 to 170 W/m(2). Mapping of forecast errors show that underestimation of GHI forecast values and large RMSE values are significant in the southern part of Japan (a subtropical region located along the Pacific Ocean), particularly during summers. In winter, overestimation of GHI forecasts is found throughout the entire Japanese archipelago. The frequency of different cloud type occurrences over the Japanese islands indicate that regional and seasonal variations in cloud types are related to relatively large GHI forecast errors. High-level cirrus clouds, mid-level altocumulus, and low-level stratus are often observed during summer, when forecasted values are underestimated, and during winter, when values are overestimated. (c) 2015 The Authors. Published by Elsevier Ltd.