Abstract
In this paper, forecasts of the daily global horizontal irradiance (GHI) obtained from the global spectral model (GSM) developed at the Japan Meteorological Agency (JMA) are evaluated using the surface-observed data at the six JMA stations for the four years during 2009-2012. Seasonal and regional characteristics of forecast errors of GSM GHI and the relationship between forecast errors and the length of the lead time of the forecasts are also investigated. The mean bias errors (MBE) of the monthly averaged GHI forecasts at Tsukuba station ranges from 0.5kWh/m2 to 2.1kWh/m2 per day and the root mean square errors (RMSE) of the forecasts ranges from 0.5 to 2.2kWh/m2 per day. Annual changes of both the MBE and the RMSE values are not large during the four years. Regional and seasonal characteristics of forecast errors are also found; negative biases in summer (positive biases in winter) are found in the Ishigakijima station, while positive biases are significant in all seasons at the other five stations. We also confirmed that RMSE values of the monthly GSM GHI are depended on the length of the lead time of the forecasts from 1-day ahead to 3-days ahead.
