Trends in direct normal irradiance in Oregon: Comparison of surface measurements and ISCCP-derived irradiance

[1] Long-term pyrheliometer measurements of direct normal irradiance at three sites in Oregon are compared to irradiance calculated from data of the International Satellite Cloud Climatology Project. An increase of direct normal irradiance of up to 1 W/m2 per year between 1980 and 2004 is found in the measured data. The increase seems to be partially caused by stratospheric aerosols from two major volcano eruptions. After removing data for the years most affected by volcanic aerosol, two of the three sites still show a significant increase in direct normal irradiance, indicating other atmospheric changes may contribute to the observed trends. From 1984–2004, for which satellite data are available, both data sets correlate very well and show an increase in direct normal irradiance of 0.3 W/m2 per year on average. In both data sets trends vary seasonally. A strong increase is observed during summer and fall. At two sites a significant decrease during winter, which is consistent in measured and satellite derived values, counteracts part of the summer increase.