Reduced ERA-I forecasting skill during Forbush decreases

Previously, week-long Forbush decreases of the atmospheric cosmic ray flux have been found to correlate with terrestrial cloud cover changes. Discussions are ongoing on whether this correlation is caused by a physical mechanism or simply a result of unlikely weather fluctuations. To gain further insight on this matter, we consider the skill of weather forecasts during Forbush decreases using data from the ERA-INTERIM forecasting system. If the cloud changes during Forbush decreases are of meteorological origin, then they should be forecasted by ERA-INTERIM at a skill comparable to any other time. On the contrary, if the cosmic ray flux is coupled to clouds, forecasts should be performing worse during Forbush decreases, since ERA-interim is insensitive to cosmic rays. We find, that ERA-INTERIM was significantly worse at predicting the total cloud cover in times of large Forbush decreases compared to outside of them, supporting the hypothesis that cosmic rays influence terrestrial cloud formation.