The cloud index (CI), the cloud occurrence frequency (Occ), and the extinction data (Ext) of the Michelson Interferometer for Passive Atmospheric Sounding instrument on board Envisat (MIPAS-E) from July 2002 to March 2004 are used to investigate a possible link between galactic cosmic rays (GCRs) and midaltitude and high-altitude clouds (CI, Occ: 9-18 km, Ext: 12-24 km). The zonally averaged data with 3 km vertical resolution are averaged over six major Forbush decrease (FD) events and subsequently correlated with the Climax neutron monitor data (CNM). In order to allow for nucleation and growth of ice particles, time lags from 0 to 5 days are applied. We find several weak but statistically significant correlations with an excess of positive cloud-GCR correlations. Introducing a time lag does not enhance the correlations significantly. Subdividing the data in a global grid with 30 degrees x 90 degrees x 3 km resolution shows higher correlations in some regions. The investigation of the individual FD events yields a heterogeneous picture. Overall, there is a weak tendency toward a positive cloud-CNM correlation. A Kolmogorov-Smirnov test shows that for time shifts from 0 to +5 days a weak GCR-cloud effect is evident in the MIPAS-E measurements. An estimation of the impact of this effect delivers that a 15% increase in CNM would result in a small decrease in CI (corresponding to an increase in cloud opacity) which is most pronounced at 9 km altitude (-9% to +0.5%). For log(Ext) a decrease of -5 to 0% is calculated at 12 km altitude which shifts toward weak positive values at higher altitudes.
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