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{C}osmic rays, cloud condensation nuclei and clouds � a reassessment using {MODIS} data

by J E Kristjánsson, C W Stjern, F Stordal, A M Fj?raa, G Myhre, K Jónasson4
Atmos. Chem. Phys. ()

Abstract

The response of clouds to sudden decreases in the flux of galactic\ncosmic rays (GCR) � Forbush decrease events � has been investigated\nusing cloud products from the space-borne MODIS instrument, which\nhas been in operation since 2000. By focusing on pristine Southern\nHemisphere ocean regions we examine areas where we believe that a\ncosmic ray signal should be easier to detect than elsewhere. While\nprevious studies have mainly considered cloud cover, the high spatial\nand spectral resolution of MODIS allows for a more thorough study\nof microphysical parameters such as cloud droplet size, cloud water\ncontent and cloud optical depth, in addition to cloud cover. Averaging\nthe results from the 22 Forbush decrease events that were considered,\nno statistically significant correlations were found between any\nof the four cloud parameters and GCR, when autocorrelations were\ntaken into account. Splitting the area of study into six domains,\nall of them have a negative correlation between GCR and cloud droplet\nsize, in agreement with a cosmic ray � cloud coupling, but in only\none of the domains (eastern Atlantic Ocean) was the correlation statistically\nsignificant. Conversely, cloud optical depth is mostly negatively\ncorrelated with GCR, and in the eastern Atlantic Ocean domain that\ncorrelation is statistically significant. For cloud cover and liquid\nwater path, the correlations with GCR are weaker, with large variations\nbetween the different domains. When only the six Forbush decrease\nevents with the largest amplitude (more than 10% decrease) were studied,\nthe correlations fit the hypothesis slightly better, with 16 out\nof 24 correlations having the expected sign, although many of the\ncorrelations are quite weak. Introducing a time lag of a few days\nfor clouds to respond to the cosmic ray signal the correlations tend\nto become weaker and even to change

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