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Journal article

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

Kristjánsson J, Stjern C, Stordal F, Fj?raa A, Myhre G, Jónasson4 K ...see all

Atmos. Chem. Phys., vol. 8, issue 24 (2008) pp. 7373-7387

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The response of clouds to sudden decreases in the flux of galactic
cosmic rays (GCR) � Forbush decrease events � has been investigated
using cloud products from the space-borne MODIS instrument, which
has been in operation since 2000. By focusing on pristine Southern
Hemisphere ocean regions we examine areas where we believe that a
cosmic ray signal should be easier to detect than elsewhere. While
previous studies have mainly considered cloud cover, the high spatial
and spectral resolution of MODIS allows for a more thorough study
of microphysical parameters such as cloud droplet size, cloud water
content and cloud optical depth, in addition to cloud cover. Averaging
the results from the 22 Forbush decrease events that were considered,
no statistically significant correlations were found between any
of the four cloud parameters and GCR, when autocorrelations were
taken into account. Splitting the area of study into six domains,
all of them have a negative correlation between GCR and cloud droplet
size, in agreement with a cosmic ray � cloud coupling, but in only
one of the domains (eastern Atlantic Ocean) was the correlation statistically
significant. Conversely, cloud optical depth is mostly negatively
correlated with GCR, and in the eastern Atlantic Ocean domain that
correlation is statistically significant. For cloud cover and liquid
water path, the correlations with GCR are weaker, with large variations
between the different domains. When only the six Forbush decrease
events with the largest amplitude (more than 10% decrease) were studied,
the correlations fit the hypothesis slightly better, with 16 out
of 24 correlations having the expected sign, although many of the
correlations are quite weak. Introducing a time lag of a few days
for clouds to respond to the cosmic ray signal the correlations tend
to become weaker and even to change

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  • J E Kristjánsson

  • C W Stjern

  • F Stordal

  • A M Fj?raa

  • G Myhre

  • K Jónasson4

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