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Weekly cycles in aerosol concentration and corresponding cycles in precipitation have been reported over Europe, but results are conflicting. To obtain a large potential effect of aerosols on precipitation we here focus on a highly polluted region on the borders between Germany, Poland and the Czech Republic. Meteorological parameters from 30 surface stations in a mix of urban, rural and remote locations were analyzed for the time period 1983--2008, using three different tests: the Kruskal-Wallis test, a spectral analysis and a comparison of the regular 7-day week to constructed 6- and 8-day weeks. We expect a clear and statistically significant weekly cycle to pass all three tests. Precipitation amount and meteorological variables associated with convective conditions, such as the frequency of heavy precipitation events and observations of rain showers, showed two-peak weekly cycles with maxima on Tuesdays and during weekends. The amplitude of the precipitation cycle increased with longitude towards the more polluted eastern part of the region, but the statistical significance of the cycles did not change correspondingly. The amplitudes of the weekly cycles were in many cases larger for the heavily polluted 1983--1987 period than for the cleaner 2004--2008 as well as the total period, but were equally often largest in the clean period. Moreover, of all the variables, periods and seasons investigated, the weekly cycles were statistically significant only for summertime values of light precipitation frequency and cloud amount, and only by one of the three tests applied. Conclusively, clear weekly cycles in meteorological variables were not found in this polluted region of Europe. [ABSTRACT FROM AUTHOR]

The response of clouds to sudden decreases in the flux of galactic cosmic rays (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 which are particularly susceptible to changes in cloud condensation nuclei (CCN) concentrations, and where a cosmic ray signal should be easier to detect than elsewhere. While previous studies on the subject 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 13 Forbush decrease events that were considered, no systematic correlation was found between any of the four cloud parameters and galactic cosmic radiation, with a seemingly random distribution of positive and negative correlations. When only the three Forbush decrease events with the largest amplitude are studied, the correlations fit the hypothesis better, with 8 out of 12 correlations having the expected sign. Splitting the area of study into several sub-regions, one sub-region in the Atlantic Ocean showed statistically significant correlations compatible with a cosmic ray-induced enhancement of CCN and cloud droplet number concentrations. However, the lack of correlation in any of the other 5 sub-regions suggests that this may be a statistical co-incidence. Introducing a time lag of a few days for clouds to respond to the cosmic ray signal did not change the overall results. Singling out low clouds of intermediate optical depth with large susceptibility did not lead to higher correlations. In conclusion, no response to variations in cosmic rays associated with Forbush decrease events was found in marine low clouds in remote regions using MODIS data. [ABSTRACT FROM AUTHOR]