7. Conclusion
[19] In the present study, we used a number of different statistical techniques and found no statistically significant weekly cycles in precipitation amount or occurrence at 219 land-based stations across the United States. Using only the summertime data also produced no statistically significant results. These results are consistent with the results of
DeLisi et al. [2001] who used three different statistical techniques and found no statistically significant weekly precipitation cycles at seven coastal cities in the northeastern United States.
[20] The number of studies looking for weekly cycles in clouds and precipitation continues to grow, yet new results sometimes confirm and sometimes contradict previous studies. For example,
Cerveny and Balling [1998] and
Bäumer and Vogel [2007] found that precipitation amount was maximum on Saturday and was minimum on Monday. In contrast, Bell et al. (submitted manuscript, 2007) found that TRMM-derived summertime precipitation rates maximized on Tuesday over the southeast US and minimized on Sunday, whereas summertime gauge data maximized on Thursday and minimized on Sunday (Bell et al., submitted manuscript, 2007). By comparison, cloud characteristics also can exhibit weekly cycles:
Jin et al. [2005] found that water-cloud effective radius peaked on Wednesday and liquid water path peaked on Friday, Saturday, and Sunday.
[21] Why our negative results (as well as those of
DeLisi et al. [2001]) should differ from the other studies is cause for concern. Of comparable concern is how to reconcile the rain-gauge data with various satellite approaches, which do not measure precipitation directly, but instead infer the precipitation from brightness temperatures that are sensitive to some of the cloud microphysical characteristics that are believed to be changed by anthropogenic effects. For example,
Spencer [1993, p. 1304] states, “based upon theory and what little is known about the relative frequencies of occurrence of rain- and cloud water, the MSU channel 1 Tb variations are probably dominated by cloud water variations, the direct contribution to rainwater being small.” Similarly, Bell et al. (submitted manuscript, 2007) state, “the 85-GHz signal is largely determined by the size and amount of ice aloft, …. It is therefore possible that the weekly cycle in TRMM rain estimates may be partially due to changes in the ice aloft that are not necessarily accompanied by such large changes in rainfall amounts at the surface.” Consequently, we question whether the satellite-derived precipitation schemes are capable of fully discerning the indirect aerosol effect. Clearly, reconciling these approaches remains a significant challenge for understanding the role of humans on clouds and precipitation.
