Via Nature News.

Preprint available [paywall].

Abstract: Tropical Rainfall Measuring Mission (TRMM) satellite estimates of summertime rainfall over the southeast U.S. are found on average to be significantly higher during the middle of the work week than on weekends, attributable to a midweek intensification of afternoon storms and an increase in area with detectable rain. TRMM radar data show a signifcant midweek increase in the echo-top heights reached by afternoon storms. Weekly variations in model-reanalysis wind patterns over the region are consistent with changes in convection implied by the satellite data. Weekly variations in rain-gauge averages are also consistent with the satellite estimates, though possibly smaller in amplitude. A midweek decrease of rainfall over the nearby Atlantic is also seen. EPA measurements of surface particulate concentrations show a midweek peak over much of the U.S. These observations are consistent with the theory that anthropogenic air pollution suppresses clouddrop coalescence and early rainout during the growth of thunderstorms over land, allowing more water to be carried above the 0±C isotherm, where freezing yields additional latent heat, invigorating the storms and producing large ice hydrometeors. The enhanced convection induces regional convergence, uplifting and an overall increase of rainfall. Compensating downward air motion suppresses convection over the adjacent ocean areas. Pre-TRMM-era data suggest that the weekly cycle only became strong enough to be detectable beginning in the 1980’s. Rain-gauge data also suggest that a weekly cycle may have been detectable in the 1940’s, but with peak rainfall on Sunday or Monday, possibly explained by the difference in composition of aerosol pollution at that time. This “weekend effect” may thus offer climate researchers an opportunity to study the regional climate-scale impact of aerosols on storm development and monsoon-like circulation.

Since I stole borrowed (fair use!) the above abstract from the paper, I won’t borrow anything else. But the first two figures in the paper are intersting. They shows the phase, amplitude and significant of the weekly PM2.5 and PM10 cycles. For the most part, there is a general agreement in the PM10 figure for the phase (which day of the week), with high significant. However, the PM2.5 figure shows less agreement among the phase, and at low significance levels. The authors attribute this to the fact that the smaller particles will take longer to settle and thus the concentration will depend more strongly on the weather.

The authors continue by stating:

Observing weekly variations in PM concentrations at the surface does not prove that there is a weekly cycle in cloud-condensation-nuclei (CCN) concentrations at the altitudes where cloud-droplet formation is taking place. The widespread nature of the cycle in surface particulate concentrations does, however, provide some support for the idea that there is a weekly cycle in CCN as well.

Because the PM particles can act as CCN, it would be logical the expect there to be a weekly cycle in cloud cover - at least if the authors postulations are correct. However, they did not publish anything on clouds. They did have much discussion on precipitation - which obviously must come from clouds. It would be interesting to see if there was a statistically significant weekly cycle in cloud cover. I would guess not, otherwise it would have been in their paper.

NatureNews attributed this quote to Dr. Bell, “We also found that the weekly cycle in average gauge-measured rainfall over the southeastern United States was not statistically significant during the decades up to the 1980s, [but since then] “the cycle has grown in strength”.

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