New Paper “Why Do Tornados And Hail Storms Rest On Weekends” By Rosenfeld and Bell 2011

There is a new paper which further documents the diversity of human climate forcings that is presented in

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,  J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union

and in the American Meteorological Society statement on

Inadvertent Weather Modification (Adopted by the AMS Council on 2 November 2010)

The new paper is

Rosenfeld, D., and T. L. Bell (2011), Why do tornados and hail storms rest on weekends?, J. Geophys. Res., doi:10.1029/2011JD016214, in press.

The abstract reads [highlights added]

This study shows for the first time statistical evidence that when anthropogenic aerosols over the eastern USA during summertime are at their weekly mid-week peak, tornado and hailstorm activity there is also near its weekly maximum. The weekly cycle in summertime storm activity for 1995-2009 was found to be statistically significant and unlikely to be due to natural variability. It correlates well with previously observed weekly cycles of other measures of storm activity. The pattern of variability supports the hypothesis that air pollution aerosols invigorate deep convective clouds in a moist, unstable atmosphere, to the extent of inducing production of large hailstones and tornados. This is caused by the effect of aerosols on cloud-drop nucleation, making cloud drops smaller and hydrometeors larger. According to simulations the larger ice hydrometeors contribute to more hail. The reduced evaporation from the larger hydrometeors produces weaker cold pools. Simulations have shown that too cold and fast-expanding pools inhibit the formation of tornados. The statistical observations suggest that this might be the mechanism by which the weekly modulation in pollution aerosols is causing the weekly cycle in severe convective storms during summer over the eastern USA.”

Excerpts from the paper read

“The results are in agreement with our previous reports of similar weekly cycles in the rainfall [Bell et al., 2008] and lightning [Bell et al., 2009a] over the USA. The cycle was ascribed there to aerosols invigorating deep convective clouds in a warm, moist atmosphere. It is therefore not too surprising to find that the invigorated clouds also produce more hail and tornados.”

“This study has shown a clear correspondence between the weekly cycle of anthropogenic aerosols and the occurrences of severe convective storms, which is highly unlikely to be a result of natural variability. The observed associations cannot serve as proof for causality. However, the results are consistent with the hypothesis that air pollution aerosols invigorate deep convective clouds in moist and unstable atmosphere,  and the possibility that they can even induce the storms to produce large hail and tornados. This is also consistent with the hypothesis that the severe storms are better organized and violent because aerosols increase the hydrometeor size, decreasing their evaporation and so weakening the negative buoyancy of the downdrafts, thereby preventing the gust front from outrunning and undercutting the updraft in the feeder clouds.  Anthropogenic emissions have caused large enhancements of aerosol loads even over the remote continents, with typical enhancements of 50–300% over remote regions of Asia, North America, and South America (Wilson et al., 2001; Chin et al., 2004; Park et al., 2006; Stier et al., 2006).Regarding this increase, it is worth pointing out that if a roughly 10% weekly variation in pollution levels is resulting in a similar change in severe storm activity, then the “background” aerosol level, which is elevated with respect to the pre-industrial level even during weekends, is also likely to be changing the storm frequency that we experience today.”

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