2011-05-10 13:12:22Preliminary Assessment of Climate Factors Contributing to the Extreme 2011 Tornadoes
Peter Miesler

Have you seen this report, whch is being waved as another proof that AGW/CO2 has nothing to do with current apparent intensification of North American Tornadoes. 

Any thoughts?


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Neither the time series of thermodynamic nor dynamic variables suggests the presence of a discernable trend during April; any small trend that may exist would be statistically insignificant relative to the intensity of yearly fluctuations. A change in the mean climate properties that are believed to be particularly relevant to severe storms has thus not been detected for April, at least during the last 30 years. Barring a detection of change, a claim of attribution (to human impacts) is thus problematic, although it does not exclude that a future change in such environmental conditions may occur as anthropogenic greenhouse gas forcing increases.10

10=Trapp, R., N. Diffenbaugh, H. Brooks, M. Baldwin, E. Robinson, and J. Pal, 2007:

2011-05-10 15:44:00
Julian Brimelow

Reputable authors there CC (Brooks, Pal and Baldwin).  I do not disagree necessarily.  This of course doesn't mean that we are not in the midst of a change, it just may be too soon to tell right now.  Remember, AGW only emerged from the backgound variability in the late nineties (don't quote me on that but it is something that I recall Hansen saying).  As I keep reminding people, this is still early days in this bizarre experiment we have elected to undertake.

My two criticisms of the report is that they did not use the Eta-interim (although they may have done that b/c it only goes back to 1989).  Also, I think one might have to use more sophisticated stats to extract a signal-- they just looked at line plots for monthly mean values, but for severe storms the day-to-day variability is key (with high CAPE and strong vertical low-level wind shear need to occur in tandem), so maybe an examination of the probability distribution functions of daily max. values of key variables such as CAPE, 0-6 km AGL bulk shear, or composite variables such as the Energy Helicity Index is required.  That might be able to tell uswhether or not there has been a change in the tails (extremes).  I do not think that this is the last word on this by any means, they say so themselves:

"´╗┐´╗┐This assessment attempts to summarize a current scientific understanding of the link between ENSO and tornadoes. The assessment is not in lieu of a more rigorous diagnosis, nor does it preempt the need for better quantification of the physical climate factors associated with tornado outbreaks. This must be the topic of future research."

I guess Grant Foster is not part of the inner sanctum at SkS is he?

2011-05-10 15:46:19
Julian Brimelow

A similar study could be undertaken using observed soundings (weathe rballoon data), rather than re-analysis data.  In fact, I was going to contact Dr. Brooks with an idea that I have along those lines.

2011-05-10 16:02:49
Rob Painting

CC - I think people confornting deniers too easily get drawn into their "framing" of events. The jury is still out whether or not global warming will increase tornado activity, despite the huge spike recently in the US. Reduced wind-shear (as the higher latitudes warm faster than the tropics) may reduce the likelihood of tornado formation, whereas the increased heat and humidity might have a greater counter-acting effect. 

The report by NOAA pretty much sums up the state of scientific knowledge regarding tornado formation, there are many, many knowledge gaps, and scientists simply don't know. There has been a marked increase in tornado frequency in the last 3 decades or so, but this is probably down to better recordkeeping and vigilance. IIRC the US, which is by far and away the best official record, only began records in the 50's and the data is so poor that reliable trends cannot be discerned from it. 

If you are in dialogue with any denier, I'd ask them which peer-reviewed study suggested tornado activity/intensity would increase in a global warming scenario. Floods, droughts, heatwaves, wildfires and more intense storms yes, but tornadoes?.

Oh, I'm sure global warming had a hand in recent events, but the level of scientific understanding just isn't there yet (AFAIK)