NOAA CSI: no attribution of climate change to tornado outbreak

All the bloviation from Think Progress, Climate Progress and the rest of those people that want to turn any disaster into a link to climate change are doing is just that: bloviating.

Told ya so. Here’s the preliminary report from NOAA’s Climate Science Investigation

Climate Change and Tornadoes

---

There is a heightened sense of interest, concern, and urgency to explain extreme events in the context of a changing climate. Preliminary estimates (as of 2 May) are of 226 tornado reports during a 24-hour period on Wednesday 27 April, and 312 tornado reports during 26-28 April. NOAA estimates this to be the largest 1-day outbreak, eclipsing the prior record of 148 twisters estimated to have occurred during 3-4 April 1974. ¹

One question on many minds concerns the role of anthropogenic climate change. Two recent national and international assessment reports have summarized the existing state of knowledge on climate change and tornadoes.

According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) :

“There is insufficient evidence to determine whether trends exist in..#small-scale phenomena such as tornadoes, hail, lightning and dust-storms.”

²

The US Climate Change Synthesis Report SAP 3.3 concludes that:

“The data used to examine changes in the frequency and severity of tornadoes and severe thunderstorms are inadequate to make definitive statements about actual changes.”

³

The report also concludes that:

“There were no significant changes in the high-intensity end of these distributions from the 1950s through the 1990s, although the distribution from 2000 and later may differ.”

The difficulties in assessing change directly from the history of tornado data is emphasized in both reports. A simple illustration of the problem is provided by the 1950-2005 time series of the number of tornadoes in Illinois, as constructed by the Illinois State Climatologist Office⁴; the number of tornado counts has more than doubled since 1990. The post-1990 increase in counts is almost entirely associated with the counts in weak (F-0) tornadoes. Factors that relate to a non-physical trend include changed instrumentation (e.g., the implementation of the WSR-88D Doppler radar after 1990), increases in population, and public awareness via enhanced spotter networks that may have contributed to more reporting.⁵ It should be further noted that a careful appraisal of historical tornado outbreaks has led scientists to recognize that secular trends in tornado occurrences before 1970 are likely more influenced by non-meteorological factors⁶, although the quality of time series even after 1970 suffer from inhomogeneities described above. At this time, the historical tornado data are not of a quality to permit a rigorous appraisal of trends, although there is a general belief that the F2+ database of stronger tornadoes is more reliable, especially over the last 30 years.

Are there indirect indications for possible changes in tornadoes that can be inferred from variables other than the tornado counts themselves? A recent special report by the WMO Working Group and Expert Team on Detection and Attribution Related to Anthropogenic Climate Change⁷ offers guidelines on attribution research. These propose that a first step toward a rigorous assessment is to determine whether a change in an event, or in variables closely related to such events, has been detected. Thus, whereas the tornado data itself is not of a quality for rigorous assessment of physical trends, perhaps trends in the conditions that are believed to be conducive for such storms can be studied.

In this regard, it has long been known that thermodynamic stability, column moisture content, and vertical wind shear are particularly relevant for discriminating non-tornadic from tornadic environments. ^8,9 Decreased thermodynamic stability (also consistent with increased convective available potential energy), increased moisture content in the atmosphere, and increased vertical wind shear within the 5km above ground layer characterize an environment more favorable for a tornado outbreak. In particular, tornadoes are much more likely to occur when both high CAPE and high shear are present. Secondarily, the presence of an elevated mixed layer (reflected in moderate values of “convective inhibition” or “CIN”) can delay the onset of convection such that when it occurs it does so more explosively and in the form of more long-lived, isolated supercells. A recent analysis of climate change projections suggests that the number of days during which meteorological conditions are conducive for severe storms may increase during latter decades of the 21st Century as a consequence primarily of increased instability, though projected decreases in vertical wind shear may oppose thermodynamic destabilization.

Here we show a preliminary analysis, covering 1979-2010, to explore only whether changes in such large-scale, time-averaged climate variables for April are detected^**. Our diagnosis attempts to reveal whether large-scale conditions may have become more favorable for violent storms to occur over the lower Mississippi Valley, the region of the recent super tornado outbreak. Various data sets are used to estimate the time variability in column precipitable water (Fig. 1, top 2 panels). These are found to be in close agreement with each other with respect to their interannual variability; the water vapor time series are dominated by strong year-to-year variations both over the Gulf of Mexico (top) source region and over the lower Mississippi Valley impact region of possible tornadic activity (second panel). A similar interpretation applies to a time series of thermodynamic stability (based on the index of convective available potential energy, CAPE) which as might be expected, varies coherently with the atmospheric water vapor content (third panel). Finally, the vector wind shear magnitude for the surface-500mb layer is also dominated by interannual variability, with little evidence for a trend during the 30-yr period (lower panel).

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

The body of knowledge regarding the possible role played by large-scale climate forcings in tornado outbreaks is rapidly evolving, and constitutes a field of study that must integrate existing expertise in meso-scale meteorology with expertise in global-scale climate dynamics. Likewise, the methods for conducting attribution science also continue to evolve, and advances on the tornado-climate linkages will require modeling capabilities beyond current tools. Despite various limitations in data and tools, it should be noted that applying a scientific process is essential if one is to overcome the lack of rigor inherent in attribution claims that are all too often based on mere coincidental associations.

This assessment attempts to summarize a current scientific understanding of the link between climate 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.

---

^** An implicit assumption, which should be verified in future research, is that a change in the monthly average conditions would indicate similar changes in the tails of the daily distribution, i.e., a slight increase in the mean monthly CAPE would result in more individual days with CAPE sufficiently high to support tornadic activity. Similarly, a future research, would explore the frequency and change over time of the joint instantaneous occurrence of the high CAPE and high shear, and possibly moderate CIN that are conducive for tornado development.

================================================================

source: http://www.esrl.noaa.gov/psd/csi/events/2011/tornadoes/climatechange.html