5.4
The enviroments of severe thunderstorms: global distribution and temporal changes (Invited)
Harold Brooks, NOAA/NSSL, Norman, OK
The question of possible changes in the distribution and intensity of severe thunderstorms and tornadoes as climate changes is one that Assessment Reports from the Intergovernmental Panel on Climate Change (IPCC) have left open as a result, in large part, of the lack of consistent reporting of events. A 2002 IPCC Workshop recommended the use of environmental proxies in place of raw reports to address the question. Work to identify consistent aspects of severe thunderstorms from within the databases has yielded some successes, such as an apparent consistency in the intensity distribution of tornadoes in many countries (Feuerstein et al. 2005). Studies using NCAR/NCEP reanalysis data to develop relationships between environments and events have produced global maps of severe thunderstorms that are qualitatively reasonable (Brooks et al. 2003, Brooks 2006).
The fundamental questions still remain, however:
1) How can relationships between environmental conditions and severe thunderstorms and tornadoes be developed that are robust globally?
2) How can the report databases be processed to provide information on the occurrence of severe thunderstorms and tornadoes over relatively long historical time periods and/or large spatial areas?
In the current work, I'll present efforts to address both questions. First, the recent creation of a European Severe Weather Database (http://www.essl.org/ESWD) has allowed us to create reanalysis-based proximity soundings for almost 200 significant severe (F2 or stronger tornado, 5 cm diameter or larger hail, convective wind gusts of at least hurricane strength) thunderstorms in Europe. These soundings can be compared to non-severe European soundings and to a set of significant severe soundings (N~1200) previously created for the US. Although there are differences in the US and European severe soundings, enough similarities exist to raise the possibility of creating a globally-robust relationship between environments and events.
The second aspect involves application of “inflation-adjustment” techniques to annual counts of non-tornadic severe thunderstorms. While Verbout et al. (2006) applied a linear inflation adjustment to tornadic reports, the non-tornadic reports require additional processing. By considering hail days on a grid with spacing 80 km and applying an exponential adjustment, a meaningful series can be created. The resulting time series for wind reports in the US shows no consistent trends over time. For hail, however, there is a decline from the late 1950s into the mid-1970s, followed by a rise in adjusted hail-grid days. The series is correlated with annual counts of favorable severe environments from the reanalysis data with r=0.50 (p<0.001), providing evidence that the reanalysis-derived environment-event relationships may be of reasonable quality, at least for the US. From that, I estimate that hail in the US has increased by a little over 1% per year since the mid-1970s. The reports have increased six-fold over that time, so that the contribution of meteorological changes to the total change in reports is about 5%. The other 95% comes from changes in report collection.
References:
Brooks, H. E., 2006: A global view of severe thunderstorms: Estimating the current distribution and possible future changes. Preprints, Syposium on the Challenges of Severe Convective Storms, Atlanta, GA, American Meteorological Society, Conference CD.
Brooks, H. E., J. W. Lee, and J. P. Craven, 2003: The spatial distribution of severe thunderstorm and tornado environments from global reanalysis data. Atmos. Res., 67-68, 73-94.
Feuerstein, B., N. Dotzek, and J. Grieser. 2005: Assessing a tornado climatology from global tornado intensity distributions. J. Clim. 18, 585–596.
Verbout, S. M., H. E. Brooks, L. M. Leslie, and D. M. Schultz, 2006: Evolution of the US tornado database: 1954-2003. Wea. Forecasting, 21, 86-93.
.Session 5, Comparisons between high-resolution regional and global models for studying climate
Thursday, 18 January 2007, 1:30 PM-4:30 PM, 206B
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