The tenth IMSC, Beijing, China, 2007 - International Meetings on ...

southern stations in the late eighties which appear to relate to changes in middle and high
cloud cover.
This work was supported by Western Australian state government projects: Indian Ocean
Climate Initiative and Strategic Research Fund for the Marine Environment.
Patterns and trends in severe storm environments using re-analysis data
Speaker: Matthew Pocernich
Matthew Pocernich
NCAR, USA
pocernic@ucar.edu
Eric Gilleland
NCAR, USA
Harold Brooks
NSSL, USA
Barbara Brown
NCAR, USA
Severe storms cannot be resolved well by climate models because they occur at too
fine of a scale and because of limitations in the capabilities of climate models. We therefore
examine the changes in frequency of large-scale conditions conducive to severe storms with
respect to space and time.
This presentation addresses three issues. First, using previously published results, the
relationships between moderately high levels of convective available potential energy (CAPE
J/kg) and vertical shear (m/s) and the occurrence of observed convective storms are reviewed.
This review supports the use of high values of the product of CAPE and shear as a proxy for
severe convective events, but cautions about the ability of this information to discern storm
types.
Second, to gather information over a greater domain of space and time, CAPE and shear
are calculated from the NCEP/ NCAR reanalysis data set using the Skew-t/Hodograph
Analysis and Research Program (SHARP) yielding data on an approximately 2-degree square
grid, 4 times daily from 1958 through 1999. <strong>The</strong> CAPE, shear and their product (CAPE*shear)
are summarized statistically and graphically. Statistically significant trends are identified using
general linear models fit to annual and monthly summaries of this data set. Issues associated
with multiple comparisons are addressed using a method that limits the false discovery rate.
Results show that in select regions significant changes in the frequencies of high CAPE*shear
events have occurred over the last 40 years.
Third, the distribution of the product of CAPE and shear are heavy tailed, with large values
being relatively rare. <strong>The</strong>refore, the generalized extreme value distribution is well suited to
model the tails of high values. In our work we model these extremes using the spatial
extension of the extRemes toolkit. Initial results are consistent with empirical extreme values,
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