Tuesday, 5 October 2004
Commonly used thermodynamic (CAPE, LI, CIN), kinematic (shear, helicity), and combined indices (Supercell Composite, Significant Tornado Parameter) are computed for each individual 00 UTC winter rawinsonde sounding taken from the Southeastern United States between 1958 and 2003. The sounding parameters are then sorted by the CPC defined ENSO phases: El Nino, Neutral, and La Nina. To begin analysis, distributions of the parameters from a subset of the sounding database, those soundings released within 3 hours and 186 kilometers of extreme severe thunderstorm events (F2 or greater tornadoes, hail 51 mm in diameter or greater, wind gusts of 33 ms-1), are examined by ENSO phase. Results indicate that sounding parameters computed in the proximity of extreme severe thunderstorms do not vary markedly from one ENSO phase to another. This confirms that the same atmospheric stratification is associated with severe thunderstorms no matter the ENSO phase. For the second part of the study, parameter distributions for all the soundings in the database are sorted by their ENSO categories. Preliminary results indicate that only subtle differences in the distributions are present amongst the three phases. During La Nina seasons, CAPE and LI have a slight tendency to be enhanced, and CIN tends to be slightly suppressed, whereas shear and helicity appear to be slightly enhanced during El Nino events. The distributions of the composite indices are still being computed at this time. Irregardless of these results, the differences between the distributions are found solely in the upper/lower 25% or less of the cases. This indicates that while ENSO does indeed affect winter severe thunderstorms over the Southeastern United States, the mesoscale details of the three dimensional atmospheric structure on any given day is much more important than ENSO to the occurrence of severe thunderstorms.
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