Analysis of severe elevated thunderstorms using DCIN and DCAPE

Abstract

A 10-year study of elevated severe thunderstorms was performed using The National Centers for Environmental Information (NCEI) Storm Report database. This research further corroborates previous studies of occurrence, frequency, and severe characteristic distributions of elevated convection with severe weather. From the aforementioned database, 55 Significant ([greater than or equal to] 5 severe storm reports) and 25 Marginally (<5 severe storm reports) severe cases occurred at least 50 statute miles away from a surface boundary within a cold sector. Previous studies have established the importance in predicting whether a downdraft has enough energy to penetrate through the subinversion layer to cause severe surface winds. This study will advance an effort in predicting severe winds from an elevated thunderstorm by implementing a tool to help measure the potential for a downdraft to penetrate through the depth of the stable surface layer by using downdraft convective available potential energy (DCAPE) and downdraft convective inhibition (DCIN). Using outputs from the RUC/RAP analyses, 2-D plan view maps of DCIN and DCAPE were created to assess elevated thunderstorms as they propagated into different environments. Additionally, point sounding analyses were used to analyze the vertical thermodynamic profile for the hour prior to, and at the location of, the first storm report. The findings of this study provide insight of a environment favoring weather with severe winds. The hypothesis is posed that if the DCIN/DCAPE ratio gets progressively smaller in the path of a thunderstorm, then one may expect a greater possibility of observing severe winds at the surface. A statistical analysis was performed to determine correlations between thermodynamic variables of cases that were Significant versus Marginal using a Mann-Whitney test due to the gamma-like distributions associated with each of the variables. The Significant case set had values of DCIN closer to zero, which is consistent with the expectation that downdrafts will be able to penetrate to the surface more easily. Also, the DCIN/DCAPE ratio of Significant cases tends to be near zero with all Significant-Wind cases having a DCIN/DCAPE ratio equal to zero. Secondly, a comparison was made between thermodynamic variables of the dominant severe-type events (hail severe-type or wind severe-type). Again, these variables exhibited a skewing of the medians closer to zero than the mean indicating a gamma-like distribution. A Mann-Whitney test was carried out again to show a comparison of the thermodynamic variables. The DCIN-Hail to DCIN-Wind comparison Mann-Whitney results show DCIN-Wind values are closer to zero indicating the downdraft is able to penetrate to the surface causing severe observed winds. Thus, comparing DCIN and DCAPE is a viable tool in determining if downdrafts will reach the surface within an elevated thunderstorm.