May 22 - July 28
Difficulties With Classifying and Analyzing the Low Level Jet in a Convection Allowing Ensemble
Emily Tinney and James Correia, Jr.
What is already known:
- The low level jet can act to increase vertical wind shear, heat, and moisture in the overnight hours, therefore having important implications for storms in the Great Plains.
- William Bonner used a 2-year jet climatology in 1968 to create a jet classification method that is still commonly used today.
- Bonner's low level jet criteria specifies thresholds of wind speed at the jet maxima and wind shear above that maxima which must be met to be classified as a low level jet.
What this study adds:
- The Bonner classification method does not properly classify the low level jet in cases with a strong synoptic wind field that makes the shear criteria difficult to meet.
- Since storms tend to form in non-quiescent conditions, a better low level jet classification method is required to analyze the relationship between storms and the low level jet in a convection allowing ensemble.
- Future low level jet classification studies should focus on using parameters such as storm relative helicity and wind shear below the wind maximum, rather than the wind shear above.
Abstract:
Common in the Great Plains, the low level jet can have important implications for storms and convective systems as it can increase heat, moisture, and vertical wind shear after sunset. Criteria created by William Bonner in 1968 have been used in numerous studies in the past five decades, and are still commonly used today. As convection allowing models become more accurate and more important in forecasting today, the relationship between the LLJ and storms can be analyzed to provide further knowledge into the impacts the LLJ may have on severe weather. This study used data from the NSSL Experimental Warn-on-Forecast System for ensembles for May 16th, 2017 to see if the Bonner criteria would be a suitable classification method for a typical severe weather event in the Great Plains. Results show that for cases with a strong background wind field such as the one examined in this study, the use of Bonner’s criteria prevents a spatially coherent jet from being classified due to a lack of wind shear. Therefore, a different method of classifying the low level jet is required to analyze the low level jet and storms in a convection allowing ensemble.