A Polar Low Genesis Potential Index
Abstract: Polar lows (PLs) are high-latitude intense maritime mesocyclones that develop near the sea-ice margin or in proximity to snow-covered continents during marine cold air outbreak events. The impacts posed by these systems to coastal and island communities, maritime and air operations, and the broader environment demand a robust understanding of the environmental factors that promote PL formation, and in turn, skillful prediction and projection of PL activity. Numerical weather prediction models have shown substantial improvements in simulating these systems but forecasting beyond 2-3 days is unrealistic in the present framework for operational forecasting of PLs. Global climate models are unable to resolve the structure and dynamics of PLs, but they can skillfully predict large-scale climate conditions. We utilize this in developing a PL genesis potential index (PGI) that relates the spatial distribution of PL genesis frequency and key climate variables in a Poisson regression framework. Our model shows a good agreement with the observations. More specifically, the PGI well represents the seasonal cycle of PL genesis frequency and captures the interannual variability of PL activity skillfully. The effects of AO and ENSO on the interannual variability of PL genesis frequency are also explored.
Ryan's Presentation: Investigating the Longevity of Significant Tornado-Producing Supercells
All significant (EF-2+) tornadoes and their environments between 2008 and 2018 in the United States are examined, with focus on parent supercell thunderstorm longevity, for a total of 1,515 tornadoes. Almost all significant tornadoes are produced by discrete supercells (66%) or clusters (27%), and they are most common in the Plains or Southeast between March and June, with a secondary maximum between December and February in the Southeast. Bulk wind shear and storm-relative helicity (SRH) are moderately correlated (r ≈ 0.35) with the total time that a supercell was discrete, the total number of significant tornadoes produced by a storm, and the total tornado path length associated with a storm. There is no correlation between storm longevity and any thermodynamic variable. Long-lived (≥ 4 hours) supercells are most frequent in the Southeast, where shear and SRH are typically greatest. Additionally, SRH is roughly 50-70 m2 s-2 greater for the longer-lived supercells (≥ 75th percentile) than the shorter-lived ones. A key finding is that excessive backing or veering of the wind shear vector above 1 km AGL is detrimental to supercell longevity.
