Feb 9, 2017

The Impact of Vertical Wind Shear on Hail Growth in Simulated Supercells

by
Eli J. Dennis and Matthew R. Kumjian
,
Journal of the Atmospheric Sciences
  • States that severe hailstorms produce over $1 billion in insured losses annually in the United States, yet details of a given storm’s hail threat (e.g., maximum hailstone size and total hailfall) remain challenging to forecast
  • Previous research suggests that, in addition to maximum updraft speed, the storm-relative airflow could be equally important for hail formation and growth
  • This study investigates how changes in environmental wind shear and subsequent changes in simulated supercell storm structure affect hail growth
  • Performs 20 idealized simulations in which the thermodynamic profile remains fixed but the environmental hodograph is systematically altered
  • Quantifies hail growth using the hail mass mixing ratio from composites of storms over the last hour of simulation time
  • Computes hailstone growth “pseudotrajectories” from these storm composites to determine favorable embryo source regions
  • Results indicate that increased deep-layer east–west shear elongates the storm’s updraft in that direction, providing:
    • (1) increased volumes over which microphysically relevant hail processes can act
    • (2) increased hailstone residence times within the updraft, and
    • (3) a larger potential embryo source region; together, these lead to increased hail mass
  • Finds that increased low-level north–south shear, which results in hodographs with increased 0–3-km storm-relative helicity, also elongates the updraft in the north–south direction
  • Finds that hail mass is reduced owing to a separation of favorable embryo source regions (which shift southward) and available hydrometeors to serve as embryos (which shift northward)