Date of this Version
Published in Quarterly Journal of the Royal Meteorological Society, 147, Issue736 (April 2021), pp. 1584-1605.
Published: 03 February 2021
A new mechanism is proposed as a potential cause for the one-third of warm season severe nocturnal convection in the U.S. Great Plains that develops in environments without the presence of air mass boundaries of fronts or mesoscale systems. This mechanism is tested in two- and three dimensional models. Results show strong ascent (~1.0 m/s) sufficient for nocturnal convection initiation, arising from interactions of mean westerly zonal wind with the vertical shear of a northern vortex and also perturbation westerly winds that are created by the Coriolis torque on the Great Plains southerly low-level jet. The interaction involving the northern vortex results in organized strong ascent on the east side of the vortex from the near surface level to the top of the model atmosphere, and also a weak upward acceleration near the center of the vortex. In simulations with westerly wind perturbations, strong and organized ascent occurs above and on the east side of the westerly perturbation winds. The upward motion in these simulations relies on both mechanical forcing from non-hydrostatic pressure perturbations and buoyant acceleration caused by interactions of the westerly zonal wind and the vertical shear in the vortex or the perturbation westerly wind. Statistical tests confirm that these interactions, not the northern vortex or westerly perturbation itself and related shear, are essential for the simulated vertical motion. Additional sensitivity analysis indicates robust ascent across a wide range of westerly perturbation or northern vortex strengths. The vertical motion profile is not sensitive to the horizontal grid spacing of the model, at least at or below 4 km, but to the morphology of westerly wind perturbations. The latter suggests where improvement could be made to increase the accuracy of model prediction of nocturnal convective storms in the U.S. Great Plains.