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Environments characterized by large values of vertical wind shear and modest convective available potential energy (CAPE) are colloquially referred to as high-shear, low-CAPE (HSLC) environments. Convection within these environments represents a considerable operational forecasting challenge. Generally, it has been determined that large low-level wind shear and steep low-level lapse rates—along with synoptic-scale forcing for ascent—are common ingredients supporting severe HSLC convection. This work studies the specific processes that lead to the development of strong surface vortices in HSLC convection, particularly associated with supercells embedded within a quasi-linear convective system (QLCS), and how these processes are affected by varying low-level shear vector magnitudes and lapse rates. Analysis of a control simulation, conducted with a base state similar to a typical HSLC severe environment, reveals that the key factors in the development of a strong surface vortex in HSLC embedded supercells are (i) a strong low- to midlevel mesocyclone, and (ii) a subsequent strong low-level updraft that results from the intense, upward-pointing dynamic perturbation pressure gradient acceleration. Through a matrix of high-resolution, idealized simulations, it is determined that sufficient low-level shear vector magnitudes are necessary for the development of low- to midlevel vertical vorticity [factor (i)], while steeper low-level lapse rates provide stronger initial low-level updrafts [factor (ii)]. This work shows why increased low-level lapse rates and low-level shear vector magnitudes are important to HSLC convection on the storm scale, while also revealing similarities between surface vortexgenesis in HSLC embedded supercells and higher-CAPE supercells.
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Lift in the vertical shear of a southerly jet embedded in a uniform westerly flow
Abstract A new mechanism is proposed as a potential cause for the one‐third of warm season severe nocturnal convection in the US 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−1), 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 centre 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 US Great Plains.
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- Award ID(s):
- 1924679
- PAR ID:
- 10442982
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Quarterly Journal of the Royal Meteorological Society
- Volume:
- 147
- Issue:
- 736
- ISSN:
- 0035-9009
- Page Range / eLocation ID:
- p. 1584-1605
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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