Gravity waves generated by the dry convective boundary layer: Two-dimensional scale selection and boundary-layer feedback

Abstract Simulations of supercell thunderstorms in a sheared convective boundary layer (CBL), characterized by quasi-two-dimensional rolls, are compared with simulations having horizontally homogeneous environments. The effects of boundary layer convection on the general characteristics and the low-level mesocyclones of the simulated supercells are investigated for rolls oriented either perpendicular or parallel to storm motion, as well as with and without the effects of cloud shading. Bulk measures of storm strength are not greatly affected by the presence of rolls in the near-storm environment. Though boundary layer convection diminishes with time under the anvil shadow of the supercells when cloud shading is allowed, simulations without cloud shading suggest that rolls affect the morphology and evolution of supercell low-level mesocyclones. Initially, CBL vertical vorticity perturbations are enhanced along the supercell outflow boundary, resulting in nonnegligible near-ground vertical vorticity regardless of roll orientation. At later times, supercells that move perpendicular to the axes of rolls in their environment have low-level mesocyclones with weaker, less persistent circulation compared to those in a similar horizontally homogeneous environment. For storms moving parallel to rolls, the opposite result is found: that is, low-level mesocyclone circulation is often enhanced relative to that in the corresponding horizontally homogeneous environment.

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Characteristics of the secondary circulations in the convective boundary layer over two-dimensional heterogeneous surfaces

10.5194/egusphere-egu2020-12391 ◽

2020 ◽

Author(s):

Zixuan Xiang ◽

Jianning Sun ◽

Jun Zou

Keyword(s):

Boundary Layer ◽

Wind Speed ◽

Coriolis Force ◽

Convective Boundary Layer ◽

Sensible Heat Flux ◽

Two Dimensional ◽

Background Wind ◽

Convective Boundary ◽

Boundary Layer Height ◽

Secondary Circulations

Large-eddy simulations are performed to investigate the effects of background wind on the secondary circulations (SCs) in the convective boundary layer. Heterogeneities are produced by a prescribed two-dimensional surface sensible heat flux pattern of chessboard-type and have a size which is a bit larger than the boundary layer height.When the wind blows along the diagonal of the chessboard-like pattern, the roll-like SCs are observed even when the background wind speed is as large as 10m/s, with whose axes are oriented along the diagonal of the pattern. Another case with wind direction along neither the diagonal nor the side of the chessboard-like pattern and weak wind speed shows the roll-like SCs still exist but lack symmetry. The SCs become much weaker and change their axes orientation when the wind speed increases.Meanwhile, the results are different when the Coriolis force is considered. When the background wind is weak, the asymmetry of the SCs become more significant with the development of boundary layer when the Coriolis force is considered, while the SCs tend to be symmetrical without the Coriolis force. When the background wind strengthens, the SCs are more difficult to maintain in the case of Coriolis force.Further analysis through rotational and divergent decomposition suggests which part contributes more to the maintenance of the SCs.

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