Vertical Shear Instability Research Articles

Magnetic and vertical shear instabilities in accretion discs

ABSTRA C T The stability properties of magnetized discs rotating with angular velocity Q o Q﷿s; zfi, dependent on both the radial and the vertical coordinates s and z, are considered. Such a rotation law is adequate for many astrophysical discs (e.g., galactic and protoplanetary discs, as well as accretion discs in binaries). In general, the angular velocity depends on height, even in thin accretion discs. A linear stability analysis is performed in the Boussinesq approximation, and the dispersion relation is obtained for short-wavelength perturbations. Any dependence of Q on z can destabilize the flow. This concerns primarily small-scale perturbations for which the stabilizing effect of buoyancy is strongly suppressed due to the energy exchange with the surrounding plasma. For a weak magnetic field, instability of discs is mainly associated with vertical shear, whilst for an intermediate magnetic field the magnetic shear instability, first considered by Chandrasekhar and Velikhov, is more efficient. This instability is caused by the radial shear which is typically much stronger than the vertical shear. Therefore the growth time for the magnetic shear instability is much shorter than for the vertical shear instability. A relatively strong magnetic field can suppress both these instabilities. The vertical shear instability could be the source of turbulence in protoplanetary discs, where the conductivity is low.

Magnetic and vertical shear instabilities in accretion discs

Magnetic and vertical shear instabilities in accretion discs

Possible Mechanisms of Clear-Air Turbulence in Strongly Anticyclonic Flows

Abstract The forecasting of clear-air turbulence (CAT) continues to be a challenging problem despite progress made in the understanding of vertical shear (Kelvin–Helmholtz) instabilities. The possible connections between horizontal anticyclonic flows and CAT are addressed. Analytical expressions are derived to show that current CAT diagnostics do not correctly account for the dynamics of strongly anticyclonic situations. In gradient-balanced anticyclonic flows, nonfrontogenetical enhancement of vertical shear may lead to CAT. A review of observations, theory, and modeling is presented to support the claim that strong anticyclonic relative vorticity can also lead to CAT through the generation of gravity wave activity by geostrophic adjustment and inertial instability. CAT diagnostics are then discussed in light of these claims. Observational work is in progress to investigate the possibility of inertial instability-triggered CAT.

孤立した山岳の風下にできる筋状雲の生成メカニズムに関する数値実験

Cloud streets are frequently observed when a cold air mass advects over a warm sea surface in the winter season; thick and long cloud streets can be sometimes seen in the lee of some islands or mountains near a coast. According to the conventional theory, the vertical shear and static instability play an important role in the formation of the thin cloud streets. For the thick cloud streets in the lee of the isolated mountain, however, another mechanism seems to be important in addition to the conventional theory. The effects of the two factors, strong static instability and topographically induced mechanical disturbance, are investigated by use of the CSU RANTS (Regional Atmospheric Modeling System) with high horizontal and vertical resolution. In the calculation, uniform stratification and wind velocity (low Froude number) are assumed at the inflow boundary. To clarify the importance of both the strong static instability and the topographically induced mechanical disturbance on the formation and maintenance of cloud streets, three kinds of numerical experiments with different sea-surface temperature were carried out, including a numerical experiment without a mountain. Cloud streets were successfully simulated in the lee of an isolated mountain near a coast, with the addition of a large sensible heat flux at the sea surface. Well developed cloud streets occur in the simulation between a pair of convective rolls below a height of 1 km over the sea. The following five results were obtained: 1) For the formation of the pair of convective rolls, both strong static instability and a topographically induced mechanical disturbance are required at the same time. 2) Strong sensible heat flux from the sea surface is the main source of the convective rolls (the buoyancy caused by the condensation process in the cloud is negligibly small). 3) The pair of convective rolls contain two sub-rolls. One is the outer roll, which has a large radius but a weak circulation, and the other is the inner roll, which has a small radius but a strong circulation. The former gathers a large amount of moisture by convergence in the lower marine boundary layer, and the latter transfers the convergent moisture to the upper boundary layer by the strong vertical motion between the rolls. 4) The pair of inner rolls form the line-shaped cloud streets, and keep them on a narrow line along the center-line of the domain. 5) Cloud streets can be simulated both by non-hydrostatic and by hydrostatic models, which implies that vertical inertia is not always important for cloud-street formation. The horizontal scale of the topography does not seem to be restricted to within the small scale, where non-hydrostatic effects are important.

Effects of Cumulus Ensemble and Mesoscale Stratiform Clouds in Midlatitude Convective Systems

Abstract Diagnostic and semiprognostic analyses are performed using OK PRE-STORM (Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central) data to examine the cumulus-environment interaction in midlatitude convective systems. The similarities and differences of the interaction processes between midlatitude and tropical convective systems are also discussed: Analyses of PRE-STORM and GATE (GARP Atlantic Tropical Experiment) data show generally larger vertical wind shear, large-scale forcing, and moist convective instability in midlatitude MCCs (mesoscale convective complexes) and squall lines than in tropical cloud clusters. It is found that the interaction mechanism based on the cumulus-induced subsidence and detrainment is capable of explaining most of the observed heating and drying under widely different environment conditions. Convective- wale downdrafts act to cool and moisten the lower troposphere in the midlatitudes as in the tropics. The quasi- equilibrium assumption between stabilizatio...

Vertical shear instability in accretion discs

Two-dimensional shear flows in the (R-z) and (φ-z) planes in thin accretion discs are studied. The vertical shear is set up by the resonant response of a disc with small tilt. It is shown that the flow is dynamically unstable, with growth rates in excess of ∼ 0.1 Ω −1 , corresponding to a growth time-scale of one orbital period at a given radius. Standard analytical results for the two-dimensional flow instability are extended to take the effect of finite-density free boundaries into account. Ray theory is used to trace the path of energy transport