Baroclinic Channel Research Articles

The Microphysical Building Blocks of Low-Level Potential Vorticity Anomalies in an Idealized Extratropical Cyclone

Abstract Diabatically generated low-level potential vorticity (PV) anomalies in extratropical cyclones enhance near-surface winds and influence the cyclone’s development. Positive and negative PV anomalies in the warm-frontal region of an extratropical cyclone, simulated with an idealized moist baroclinic channel model, are investigated to identify the microphysical processes that produce them. Using a novel method based on backward trajectories from the PV anomalies, the contribution of different microphysical processes to the formation of the anomalies is quantified. It is found that, for each anomaly, typically one specific microphysical process takes the leading role in its diabatic generation. A large but rather weak low- and midlevel positive anomaly is produced by depositional growth of ice and snow. Two smaller but stronger positive anomalies at lower levels are generated mainly by in-cloud condensational heating at the warm front and below-cloud rain evaporation and snow melting 200 km farther north. In addition, near-surface negative anomalies are produced by snow melting and snow sublimation. In summary, this idealized study reveals that (i) a variety of microphysical processes are involved in generating the complex mesoscale PV structures along the warm front; (ii) the model representation of these processes, some of them still insufficiently understood and parameterized, therefore matters for an accurate prediction of these features; (iii) below-cloud processes are also relevant for PV anomalies located in clouds, owing to accumulation of diabatic PV tendencies along ascending air parcels; and (iv) the diabatic history of the air parcels is essential in order to explain the observed PV pattern.

An Initialization Method for Idealized Channel Simulations

Abstract Idealized model simulations have long been established as valuable tools to gain insight into atmospheric phenomena by providing a simplified, easier to comprehend version of the complex atmospheric system. A specific subgroup of idealized simulations, such as baroclinic channel models, requires the initialization of the model with balanced atmospheric fields to investigate the evolution of an introduced perturbation. The quality of these simulations depends on the degree of balance of the initial state, as imbalances result in geostrophic and hydrostatic adjustment processes that potentially skew the results. In this paper, a general method to create geostrophically and hydrostatically balanced initial conditions is introduced. The major benefit of this method is the possibility to directly define a basic state wind field with the pertinent atmospheric fields being derived given appropriate boundary conditions. Application of the method is exemplified by constructing initial conditions for a baroclinic test case with WRF and analyzing a perturbed and unperturbed numerical simulation. The unperturbed simulation exhibits weak inertia–gravity wave activity and minimal adjustment of the initial state during a 5-day simulation, which confirms the high degree of initial balance provided by the initialization technique. In the perturbed simulation, baroclinic instability is initiated, resulting in a cyclogenesis event similar to previous idealized baroclinic channel simulations. The proposed method is compared with initial conditions formulated in a Boussinesq framework, illustrating the difference in imbalances and their effect on perturbation growth.

Idealised simulations of polar low development in an Arctic moist‐baroclinic environment

This study examines the influence of moisture, baroclinicity, and static stability on developing disturbances at high latitudes by utilising an idealised baroclinic channel model. The set‐up is composed of environmental baroclinicity defined by a zonally uniform jet in thermal wind balance with a meridional temperature gradient and moisture content defined by relative humidity profiles. Initiation of disturbance growth is achieved by superimposing a weak, surface‐based warm‐cored cyclonic disturbance to the set‐up. The experiments show that the disturbance is able to amplify within such an environment in the absence of an upper‐level perturbation, surface fluxes, friction, or radiation. Separation between developing and non‐developing disturbances is feasible by considering the baroclinic and diabatic contributions to eddy available potential energy. Developing disturbances show a clear diabatic dominance during the early stage of development, whereas experiments lacking this diabatic boost fail to intensify within a 3‐day time window. A comparison with the conceptual framework of the Diabatic Rossby Vortex (DRV) growth mechanism provides insight into the dynamical pathway potentially underlying the enhanced amplification. The emerging disturbance qualitatively resembles the postulated DRV structure, the major difference being a decreased depth in comparison with simulated and observed DRVs in midlatitudes. A suit of sensitivity experiments examines the range of atmospheric conditions in which enhanced amplification by diabatic processes is possible. Threshold values for moisture content and isentropic slopes are identified.

Two finite-volume unstructured mesh models for large-scale ocean modeling

Two approaches pertaining to modeling large-scale ocean circulation on unstructured meshes are described. Both use the finite-volume ideology, unstructured surface triangular mesh and geopotential vertical coordinate, and promise better numerical efficiency than P1–P1 finite element models. The first one is formulated on median-dual control volumes for all variables and presents a finite-volume implementation of P1–P1 finite-element discretization (A-grid). The second one differs by the cell-centered placement of horizontal velocities (quasi-B-grid). Two practical tasks have to be solved to ensure their stable performance in long-term simulations. For triangular A-grids, it is the stabilization against pressure modes triggered by the stepwise bottom topography. The proposed solution preserves volume and tracers by introducing a composite representation for the horizontal velocity (with an elementwise-constant velocity correction). The quasi-B-grid setup is free of pressure modes but requires efficient filtering and dissipation in the momentum equation because of its too large velocity space. Implementations of momentum advection and viscosity that serve this goal are proposed. Both setups show stable performance and similar numerical efficiency, as exemplified by simulations of a baroclinic channel flow and circulation in the North Atlantic.

Impact of different initial conditions on the growth of polar lows: idealised baroclinic channel simulations

AbstractThe development of a polar low is simulated in an ideal baroclinic channel with the objective of studying the relative influence of different initial conditions on certain characteristics of the polar low. The basic state in the channel has a baroclinic jet at the tropopause level superposed by potential vorticity anomalies. The upper‐level perturbation leads to the genesis of a polar low through baroclinic instability. During its growth, the polar low is driven by a combination of baroclinic and convective processes as the vertical motion associated with the polar low is found to be forced simultaneously by the adiabatic and diabatic omega‐forcing terms in the quasi‐geostrophic omega equation. The degree of baroclinicity, surface heating and the scale of the upper‐level anomaly were each reduced, and static stability increased separately in a series of sensitivity experiments. The results show that the pattern of the vertical motion, the growth rate and phase speed of the polar low are highly sensitive to the modifications in the background conditions. In particular, the surface temperature and baroclinicity appear to be crucial in determining the strength of the vertical motion associated with the polar low. The scale and structure of the polar low are more vulnerable to the scale of the upper‐level anomaly and initial baroclinicity than to the rest of the parameters tested. In all the sensitivity experiments, the formation of the polar low gets delayed and its intensity, in terms of the surface pressure, reduced due to the modified initial conditions. The reduced intensity suppresses the tendency for a vertical coupling of upper‐ and lower‐level features of the polar low. Copyright © 2011 Royal Meteorological Society

Simulations of Subtropical Cyclones in a Baroclinic Channel Model

Abstract The present study considers a variety of cyclone developments that occur in an idealized, baroclinic channel model featuring full condensation heating effects over an ocean with prescribed sea surface temperature variation. The geostrophic basic-state jet is specified by the tropopause shape, and horizontal shear is included by specifying the meridional variation of zonal wind on the lower boundary. The horizontal shear induces anticyclonic wave breaking of baroclinic waves. Normal mode perturbations are computed using a “fake-dry” version of the model but integrated forward using full physics. Low-latitude moist convection is particularly strong in simulations with strong surface easterlies that destabilize the troposphere through water vapor fluxes from the ocean surface. Deep convection produces a locally elevated dynamic tropopause and an associated anticyclone. This modified zonal flow supports moist baroclinic instability. The resulting cyclones, identified as subtropical cyclones, occur in deep westerly vertical wind shear but are nearly devoid of lower-tropospheric baroclinicity initially. These systems are distinguished from baroclinically dominated secondary cyclones that also form at relatively low latitudes in the simulations. For weak jets and strong subtropical surface easterlies, subtropical cyclone development dominates formation on the midlatitude jet. For strong westerly jets or weak horizontal shear, the situation is reversed and the midlatitude baroclinic wave can help or hinder the ultimate intensification of the subtropical cyclone. The similarity of this cross-latitude influence to the extratropical transition of tropical cyclones is noted.

Low-Frequency Variability in a Turbulent Baroclinic Jet: Eddy–Mean Flow Interactions in a Two-Level Model

Abstract The origin of low-frequency variability in the midlatitude jet is investigated using a two-level baroclinic channel model. The model state fields are separated into slow and fast components using intermediate time- scale averaging. In the equation for the fast variables the nonlinear wave–wave interactions are parameterized as a stochastic excitation. The slowly varying ensemble mean eddy fluxes obtained from the resulting stochastic turbulence model are coupled with the slowly varying mean flow dynamics. This forms a coupled set of deterministic equations on the slow time scale that governs the dynamics of the eddy–mean flow interaction. The equilibria of this coupled system are found as a function of the excitation strength, which controls the level of turbulence. At low levels of turbulence the equilibrated flow with zonally symmetric mean forcing remains zonally symmetric, but as excitation increases it undergoes zonal symmetry-breaking bifurcations. Time-dependent flows arising from these bifurcations take the form of westward-propagating wavelike structures resembling blocking patterns. Features of these waves are characteristic of blocking in both observations and atmospheric general circulation model simulations including retrogression, eddy variance concentrated upstream of the waves, and eddy momentum flux forcing the waves.

Wave-induced topographic formstress in baroclinic channel flow

Large-scale zonal flow driven across submarine topography establishes standing Rossby waves. In the presence of stratification, the wave pattern can be represented by barotropic and baroclinic Rossby waves of mixed planetary topographic nature, which are locked to the topography. In the balance of momentum, the wave pattern manifests itself as topographic formstress. This wave-induced formstress has the net effect of braking the flow and reducing the zonal transport. Locally, it may lead to acceleration, and the parts induced by the barotropic and baroclinic waves may have opposing effects. This flow regime occurs in the circumpolar flow around Antarctica. The different roles that the wave-induced formstress plays in homogeneous and stratified flows through a zonal channel are analyzed with the BARBI (BARotropic-Baroclinic-Interaction ocean model, Olbers and Eden, J Phys Oceanogr 33:2719–2737, 2003) model. It is used in complete form and in a low-order version to clarify the different regimes. It is shown that the barotropic formstress arises by topographic locking due to viscous friction and the baroclinic one due to eddy-induced density advection. For the sinusoidal topography used in this study, the transport obeys a law in which friction and wave-induced formstress act as additive resistances, and windstress, the effect of Ekman pumping on the density stratification, and the buoyancy forcing (diapycnal mixing of the stratified water column) of the potential energy stored in the stratification act as additive forcing functions. The dependence of the resistance on the system parameters (lateral viscosity ε, lateral diffusivity κ of eddy density advection, Rossby radius λ, and topography height δ) as well as the dependence of transport on the forcing functions are determined. While the current intensity in a channel with homogeneous density decreases from the viscous flat bottom case in an inverse quadratic law ~δ –2 with increasing topography height and always depends on ε, a stratified system runs into a saturated state in which the transport becomes independent of δ and ε and is determined by the density diffusivity κ rather than the viscosity: κ/λ 2 acts as a vertical eddy viscosity, and the transport is λ 2/κ times the applied forcing. Critical values for the topographic heights in these regimes are identified.

Air-Sea Interaction Influences on the Structure and intensification of an Idealized Marine Cyclone

Abstract Three numerical simulations of cyclogenesis in a baroclinic channel model with realistic physical parameterizations are compared to examine the influence of surface heat and moisture fluxes on the structure and development mechanisms of an idealized midlatitude cyclone. Identical atmospheric initial conditions are used in the three numerical simulations, which differ only in their surface flux conditions. The impact of the surface heat and moisture fluxes on the thermal structure and vertical circulation in the model cyclone is examined by comparing the geopotential height, temperature, static stability, surface convergence and wind stress as well as the surface heat and moisture flux distributions in the three simulations. Boundary layer stratification effects on convergence are examined and the associated changes in the boundary layer-free atmosphere interaction are compared. Results indicate that the direct influence of the surface beat and moisture fluxes is limited to the boundary layer stru...

Numerical Experiments on the Influence of Orography on Cyclone Formation with an Isentropic Primitive Equation Model

Numerical integration experiments with a primitive equation model in isentropic coordinates are performed to investigate the influence of orography on cyclone formation. Results obtained in the presence of an isolated mountain in a baroclinic channel are shown. An ideal simplified experiment reproducing cyclogenesis in the lee of the Alps is also presented.