Abstract
AbstractSecondary disturbances spawning frontal waves along the fronts of mature midlatitude low‐pressure systems were identified decades ago from satellite images and during field campaigns. Today's flagship supercomputers allow performing simulations at kilometer‐scale resolution on computational domains covering the entire lifecycle of synoptic‐scale systems and thus enable explicit representation of small‐scale disturbances embedded in large‐scale circulations. Here we demonstrate these capabilities in two different types of kilometer‐scale simulations. The first is a 10‐day‐long near‐global simulation of an idealized moist baroclinic wave, performed at 1 km grid spacing and employing 16,001 × 36,006 × 60 grid points. The second is a real‐case simulation of an extratropical low‐pressure system, driven by the European Centre for Medium‐Range Weather Forecasts's operational analysis. At kilometer‐scale resolution, both simulations display clear evidence of embedded mesoscale vortices spawning along frontal systems of mature extratropical cyclones. The vortices appearing in the real‐case simulation can also be identified in satellite imagery of the system. The simulated developments are due to a barotropic instability mechanism and driven by strong low‐level horizontal wind shear. While the simulation of the frontal systems is amenable at model resolutions around 10–50 km, the instability mechanism itself relies on the representation of a narrow shear zone, requiring about 5 times finer resolution. Results suggest that the flow regimes suppressing or fostering barotropic vortices can coexist in the same synoptic system. Far away from the cyclone core, the instability is suppressed by deformation associated with the large‐scale flow, while close to the mature cyclone core, the narrow frontal structure becomes unstable.
Highlights
The midlatitude atmospheric dynamics are strongly affected by fluid-dynamical instabilities of barotropic and baroclinic type
Secondary Instability in an Idealized Moist Baroclinic Wave In Fuhrer et al (2018), we have presented the evolution of the growing baroclinic wave within an unstable westerly flow
In idealized and real-study simulations at kilometer-scale resolution, we detected a secondary barotropic instability mechanism leading to mesovortices forming along fronts of occluding extratropical low-pressure systems: First we considered the simulation of an idealized moist baroclinic wave, following a moist version of the configuration by Jablonowski and Williamson (2006)
Summary
The midlatitude atmospheric dynamics are strongly affected by fluid-dynamical instabilities of barotropic and baroclinic type. The idealized simulations of the current study consider a near-global model domain and represent the evolution of a growing baroclinic wave to a mature cyclone, the subsequent formation and growth of barotropic wave trains on the narrow cold front, and the formation of mesovortices within the cyclone core. This sequence of events is facilitated by the use of a free-slip lower boundary condition, consistent with the setup of the idealized baroclinic wave (Jablonowski & Williamson, 2006; Park et al, 2013), which enables large near-surface horizontal wind shears.
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