BAROCLINIC INSTABILITY IN SATURATED ENVIRONMENT
| Time evolution of mean zonal state during the development of a baroclinic wave 5000 km long. | |||
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| Initial mean state | 300 hrs | 450 hrs | 600 hrs |
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Colors: zonal wind. Contours of zonal wind every 2 m/s; solid: positive values; short dash: zero and negative values. Contours of potential temperature every 5 K, long dashed. Domain: abscissa: rectified latitude, 12000 km, South to the left; ordinate: geometric height, 10 km. Upper panel: dry experiment. Lower panel: moist experiment.
AVI file (divx5) - 1.1M |
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(Maurizio Fantini)
Baroclinic instability is the most important mechanism of development of midlatitude cyclones, and it is often present, in conjunction with organized convection, in high-latitude events (polar lows, comma clouds). The intrinsic spatial scale of baroclinic cyclones ranges in the thousands of kilometers (synoptic scale), but its influence is felt on all the sub-synoptic-scale phenomena that characterize its internal structure (fronts, squall lines, precipitation bands) or that evolve in the environmental conditions provided by the synoptic cyclone (organized convective activity, severe storms). On the other extreme of the spatial scales involved, baroclinic cyclones influence the planetary scale by contributing to the maintenance of the observed mean state of the atmosphere. In this sense baroclinic cyclones are mediators between the long time-and-space scales characteristic of climate change and the everyday perception of weather phenomena and their impact.
The presence of water vapor in the atmosphere changes the spatial structure and time evolution of baroclinic cyclones and therefore modifies both the short scales of motion, by changing the environment in which convection develops, and the planetary scales which experience the latitudinal redistribution of energy extracted from latent heat release. The purpose of the kind of process modeling we perform is to identify the effects of latent heat release on baroclinic waves throughout the lifecycle, from exponential modal growth through finite amplitude to nonlinear equilibration and decay, in simplified conditions which isolate the desired mechanism.
We show here an illustration of the influence of baroclinic cyclones on the mean state of the atmosphere, by comparing the evolution of the mean zonal wind and potential temperature while a baroclinic wave (5000 km long) develops (along the axis normal to the page) in a dry (upper panel) or a saturated environment (lower panel). The experiments are performed with a PE numerical model, in a idealized geometrical configuration (Cartesian geometry, f-plane, periodic boundaries) and with a simplified representation of latent heat release. The initial mean state is a Hoskins-West jet flanked by unsheared regions that initially confine the evolution of the perturbation to the central third of the domain of integration.
It is seen here that the dry evolution is perfectly symmetric thanks to the idealized setup chosen. The baroclinicity initially located at the center of the domain is partly destroyed in the baroclinic lifecycle (the conversion of zonal potential energy to eddy potential energy and eddy kinetic energy, which is eventually dissipated) and partly expelled symmetrically to the north and south of the original jet. In the moist evolution, on the other hand, the latitudinal variation of saturation mixing ratio, corresponding to the latitudinal gradients of temperature and pressure, breaks the symmetry, and the baroclinic wave generates latitudinal structures which are more intense on the Northern flank of the initial jet. The energy obtained from latent heat release is organized by the synoptic-scale flow and distributed partly to eddy energy (faster evolution of the cyclone - not shown), partly to mean zonal wind (notice the central core of the jet more intense in the moist experiment, although the vertical shear is reduced) and partly to the potential energy of the reconstituted baroclinic belt, to the North of the original one, which can become newly unstable to baroclinic instability.