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Turbulence is a dominant feature of atmospheric motions. Despite its chaotic nature, it is observed to create and maintain coherent structures such as zonal jets and large-scale waves in the atmosphere. Previous studies on the self-organization of turbulence in a baroclinic two-layered fluid have shown that large-scale coherent structures emerge out of a homogeneous turbulent field through a collective type of instability with a preference for barotropic flows. That is, the barotropic coherent structures reorganize the turbulence in such a way as to reinforce themselves through a positive feedback and emerge in the flow. In this work, a statistical framework (S3T) is utilized in order to study the vorticity and thermal flux feedbacks underlying this instability. It is found that the feedbacks produced by the organization of incoherent barotropic and baroclinic eddies by the coherent emergent structures differ. For large stratification, the feedback that results from the organization of barotropic eddies by baroclinic coherent structures is negative and completely cancels out the positive feedback produced by the organization of the baroclinic eddies. On the contrary, the feedback resulting from the organization of the barotropic and the baroclinic turbulent eddies by the barotropic coherent structures do not cancel each other for most scales and lead to the emergence of these structures in the flow. The spatial features of the emerging coherent structures depend on the value of the non-dimensional planetary vorticity gradient. For small values of the gradient, the maximum value of the feedback is found for zonal jets, while for large values of the gradient, the maximum value of the feedback is found for Rossby waves. [ABSTRACT FROM AUTHOR]

Macroscale turbulence in the atmosphere is observed to be self-organized into large-scale structures such as zonal jets and robust waves and vortices. A simple model containing the relevant dynamics of turbulence self-organization is quasi-geostrophic turbulence in a stably stratified atmosphere approximated with a single-layer fluid on a beta-plane. Numerical simulations of this model have shown the dominance of Rossby waves, zonal jets and robust vortices in different regions of the parameter space. In this work, we perform numerical integrations of this model and focus on the regime in which robust large-scale vortices dominate the flow. The goal is to identify the Lagrangian coherent vortices that trap the same air masses in their core throughout their life cycle and to obtain their characteristics. The vortices are identified using an objective algorithm based on the Lagrangian-averaged vorticity deviation calculated using the advection of Lagrangian particles by the flow. Long-lived vortices with scales comparable to the deformation scale are found with a symmetry between cyclones and anti-cyclones as expected from the simplified dynamics of the model. The scale as well as the life span of the vortices are also found to increase alongside an increase in the strength of turbulence. [ABSTRACT FROM AUTHOR]

Baroclinic instability is one of the main mechanisms for the formation of synoptic scale systems. Previous studies examined the exponential growth of small perturbations for a stably stratified troposphere in the case of a constant meridional temperature gradient ignoring the stratosphere (Eady's model). However, since stratospheric flow also affects to some extent the motions in the troposphere, in this work we investigate the effect of stratospheric wind shear on baroclinic instability using the tools of Generalized Stability Theory (GST). GST is a linear stability theory that addresses both the exponential growth of perturbations that is pertinent in the large time asymptotic limit and the transient growth of perturbations at finite time. The optimal initial perturbations leading to the largest growth over a specified time interval are calculated for three main cases of stratospheric shear: positive, zero and negative shear over the stratosphere. It is found that the inclusion of stratospheric shear in all three cases decreases perturbation growth and influences the scale of the structures that will dominate the flow. For optimizing times of the order of a week, the development of systems with larger spatial scale compared to the prediction of the Eady model is expected, while for optimizing times of the order of a day, smaller scale systems are found to develop. [ABSTRACT FROM AUTHOR]

The study of the conditions under which a stratified shear flow becomes turbulent is important, as turbulence is the source of mixing and dissipation in the atmosphere and can significantly influence the momentum and temperature structure of the atmospheric circulation. Oftentimes, the density structure of atmospheric flows is organized in thick layers of constant density separated by thin layers of sharp density gradients. It has been shown by previous studies that such multilayered flows can become unstable under shear. In this work, we investigate Taylor-Caulfield Instability (TCI), which occurs in a three-layer fluid moving with a constant shear flow. Previous studies examined the instability under the Boussinesq approximation, which is not expected to hold in cases of sharp density gradients. The non-Boussinesq limit is therefore investigated in this work. TCI is studied using the classical perturbation theory, that is by examining the evolution of small perturbations to the base flow. The wavelength of the waves expected to dominate the flow as well as the time in which these waves will emerge are calculated. In addition, the characteristics of the unstable waves are studied under a variety of conditions for the shear and the stratification. It is found that under the Boussinesq approximation, the wavelength of the instability waves is underestimated and the time for the evolution of the waves is overestimated. [ABSTRACT FROM AUTHOR]