Investigation of the Influence of Stratospheric Shear on Baroclinic Instability

Abstract

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.

Keywords

• baroclinic instability
• transient growth
• stratospheric flow
• generalized stability theory
• Eady model