Stratospheric and mesospheric data assimilation: the role of middle atmospheric dynamics

Abstract

The middle atmosphere refers to the stratosphere and mesosphere and features dynamics and circulationsthat are fundamentally different from those of the troposphere. The large-scale meridional circulations inthe middle atmosphere operate on seasonal and longer time scales and are largely forced by the breakingof upward propagating waves. The winter stratosphere is dominated by large-scale waves and a polarvortex which confines constituents and which is sometimes punctuated by stratospheric sudden warmings.In contrast, the summer stratosphere is quiescent. Meanwhile, the meridional circulation in themesosphere is mainly driven by the breaking of a broad spectrum of gravity waves that have propagatedupward from the troposphere. These facets of middle atmosphere dynamics have implications for, andpose unique challenges to, data assimilation systems whose models encompass this region of theatmosphere. In this work, we provide an overview of middle atmosphere data assimilation in the contextof the dynamics of this region. The purpose is to demonstrate how the dynamics can be used to explainthe behavior of data assimilation systems in the middle atmosphere, and also to identify challenges inassimilating measurements from this region of the atmosphere. There are two overarching themes.Firstly, we consider the vertical propagation of information through waves, resolved and parameterized,and background error covariances. Secondly, we delve into the dynamical sources of model errors andtechniques for their estimation.