Upstream development in idealized baroclinic wave experiments

Abstract

An idealized dry primitive equation model on the f-plane is used to study upstream (and downstream) baroclinic wave development. The simulations are initiated with localized finite amplitude and vertically evanescent perturbations, specified either as upper-level potential vorticity or surface potential temperature anomalies. The nonlinear evolution of these nonmodal perturbations leads to the generation of large-scale upper-level induced primary and downstream surface cyclones, and distinctively smaller, shallow and more slowly intensifying upstream systems. It is shown that in particular the genesis and evolution of upstream cyclones is highly sensitive to the scale of the initial perturbation. Narrow upper-level troughs (or zonally confined surface temperature anomalies) are favorable for upstream development, whereas no or only weak upstream activity occurs with broad planetary-scale troughs (or zonally extended surface temperature anomalies) as initial perturbations. It is proposed that this sensitivity property of upstream development is qualitatively related to the dispersion characteristics of surface edge waves. The shortcomings of the present approach are discussed, and some consideration is given to the occurrence of upstream cyclogenesis in the real atmosphere, to the relationship with earlier concepts of secondary cyclogenesis, and to possible implications for the issue of predictability of extratropical weather systems.