Upstream development in idealized baroclinic wave experiments

Abstract

An idealized dry primitive equation model on the f-plane is used to study upstream (anddownstream) baroclinic wave development. The simulations are initiated with localized finiteamplitude and vertically evanescent perturbations, specified either as upper-level potential vorticityor surface potential temperature anomalies. The nonlinear evolution of these nonmodalperturbations leads to the generation of large-scale upper-level induced primary and downstreamsurface cyclones, and of distinctively smaller, shallow and more slowly intensifyingupstream systems. It is shown that in particular the genesis and evolution of upstream cyclonesis highly sensitive to the scale of the initial perturbation. Narrow upper-level troughs (or zonallyconfined surface temperature anomalies) are favorable for upstream development, whereas noor only weak upstream activity occurs with broad planetary-scale troughs (or zonally extendedsurface temperature anomalies) as initial perturbations. It is proposed that this sensitivity propertyof upstream development is qualitatively related to the dispersion characteristics of surfaceedge waves.The shortcomings of the present approach are discussed, and some consideration is given tothe occurrence of upstream cyclogenesis in the real atmosphere, to the relationship with earlierconcepts of secondary cyclogenesis, and to possible implications for the issue of predictabilityof extratropical weather systems.