On the mechanism of cyclogenesis as deduced from vertical axis tilts

The relative strengths of 2 mechanisms to explain early stages of cyclogenesis are examined with an initial value model. Either mechanism can be emphasized by different choices of initial trough axis configuration. In our model, if initial conditions have an upper level trough upstream of the surface trough, and both troughs have little tilt, the growth is primarily by normal mode instability. Some evidence is presented suggesting that observed troughs are initially separate and have little along-flow tilt. A case study is shown where an observed storm develops while maintaining a fixed structure for a short period of time. The evidence suggests that the normal mode concept is a relevant motif for cyclogenesis.