# Large Scale Flow

### Norwegian or Shapiro-Keyser Cyclone? The Large-Scale Flow.

What are the criteria that determine whether the evolution of a cyclone follows the Norwegian or the Shapiro-Keyser cyclone model?

The evolution of mid-latitude cyclones is influenced by dynamical factors such as the large-scale flow, surface friction, diabatic heating and physiography. David Schultz, Daniel Keyser and Lance Bosart showed in 1997 that one of these factors, the large-scale flow, has a major effect on cyclone type. They concluded that:

1) The Norwegian cyclone model typically applies to cyclones developing within diffluent, high-amplitude background flows (see Figure 1). These cyclones are characterized by:

- A strong cold front
- The classical occlusion as a merging of cold and warm fronts
- A weak warm front or no warm front at all

Figure 1: A Norwegian cyclone (SEVIRI IR10.8 μm, 3 April 2019; 00:00 UTC). Cyan isolines depict the geopotential at 500 hPa and black isolines the mean surface pressure

2) The Shapiro-Keyser cyclone model typically applies to cyclones evolving within confluent, low-amplitude background flows (see Figure 2). These cyclones are characterized by:

- The cold and warm fronts following a T-bone pattern
- A cold frontal fracture (i.e., the cold front is not attached to the warm front)
- A strong warm front where one would expect the occlusion

Figure 2: A Shapiro-Keyser cyclone (SEVIRI IR10.8 μm, 5 October 2019, 03:00 UTC). Cyan isolines depict the geopotential at 500 hPa and black isolines the mean surface pressure

Note:

The upper-level flow determines the low-level frontal structure. A cyclone can transition from one type to the other as the associated upper-level flow evolves.