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

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