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Norwegian Frontal modal by J. Bjerknes (3D)
The METEO studying group (Andrea, Titouan, Camilla) had a short review about different items in the midday break on Friday (17th Nov) ... Andrea introduced a 3D picture of the Norwegian weather model explaining the frontal systems (related to polar front) by legendary Norwegian born US meteorologist Jacob Bjerknes.

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[Image: norwegian-model-1.jpg]
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Details here :

A good 3D perspective explaining also the cloud system ...

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Some more interesting infos ...

Features analyzed on surface maps (e.g., low pressure systems, fronts) convey basic information about the type and intensity of weather present. Due to the sparseness of marine surface observations, however, the analysis of surface features in between observations often (1) is inferred from other information such as satellite imagery, (2) is drawn according to the analysts' previous experience, or (3) is envisioned in terms of a conceptual model. This paper illustrates a method to assist in surface analysis by incorporating information from the large-scale flow and using conceptual models of marine midlatitude cyclone structure and evolution.

Those who regularly perform surface frontal analyses of midlatitude cyclones often recognize that these storms can undergo a variety of different evolutions (e.g., Smigielski and Mogil 1995; Sienkiewicz 1996). Two of these evolutions have been observed frequently enough to attain conceptual-model status: the Norwegian and Shapiro-Keyser cyclone models (Fig. 1).

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Figure 1: Conceptual models of cyclone evolution showing lower-tropospheric geopotential height and fronts (top), and lower-tropospheric potential temperature (bottom). (a) Norwegian cyclone model: (I) incipient frontal cyclone, (II) and (III) narrowing warm sector, (IV) occlusion; (b) Shapiro-Keyser cyclone model: (I) incipient frontal cyclone, (II) frontal fracture, (III) frontal T-bone and bent-back front, (IV) frontal T-bone and warm seclusion. Panel (b) is adapted from Shapiro and Keyser (1990, their Fig. 10.27) to enhance the zonal elongation of the cyclone and fronts and to reflect the continued existence of the frontal T-bone in stage IV. The stages in the respective cyclone evolutions are separated by approximately 6-24 h and the frontal symbols are conventional. The characteristic scale of the cyclones based on the distance from the geopotential height minimum, denoted by L, to the outermost geopotential height contour in stage IV is 1000 km. From Schultz et al. (1998, Fig. 15).

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Figure 2: Schematic illustrating confluence at the jet-entrance region and diffluence at the jet-exit region. Strongest wind speed is denoted by arrow. Geopotential height lines are solid.

The Norwegian cyclone model (Fig. 1a), so named to honor the Norwegian meteorologists (e.g., Bjerknes, Bergeron, and Solberg) who first conceptualized the typical life cycle of midlatitude cyclones in the 1910s and 1920s, presents the evolution of a cyclone from an incipient frontal wave with cold and warm fronts (stage I), to a deepening cyclone with a narrowing warm sector as the cold front rotates around the cyclone faster than the warm front (II and III), and finally to a mature cyclone with an occluded front (IV). Typically, a Norwegian cyclone is oblong, oriented roughly north-south with the cold front more intense and longer than the weak and "stubby" warm front.

This investigation shows that cyclones embedded in diffluent large-scale flow tend to develop structures and follow evolutions similar to the Norwegian cyclone model, with a strong cold front, weak warm front, and narrowing warm sector to form an occluded front. In contrast, cyclones embedded in confluent large-scale flow tend to develop structures and follow evolutions similar to the Shapiro-Keyser cyclone model, with a strong warm front, weak cold front, and T-bone frontal structure.

Because the large-scale flow is not constant in time, cyclones can change from one type to another. For example, many initially Shapiro-Keyser-like cyclones may develop occluded fronts late in their life cycles, becoming more Norwegian-like, as the initially confluent flow becomes more diffluent during cyclogenesis.

Whereas limited surface data may make detailed analysis of the frontal structure of marine cyclones difficult, the large-scale flow may provide some guidance in recognizing these different cyclone evolutions and anticipating the relative strengths and orientations of the fronts. The Marine Prediction Center currently issues analyses and forecasts of the 500-mb flow, available on the World-Wide Web ( These graphics products can be used to infer the nature of the surface cyclones embedded within that flow.

There are other factors on the mesoscale that affect the strength of low-level fronts, so this proposed method cannot be used independently of other guidance. Nevertheless, it has shown some utility at the Marine Prediction Center as one of their analysis tools (e.g., Sienkiewicz 1996) and may be of use to other mariners and surface analysts.


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