Midlatitude Disturbances

Theoretical ideas about the atmosphere and its weather systems evolved in part through the needs of nineteenth-century mariners for information about winds and storms, especially predictions of future behaviour. At low levels in the westerly belt (approximately 40° to 70° latitude) there is a complex pattern of moving high and low pressure systems, while between 6000 m and 20,000 m there is a coherent westerly airflow. Dove (1827 and 1828) and Fitz Roy (1863) supported the 'opposing current' theory of cyclone (i.e. depression) formation, where the energy for the systems was produced by converging airflow. Espy (1841) set out more clearly a convection theory of energy production in cyclones with the release of latent heat as the main source. In 1861, Jinman held that storms develop where opposing air currents form lines of confluence (later termed 'fronts'). Ley (1878) gave a three-dimensial picture of a low-pressure system with a cold air wedge behind a sharp temperature discontinuity cutting into warmer air, and Abercromby (1883) described storm systems in terms of a pattern of closed isobars with typical associated weather types. By this time, although the energetics were far from clear, a picture was emerging of mid-latitude storms being generated by the mixing of warm tropical and cool polar air as a fundamental result of the latitudinal gradients created by the patterns of incoming solar radiation and of outgoing terrestrial radiation. Towards the end of the nineteenth century two important European research groups were dealing with storm formation: the Vienna group under Margules, including Exner and Schmidt; and the Swedish group led by Vilhelm Bjerknes. The former workers were concerned with the origins of cyclone kinetic energy which was thought to be due to differences in the potential energy of opposing air masses of different temperature. This was set forth in the work of Margules (1901), who showed that the potential energy of a typical depression is less than 10 per cent of the kinetic energy of its constituent winds. In Stockholm V. Bjerknes' group concentrated on frontal development (Bjerknes, 1897, 1902) but its researches were particularly important during the period 1917 to 1929 after J. Bjerknes moved to Bergen and worked with Bergeron. In 1918 the warm front was identified, the occlusion process was described in 1919, and the full Polar Front Theory of cyclone development was presented in 1922 (J. Bjerknes and Solberg). After about 1930, meteorological research concentrated increasingly on the importance of mid- and upper-tropospheric influences for global weather phenomena. This was led by Sir Napier Shaw in Britain and by Rossby, with Namias and others, in the USA. The airflow in the 3-10 km high layer of the polar vortex of the northern hemisphere westerlies was shown to form large-scale horizontal (Rossby) waves due to terrestrial rotation, the influence of which was simulated by rotation 'dish pan' experiments in the 1940s and 1950s. The number and amplitude of these waves appears to depend on the hemispheric energy gradient, or 'index'. At times of high index, especially in winter, there may be as few as three Rossby waves of small amplitude giving a strong zonal (i.e. west to east) flow. A weaker hemispheric energy gradient (i.e. low index) is characterized by four to six Rossby waves of larger amplitude. As with most broad fluid-like flows in nature, the upper westerlies were shown by observations in the 1920s and 1930s, and particularly by aircraft observations in the Second World War, to possess narrow high-velocity threads, termed 'jet streams' by Seilkopf in 1939. The higher and more important jet streams approximately lie along the Rossby waves. The most important jet stream, located at 10 km, clearly affects surface weather by guiding the low pressure systems which tend to form beneath it. In addition, air subsiding beneath the jet streams strengthens the subtropical high pressure cells.

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