. Compendium of meteorology. Meteorology. EXTRATROPICAL CYCLONES 583 placement polewards. This latter development, which does not start until the cyclone is past the nascent stage, gains in magnitude with the growth of the cyclone. In the westerly wave of the upper troposphere, as represented in Fig. 1, the air enters the cyclone from the northwest and leaves it toward the east, across the wave crest ahead of the surface cyclone. As long as the upper wave does not degenerate, the relative vortic- ity changes sign at the longitude of the inflection points, thus changing from anticyclonic to cyc
. Compendium of meteorology. Meteorology. EXTRATROPICAL CYCLONES 583 placement polewards. This latter development, which does not start until the cyclone is past the nascent stage, gains in magnitude with the growth of the cyclone. In the westerly wave of the upper troposphere, as represented in Fig. 1, the air enters the cyclone from the northwest and leaves it toward the east, across the wave crest ahead of the surface cyclone. As long as the upper wave does not degenerate, the relative vortic- ity changes sign at the longitude of the inflection points, thus changing from anticyclonic to cyclonic relative vorticity in the middle of the upper zone of convergence, and from cyclonic to anticyclonic in the middle of the upper zone of divergence. This vorticity change by divergence is supported by the effect of meridional advection. The factor (f + 2fi sin . Any small wave impulse may then develop into meandering wave patterns. It is obvious that the meandering phenomenon, once started in regions of excessive anticyclonic vorticity in the upper atmosphere, will also have a profound in- fluence on the total cyclone picture down to the ground. The deepening of the upper wave trough is associated with the deepening of the cyclonic vortex underneath, and usually also entails a southward component added to the normal eastward displacement of the cyclone. In all cases of such deepening the initial upper dis- turbance must start through the build-up of excessive anticyclonic curvature on the wave crest to the west of the cyclone. Above the level of nondivergence, the divergence term and the meridional advection term in (6) are of the same phase, so that the additional terms in dvjdy and dv^/dx are not likely to affect the general pattern of d^/dt very much. The two levels where their influence may be expected to count are (1) close to the level of nondivergence, and (2) at the localities where. Fig. 5.—Successive (1—> 3) degeneration of sinusoidal wave- pattern caused
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