Frontal LiftingFrontal lifting is the term applied to the processrepresented on a front when the inclined surfacerepresents the boundary between two air masses ofdifferent densities. In this case, however, the sloperanges from 1/20 to 1/100 or even less. The steeper thefront, the more adverse and intense its effects, otherfactors being equal. These effects were discussed indetail in the AG2 TRAMAN, volume 1.Vertical StretchingSince it is primarily from properties of thehorizontal wind field that vertical stretching isdetectable, it is more properly called convergence. Thisterm will be used hereafter.The examples of convergence and divergence,explained in the foregoing, are definite and clear cut,associated as they are with the centers of closed flowpatterns. Less easily detected types of convergence anddivergence are associated with curved, wave-shaped, orstraight flow patterns, where the air is moving in thesame general direction. Variations in convergence anddivergence are indicated in figures 4-1, 4-2, 4-3, and 4-4by means of the following key:Figure 4-2.-Convergence and divergence in meridional flow.The left side of figure 4-1 illustrates longitudinalconvergence and divergence; the right side illustrateslateral convergence and divergence. Many morecomplicated situations can be analyzed by separationinto these components.It can be shown mathematically and verifiedsynoptically that a fairly deep layer of air moving witha north-south component has associated convergence ordivergence, depending on its path of movement. Infigure 4-2 the arrows indicate paths of meridional flowin the Northern Hemisphere. In general, equatorwardflow is divergent unless turning cyclonically, andpoleward flow is convergent unless turninganticyclonically.The four diagrams of figure 4-3 represent theapproximate distribution of convergence anddivergence in Northern Hemispheric cyclones andanticyclones.For moving centers, the greatestconvergence or divergence occurs on or near the axisalong which the system is moving. The diagrams offigure 4-3 show eastward movement, but they applyregardless of the direction of movement of the center.Figure 4-1.-Longitudinal and lateral convergence anddivergence.Figure 4-3.-Convergence and divergence in lows and highs.4-2
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