Diffluence is essentially the opposite of confluence (a concept you studied in Meteo 101). For example, the figure below shows a confluent and a diffluent pattern of 500-mb height lines. Focus on the diffluent pattern. Note that the spreading of the height lines (from west to east) dictates that there is speed convergence at work. Indeed, air tends to pile-up because wind speeds decrease from west to east (think of speeding cars on an interstate highway funneling traffic onto two, unpaved country roads that branch off to the northeast and southwest - there will be a traffic jam). On the other hand, as the spacing between height lines increases from west to east, directional divergence (the horizontal spreading of a stream of air) also comes into play. Which one governs? It's impossible to say just be looking at the overall pattern. So we turn to diagnostic equations to assess mass convergence and mass divergence (as opposed to prognostic equations that we can integrate with respect to time). The quasi-geostrophic vorticity equation is one of the diagnostic equations meteorologists can use to infer mass divergence, but it is beyond the scope of Meteo 361.
| It is nearly impossible to visually assess whether confluence (left) and diffluence (right) translate to mass convergence or mass divergence. In a diffluent pattern, there is speed convergence and directional divergence. Which one governs? It's impossible to say just by looking at the overall pattern. |
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