Downward control principle

This expression describes the so-called downward control principle (Haynes et al., 1991). It shows that in the extratropics (f 7 0), the vertical velocity w (and hence the vertical transport) at a given... 

Downward-control principle An analytical result stating that the steady-state, zonal-mean vertical velocity at a given level is a function of the Eliassen-Palm flux divergence integrated vertically between the level in question and the top of the atmosphere. 

This expression is often referred to as the downward control principle, because it states that the zonal-mean vertical velocity at altitude z depends on the integrated divergence of the V F from the top of the atmosphere to the altitude in question. 

In principle, several channels of information could be transmitted simultaneously with the continuous wave technique. In particular, a downward channel to control the tool modes and an upward channel to bring up the information. 

The direction of the vertical hydraulic gradients across the river bed, which control water exchange (same principle as for aquitards downward gradients bring the river to feed the aquifer and upward gradients do the reverse). 

It is not obvious why (13.1.31) is called an electrocapillary equation. The name is a historic artifact derived from the early application of this equation to the interpretation of measurements of surface tension at mercury-electrolyte interfaces (1-4, 6-8). The earliest measurements of this sort were carried out by Lippmann, who invented a device called a capillary electrometer for the purpose (9). Its principle involves null balance. The downward pressure created by a mercury column is controlled so that the mercury-solution interface, which is confined to a capillary, does not move. In this balanced condition, the upward force exerted by the surface tension exactly equals the downward mechanical force. Because the method relies on null detection, it is capable of great precision. Elaborated approaches are still used. These instruments yield electrocapillary curves, which are simply plots of surface tension versus potential. 

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