Nonlocal Instability

So far we have assumed constant values of the environmental lapse rate ELR (i.e. straight lines in Figs 7.3 and 7.5), to be compared with the SALR or DALR, according to whether the air is saturated or not. Such a comparison indicates the local static stability. However, the ELR is rarely constant in practice, so measurements do not yield a straight line but a curving line as in Figure 7.1 and Figure 7.6. The practical consequence of this is that the occurrence of convection does not depend on the local static instability of individual layers of the atmosphere, but on the entire profile over an appreciable depth. This can be shown by considering Figure 7.6, taking various layers in turn (Note 7.F). Such a consideration reveals that a parcel of air at the level of J and K in Figure 7.6 will rise as far as the level of neutral buoyancy (LNB) or 'cloud top', despite some static stability. Static stability of the layer between J K and the LNB would be inferred from the difference between the slopes of the ELR and SALR lines up from J and K, respectively. The reason for the actual

Dalr Salr Elr
Figure 7.5 Temperature profiles of atmospheres with various degrees of moisture and static stability. The three bold lines show the DALR and two SALRs.

Figure 76 The track of an air parcel lifted from the surface, plotted on a skew T—log p diagram. The bold line is a hypothetical sounding (ELR). The dewpoint sounding is not shown, though the dewpoint (Td) at the surface is plotted. SMR is a saturation-mixing-ratio line, CAPE is the 'convective available potential energy', LCL is the lifting condensation level, and LNB is the level of neutral buoyancy.

Figure 76 The track of an air parcel lifted from the surface, plotted on a skew T—log p diagram. The bold line is a hypothetical sounding (ELR). The dewpoint sounding is not shown, though the dewpoint (Td) at the surface is plotted. SMR is a saturation-mixing-ratio line, CAPE is the 'convective available potential energy', LCL is the lifting condensation level, and LNB is the level of neutral buoyancy.

instability is that an air parcel at what is labelled the level of free convection rises spontaneously along the SALR curve all the way to the LNB, because the parcel (at temperatures along the SALR curve) is always warmer than the environment, shown by the ELR curve.

As temperature comparison (rather than relative slope) is really the basis of assessing overall atmospheric instability, we use this in describing the instability of the part that is chiefly the arena of weather, from the ground to about 5 km elevation (Note 7.G).

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