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FIGURE 4.19. A schematic of tropospheric temperature profiles showing the dry adiabat, a typical wet adiabat, and a typical observed profile. Note that the dry adiabatic ascent of a parcel is typically cooler than the surroundings at all levels, whereas the wet adiabat is warmer up to about 10 km. The wet and dry lapse rates are close to one another in the upper troposphere, where the atmosphere is rather dry.

6r s is also known as the pseudo-adiabatic lapse rate.

v p destabilized by the presence of moisture, i.e., a saturated atmosphere is unstable if dT<_ r dz s'

where rs < rd. The resulting instability is known as conditional instability, since it is conditional on the air being saturated. The tropical troposphere is close to neutrality with respect to moist convection, meaning that it has dT/dz — -rs (see below).

Lines that show the decrease in T of a parcel of air which is rising/sinking in the atmosphere under saturated adiabatic conditions are called saturated adiabats. As we now describe, we can define a temperature-like quantity that is conserved in moist processes and plays an analogous role to that of potential temperature in dry convection.

4.5.3. Equivalent potential temperature

Moist thermodynamics is complicated, but it is relatively straightforward to define a potential temperature that is conserved in moist processes. This quantity, known as equivalent potential temperature, 6e, tends to be mixed by moist convection, just as dry potential temperature, 6, is mixed in dry convection.

We begin from the first law, Eq. 4-26. Making use of p = pRT and k = R/Cp, it can be rearranged thus:

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