5.2.1 Adiabatic Temperature Change
Imagine a parcel of air with a mass of 1 kg, which occupies a volume of about 1 m3, has an impermeable surface, and no heat exchange with the surrounding air. Where there is no energy gain or loss to the environment, any change in the volume of that parcel of air will result in a change in the internal energy of the parcel. This type of temperature change is said to be adiabatic.
Why is this process important in meteorology? As a rising air parcel encounters lower pressure, its volume increases and its internal energy decreases because energy is needed to expand against the surrounding air. A reduction in the internal energy per unit volume causes the temperature to fall at what is known as the dry adiabatic lapse rate (0.98°C per 100 m ≈ 10°C per km). If the temperature continues to decrease until the parcel reaches its dewpoint temperature, condensation occurs. Further cooling occurs at the saturated adiabatic lapse rate, which is a slower rate of cooling because of the latent heat release that occurs with condensation.
When an air parcel sinks, it is compressed by the surrounding air. As the volume decreases, the internal energy increases, the temperature increases, and the relative humidity decreases.
Sometimes it is convenient to use the potential temperature, θ instead of the temperature because every parcel of air has a unique value of potential temperature that remains the same while the parcel is rising or sinking dry adiabatically. With a saturated air parcel, we assume that all condensate will eventually fall out. This process is referred to as pseudo-adiabatic (some latent heat is removed with the precipitation) and the equivalent potential temperature, θe, remains the same. In order to predict heavy precipitation from convection, forecasters look at the movement of the air with high values of θe. For example, in Fig. 5.8, heavy rainfall occurred within the axis of maximum θe at 850 hPa.7 In this case, the maximum rainfall and deadly flash floods were produced where the high θe air was lifted along the mountains in eastern Jamaica.