As we saw in Section 3.1.2, the thermal structure expected on the basis of radiative forcing alone has a temperature discontinuity at the ground, as illustrated in Fig. 3.4; the radiative equilibrium temperature of the ground is considerably warmer than that of the air above. This profile is unstable and convection will occur. Convective motions will transport heat upward from the surface; when the air parcels mix with the environment (as they will), they increase the environmental temperature until the environment itself approaches a state of neutrality, which, in the moist tropical atmosphere, is one of constant moist potential temperature. The tropical troposphere is indeed observed to be close to neutral for moist convection (cf. Fig. 4.9) where convection reaches up to the tropopause at height zT. The whole tropical troposphere is in a state of radiative-convective equilibrium, with a convectively determined state below the tropopause and a radiative equilibrium state above, as sketched in Fig. 3.4.
How the temperature structure of the tropics is conveyed into middle latitudes is less certain. It seems unlikely that local convection is the primary control of the vertical temperature structure in middle latitudes. Here transport by larger scale systems plays an important role. In Chapter 8 we will discuss the role of mid-latitude weather systems in transporting heat, both vertically and horizontally.
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