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The troposphere is the region of the atmosphere characterized by low static stability, vigorous diabatic mixing, and widespread condensational heating in clouds. Previous research has argued that in the tropics, the upper bound on tropospheric mixing and clouds is constrained by the rapid decrease with height of the saturation water vapor pressure and hence radiative cooling by water vapor in clear-sky regions. Here the authors contend that the same basic physics play a key role in constraining the vertical structure of tropospheric mixing, tropopause temperature, and cloud-top temperature throughout the globe. It is argued that radiative cooling by water vapor plays an important role in governing the depth and amplitude of large-scale dynamics at extratropical latitudes.climate dynamics | climate change | cloud feedbacks | extratropical dynamics | general circulation T he defining difference between the troposphere (the turning sphere) and the stratosphere (the layered sphere) is the amplitude of mixing within each region. The troposphere is marked by vigorous diabatic motions (i.e., motions that flux heat across isentropic surfaces); the stratosphere is characterized by relatively weak diabatic mixing.The top of the troposphere coincides closely to the level above which clouds rarely penetrate. In the tropics, the upper bound on tropospheric clouds is believed to be strongly constrained by the unique vertical structure of radiative cooling by water vapor. Saturation vapor pressure decreases rapidly with temperature in the tropical upper troposphere in accordance with the ClausiusClapeyron relationship. As the saturation vapor pressure decreases, so do water vapor concentrations, and hence so does the amplitude of the clear-sky radiative cooling (1, 2) and its associated downward mass flux (3). From continuity, the thermodynamic limit on sinking motion in clear-sky regions extends to rising motion in regions of deep convection (3). Because the rising motion in regions of deep convection is limited by saturation vapor pressures (and thus temperatures) in clear-sky regions and the static stabilities in clear-sky and cloudy regions of the tropics are comparable, it follows that cloud-top temperatures in the tropics are strongly constrained by the basic thermodynamics that govern clear-sky water vapor concentrations (3, 4).The physical linkages between clear-sky radiative cooling, the depth of tropospheric mixing, and clouds have been applied specifically to the case of tropical anvil clouds. Here we contend that the same physical linkages play an even broader role in the global atmosphere circulation. It is argued that clear-sky cooling by water vapor constrains the depth of tropospheric mixing, cloud-top temperatures, and the amplitude of large-scale atmospheric dynamics throughout the global atmosphere.A Thermodynamic Constraint on the Depth of the Troposphere At steady state, the thermodynamic energy equation can be expressed in pressure coordinates asWhere V · ∇ h T is the horizontal advection of temperature, ...

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