Land-Atmosphere Interaction and Radiative-Convective Equilibrium

Cronin, T.
PhD Thesis, Earth, Atmospheric and Planetary Sciences, MIT

I present work on several topics related to land-atmosphere interaction and radiative- convective equilibrium: the first two research chapters invoke ideas related to land- atmosphere interaction to better understand radiative-convective equilibrium; the last two research chapters use the framework of radiative-convective equilibrium to better understand land-atmosphere interaction.

First, I calculate how averaging the incident solar radiation can lead to biases in idealized climate models. I derive an expression for the absorption-weighted solar zenith angle, which minimizes the bias in mean absorbed shortwave radiation, and I find that it is closely matched by the insolation-weighted zenith angle. Common use of daytime-weighted zenith angle likely leads to high biases in albedo by ∼3%.

Second, I explore the time scales of approach to radiative-convective equilibrium with both a simple linearized two-variable model, and a single-column model with full physics. I show that there is a long time scale of approach to radiative-convective equilibrium that is order hundreds of days even when the surface heat capacity van- ishes. The impact of water vapor on the effective atmospheric heat capacity can more than double this time scale for warm temperatures and low surface heat capacities.

Third, I develop an analytic theory for the sensitivity of near-surface temperature to properties of the land surface. I show that the theory compares well against a simple numerical model of the coupled boundary layer-surface system, as well as a more complex two-column model, and discuss application of the theory to questions of how changes in land use or ecosystem function may affect climate change.

Finally, I find that the diurnal cycle of convection is important for the spatial dis- tribution of rainfall in idealized simulations of radiative-convective equilibrium with a cloud-resolving model. In a region that is partly an island and mostly ocean, pre- cipitation over the island falls primarily in a regular, strong, afternoon thunderstorm, with a time-mean rainfall rate more than double the domain average. I explore mech- anisms for this island rainfall enhancement, investigate the importance of island size for my results, and find that the upper troposphere warms with the inclusion of an island, which may have implications for the large-scale tropical circulation.

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