Local Land–Atmosphere Interactions: Exploring the Terrestrial Leg with “Little Omega”

Land-atmosphere coupling involves the interactions between the land-surface and the overlying atmospheric boundary layer (ABL), with effects on the free atmosphere above, and the associated downstream impacts on clouds, convection and precipitation. We focus on the "terrestrial leg" of land-atmosphere coupling, i.e. the near-surface land-atmosphere coupling where changing soil moisture effects surface fluxes. (The "atmospheric leg" involves changes in surface fluxes and the effects on the ABL, and those downstream impacts.) We examine this terrestrial leg with an analytical development, and using fluxnet data sets to characterize these interactions on the local scale. The change in evaporative fraction with changing soil moisture is an indicator of the strength of coupling between the soil/surface and the near-surface atmosphere, that is, for strong (weak) coupling, a given change in soil moisture yields a large (small) change in evaporative fraction. The coupling depends on a number of different conditions and processes, i.e. the nature of the surface-layer turbulence, to what degree the surface is vegetated and by what type, what the soil texture is, and how plant transpiration and soil hydraulic & thermal processes change with changing soil moisture.