Coupling between CO2, water vapor, temperature, and radon and their fluxes in an idealized equilibrium boundary layer over land -: art. no. D18103

We propose a new approach for relating concentration measurements in the atmospheric boundary layer to surface fluxes based on a simple equilibrium boundary layer model (an extension of Betts [2000]). This is a major shift from the traditional focus on the growth of the daytime dry boundary layer. We show equilibrium solutions for the diurnally averaged properties of the boundary layer that link the mixed layer equilibrium (on timescales longer than a day) of potential temperature, water vapor, CO2 (and other trace gases such as radon) with an interactive cloud layer and with the surface energy, water, and carbon balance. We examine these processes as a function of a set of external parameters: soil water content, which directly impacts respiration and photosynthesis, and hence transpiration; the surface net shortwave, directly linked to net radiation, which is also coupled to photosynthesis and transpiration; and the radiative cooling of the mixed layer (ML), which in the equilibrium model directly affects the surface sensible heat. We also show solutions where the net shortwave and radiative cooling are coupled to the cloud field. Our other variable model parameters are the properties of air entrained into the CBL, the midtropospheric values of vapor mixing ratio, CO2 and radon, and the lapse rate above cloud base. We considered two idealized ecosystems: forest (based on observations in Wisconsin) and grassland (using parameter estimates from the literature) to show how the vegetation model affects the ML equilibrium. We show how the mass transport out of the subcloud layer and the mass exchange with the free troposphere couples the mixed layer equilibrium of water vapor, CO2, and radon with the corresponding surface fluxes. We suggest that regional ML budgets may give useful constraints on regional carbon budgets, and that the coupling with the cloud field is a fundamental part of the ML equilibrium.