Radiative Cooling Effects within and above the Nocturnal Boundary Layer

For representative tropospheric profiles of water vapor, CO2 and temperature we have calculated in situ longwave radiative flux divergence for use in a simplified second-order closure model of nocturnal boundary-layer evolution. The time evolution of “bulk” boundary-layer parameters is little affected by radiational cooling, as seems to be the cast for stress, velocity variance and diffusivity profiles within this layer. In contrast the w′θ′ profile adjusts itself in response to radiative flux divergence which, within the surface layer, depends on surface emissivity, surface temperature and boundary-layer humidity. Under conditions of strong radiative cooling near the surface an elevated minimum in w′θ′ (at heights up to 0.1 h, h being boundary-layer height) exists, and the cooling produces significant effects on the nondimensional, surface-layer gradients ΦH and ΦM, particularly ΦM. Thermodynamically the boundary layer develops a three-layer structure—in the lowest (0.1 h thick) and uppermost (0.2 h thick) layers radiative cooling dominates the total cooling, while in the central layer occupying most of the boundary layer (0.7 h thick) turbulent cooling dominates. At h itself radiative cooling is a significant fraction of the surface cooling rate. Radiative effects are greatest above the boundary layer where large gradient Richardson numbers am generated. Consequently, turbulence in this region decays rapidly after transition, while in the absence of such effects a much slower decay occurs.