The Role of Radiation in the Surface Flux Budget of the Nocturnal Boundary Layer: A Question of Timescale

The role of longwave (LW) radiation in the surface flux budget of the nocturnal boundary layer (NBL) is not fully understood. As the surface cools, the upward flux from the surface is reduced, but since a significant fraction of the downward LW flux reaching the surface may originate from within the NBL itself, this flux also decreases, so that the reduction in the net LW flux is smaller than that in the upward LW flux. This reduces the effectiveness of LW radiation in arresting the cooling of the surface, a phenomenon sometimes referred to as runaway cooling. On the other hand, direct simulation of Arctic boundary layers shows that in some cases, after an initial transient, LW radiation can quite effectively limit the decrease in surface temperature.

By considering a simple model of the radiative boundary layer, scaling relations can be derived to provide a unified conceptual understanding of the relevant processes. Calculations of radiative fluxes are performed to quantify the effectiveness of longwave radiation in retarding the cooling of the surface in different atmospheres. A natural timescale for radiative effects then emerges. This is smaller in shallow dry boundary layers than in deep moist ones. On this basis, it is suggested that it is not so much that LW radiation is incapable of arresting the cooling of the surface, but rather that in some cases the relevant radiative timescale is large in relation to other timescales of the system and therefore that the effect of LW radiation can be masked.