Coupling between radiative flux divergence and turbulence near the surface

Near-surface turbulent flux and radiation divergence field observations are analysed over a grass-covered surface located at the Wageningen observatory, the Netherlands. Net radiative flux divergence appears to be a large component of the energy budget near the surface, accounting for a cooling rate of several tens of degrees per day. Long-wave radiation divergence dominates this radiation divergence flux. We show here that long-wave flux divergence near the surface is strongly coupled to sensible heat flux, except in high relative humidity (>90%) and foggy conditions. The net long-wave radiative flux divergence exhibits a sharp gradient within the first 20 m above the surface. This flux divergence is itself strongly coupled to sensible heat flux through adjustments in surface-layer profiles. Nonetheless, no systematic effect of radiation is witnessed on the turbulent temperature spectrum so that the main effect of near-surface radiation is on the mean heat budget. As typical flux-gradient relationships are derived based on observations taken in the near-surface boundary-layer region where sharp long-wave divergence is present, we suspect that those relationships implicitly represent some of the long-wave divergence term. A modification of Monin–Obukhov Similarity Theory to include the effect of radiative divergence is proposed and discussed. This calls for independent measurements of turbulent fluxes and radiative flux divergence near the surface to re-derive turbulent flux-gradient relationships independently of radiative effects.

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