Contrasting turbulent transport regimes explain cooling effect in a semi-arid forest compared to surrounding shrubland

Abstract

Efficiency and energetics of the turbulent transport in the canopy sublayer of a semi-arid pine forest and the atmospheric surface layer of a sparse desert-like shrubland are investigated from ultrasonic anemometer measurements during the summer dry season. The results show that an increased sensible heat flux over the forest canopy is generated by more energetic turbulence at small and large scales, but the transport process itself is equally efficient compared to the neighbouring shrubland. In contrast, the turbulent momentum flux over the forest canopy is caused by more efficient and energetic momentum transport at large scales. The more energetic turbulence appears to reduce the aerodynamic resistance to heat transfer of the semi-arid forest, which enables the increased sensible heat flux at a lower surface temperature. The results help explain the observed differences in diurnal variation of statistical moments of turbulent quantities that are caused by the interaction between radiative forcing, the background wind and the different turbulence production regimes of the forest and the shrubland. Lastly, the results also explain the observed cooler nighttime temperatures and quicker formation of a residual layer at the forest site.