Wind Regimes above and below a Temperate Deciduous Forest Canopy in Complex Terrain: Interactions between Slope and Valley Winds

Abstract

Abstract:The thermally driven wind over mountainous terrains challenges the estimation of CO2 exchange between forests and the atmosphere when using the eddy covariance technique. In this study, the wind regimes were investigated in a temperate deciduous forested valley at the Maoershan site, Northeast China. The wind direction above the canopy was preferentially up-valley in the daytime and down-valley in the nighttime, corresponding to the diurnal patterns of above-canopy temperature gradient and stability parameter. In both leaf-on and -off nighttime, a down-valley flow with a maximum velocity of 1~3 m•s −1 was often developed at 42 m above the ground (2.3-fold of the canopy height). However, the below-canopy prevailing wind was down-slope in the night, contrast to the below-canopy temperature lapse and unstable conditions. This substantial directional shear illustrated shallow slope winds were superimposed on larger-scale valley winds. As a consequence, the valley-wind component becomes stronger with increasing height, indicating a clear confluence of drainage flow to the valley center. In the daytime, the below-canopy wind was predominated down-slope due to the temperature inversion and stable conditions in the leaf-on season, and was mainly up-valley or down-slope in the leaf-off season. The isolation of momentum flux and OPEN ACCESSAtmosphere 2015, 6 61 radiation by the dense canopy played a key role in the formation of the below-canopy unaligned wind and inverse stability. Significant lateral kinematic momentum fluxes were detected due to the directional shear. These findings suggested a significant interaction between slope and valley winds at this site. The frequent vertical convergence / divergence above the canopy and horizontal divergence/convergence below the canopy in the nighttime / daytime is likely to induce significant advections of trace gases and energy flux.

Improving the CO2 storage measurements with a single profile system in a tall-dense-canopy temperate forest

“…The decoupling suppressed the vertical mixing and exchange of mass and energy (Burns et al, 2011;van Gorsel et al, 2011), thus might changed the gradient and time scale of scalar concentration measurements for the F s calculation. However, the occurrence and depth of decoupling sublayers are highly dependent on the canopy structure and atmospheric stability (Burns et al, 2011;Wang et al, 2015). For forests with sparse understory and clear trunk space, the decoupling sublayer was often constrained within the subcanopy (Heinesch et al, 2007;van Gorsel et al, 2011) and might increase its depth up to the whole canopy at very stable conditions (Alekseychik et al, 2013).…”

“…For forests with sparse understory and clear trunk space, the decoupling sublayer was often constrained within the subcanopy (Heinesch et al, 2007;van Gorsel et al, 2011) and might increase its depth up to the whole canopy at very stable conditions (Alekseychik et al, 2013). For forests with dense canopy, the decoupling regime might be more complex and even extend to the daytime due to the radiative heating/cooling and thermal isolation of the canopy (Froelich and Schmid, 2006;Pypker et al, 2007;Wang et al, 2015). At our site, the above canopy decoupling (wind direction shear between height levels beyond a threshold of 20 • ) had a probability of 22% in the daytime and 39% in the nighttime, and the below-canopy misalignment occurred with a frequency of more than 70% in both daytime and nocturnal occasions.…”

“…So it is difficult to design a sampling system according to the variations in depth and timing of decoupling. However, the topography and canopy structure might have general implications for the regime of decoupling (van Gorsel et al, 2011;Wang et al, 2015), which might be helpful for design profile systems for F s estimates.…”

Measuring Vegetation Phenology with Near-Surface Remote Sensing in a Temperate Deciduous Forest: Effects of Sensor Type and Deployment

“…The eddy flux tower was established on the northwest-facing (~296 • ) slope in a temperate broadleaved deciduous forest, with an average slope of~9 • . The wind direction above the canopy is preferentially up-valley (northeast) in the daytime and down-valley (southwest) in the nighttime [40]. The tower is 48 m high, and the dominant canopy is~20 m high.…”

“…mandshurica [41]. The tree biomass was 154 Mg ha −1 ; and the maximum leaf area index estimated with the litterfall collection method was 6.5 m 2 m −2 for the period of 2008, of which the understory contributed to 27% [40].…”