Flow divergence and density flows above and below a deciduous forest

Froelich, Norma; Schmid, Hans‐Peter; Grimmond, Sue; Su, Hong-Bing; Oliphant, A. J.

doi:10.1016/j.agrformet.2005.09.005

Cited by 31 publications

(37 citation statements)

References 26 publications

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“…These situations are quite different with those of open canopy forest or bare ground. Beneath such dense and high forest canopies, persistent subcanopy nighttime upward and daytime downward flow patterns are observed [10,30], which contradicts the typical thermally driven winds [15]. Another distinct feature of dense canopy wind is the core of drainage flow may be beyond the canopy height [29,31].…”

Section: Introductionmentioning

confidence: 99%

“…However, the accuracy of EC measurements of NEE has been questioned particularly over tall vegetation in complex terrains [2][3][4][5][6][7] and even over various forests with relative flat terrains [7,8]. One of the key factors that results in uncertainties in measuring scalar exchanges between the atmosphere and forests is the complexity of airflow above and within the forest canopy [4,[9][10][11]. Understanding mechanisms of airflow above and within a forest canopy provides a basis for interpreting nighttime CO2 fluxes above the forest canopy [10,[12][13][14].…”

Section: Introductionmentioning

confidence: 99%

“…One of the key factors that results in uncertainties in measuring scalar exchanges between the atmosphere and forests is the complexity of airflow above and within the forest canopy [4,[9][10][11]. Understanding mechanisms of airflow above and within a forest canopy provides a basis for interpreting nighttime CO2 fluxes above the forest canopy [10,[12][13][14].…”

Section: Introductionmentioning

confidence: 99%

“…Daytime coupling/decoupling of airflows above and below forest canopy was investigated for a few studies [10,13]. However, because of the site-specific wind regime, it needs more researches to gain a general perspective on canopy winds in a complex topography.…”

Section: Introductionmentioning

confidence: 99%

“…Most studies on wind regimes focus on nocturnal flows particularly in the leaf-on season, and few account for seasonal variations in thermally driven flows at small scales [10,31,32]. Daytime coupling/decoupling of airflows above and below forest canopy was investigated for a few studies [10,13].…”

Section: Introductionmentioning

confidence: 99%

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

Wang

2014

Atmosphere

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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.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Wang

2014

Atmosphere

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Momentum balance of katabatic flow on steep slopes covered with short vegetation

Oldroyd

Katul

Pardyjak

et al. 2014

Geophysical Research Letters

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Katabatic flows over alpine mountainous terrain differ from their forested or bare slope counterparts due to the presence of well‐ventilated, short vegetation. The impact of a grass canopy and larger‐scale pressure perturbations on the one‐dimensional mean momentum balance is explored via theory and field measurements. The model presented here reproduces the measured velocity jet shape and turbulent flux gradients. These two features imply that even when Monin‐Obuhkov similarity theory breaks down, its use for a stability adjusted mixing length remains effective to first order. Results reveal that outer layer pressure effects can be significant under low‐speed wind conditions at the top of the thin katabatic layer when larger variations in the wind direction are observed. An analytical expression to estimate the jet height, which can be utilized in large‐scale weather prediction models, shows the importance of including canopy effects for the thin katabatic flow region above the vegetation.

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The role of forest cover and valley geometry in cold‐air pool evolution

Kiefer

Zhong

2015

JGR Atmospheres

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This study utilizes a version of the Advanced Regional Prediction System (ARPS) with a canopy submodel (ARPS‐CANOPY) to evaluate the sensitivity of cold‐air pool evolution to forest canopy density and valley geometry and elucidate the underlying processes. Numerical experiments are conducted with forest canopy structures spanning from bare ground to dense canopies and terrain configurations ranging from small, shallow valleys to broad, deep valleys. In a set of experiments in which forest canopy density is varied, the minimum potential temperature in the cold‐air pool is found to be as much as 15 K warmer with a dense canopy than with no canopy. Analysis of the thermodynamic budget reveals weaker cooling rates in the dense canopy case than the sparse canopy case, with differences in cooling rates persisting through the duration of the simulation. An additional experiment in which cooling of canopy elements and the influence of the canopy on the ground radiation budget are neglected highlights the important role the canopy plays in limiting radiative loss from the ground surface along the sidewall and valley floor. In experiments in which valley geometry is varied, the cold‐air pool is found to be strongest in a medium valley (10 km wide, 187.5 m deep) and weakest in a small valley (2 km wide, 37.5 m deep). Analysis of along‐slope buoyancy suggests that downslope flow‐driven cooling in a medium valley is more efficient than in a large valley (30 km wide, 500 m deep).

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Flow divergence and density flows above and below a deciduous forest

Cited by 31 publications

References 26 publications

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

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

Momentum balance of katabatic flow on steep slopes covered with short vegetation

The role of forest cover and valley geometry in cold‐air pool evolution

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