Nighttime free convection characteristics within a plant canopy

Jacobs, A.F.G.; Boxel, J.H. van; El-Kilani, R.M.M.

doi:10.1007/bf00712176

Cited by 49 publications

(27 citation statements)

References 12 publications

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“…In contrast, at nighttime these conditions were reversed. Similar diurnal cycles of stratifications are observed for other canopies (see Jacobs et al, 1994;Kruijt et al, 2000;Nemitz et al, 2000), and are known to decouple the lower canopy from the air layers above (see Fig. 4d).…”

Section: Thermal Stratificationsupporting

confidence: 78%

“…3h). The soil released stored heat as thermal plumes during nighttime that drove an in-canopy nighttime convection, which reached up to the height of the temperature inversion as found by Dupont and Patton (2012) or Jacobs et al (1994). The τ tr (L 1 ) maximum of 200 s from 08:00 to 13:00 CET could accordingly be attributed to positive T (L 1 ) values at that time indicating a stable stratification.…”

Section: Thermal Stratificationmentioning

confidence: 92%

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Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO2–O3 triad above and within a natural grassland canopy

et al. 2015

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Abstract. The detailed understanding of surface-atmosphere exchange fluxes of reactive trace gases is a crucial precondition for reliable modelling of processes in atmospheric chemistry. Plant canopies significantly impact the atmospheric budget of trace gases. In the past, many studies focused on taller forest canopies or crops, where the bulk plant material is concentrated in the uppermost canopy layer. However, within grasslands, a land-cover class that globally covers vast terrestrial areas, the canopy structure is fundamentally different, as the main biomass is concentrated in the lowest part of the canopy. This has obvious implications for aerodynamic in-canopy transport, and consequently also impacts on global budgets of key species in atmospheric chemistry such as nitric oxide (NO), nitrogen dioxide (NO 2 ) and ozone (O 3 ).This study presents for the first time a comprehensive data set of directly measured in-canopy transport times and aerodynamic resistances, chemical timescales, Damköhler numbers, trace gas and micrometeorological measurements for a natural grassland canopy (canopy height = 0.6 m). Special attention is paid to the impact of contrasting meteorological and air chemical conditions on in-canopy transport and chemical flux divergence. Our results show that the grassland canopy is decoupled throughout the day. In the lowermost canopy layer, the measured transport times are fastest during nighttime, which is due to convection during nighttime and a stable stratification during daytime in this layer. The inverse was found in the layers above. During periods of low wind speed and high NO x (NO+NO 2 ) levels, the effect of canopy decoupling on trace gas transport was found to be especially distinct. The aerodynamic resistance in the lowermost canopy layer (0.04-0.2 m) was around 1000 s m −1 , which is as high as values determined previously for the lowest metre of an Amazonian rain forest canopy. The aerodynamic resistance representing the bulk canopy was found to be more than 3-4 times higher than in forests. Calculated Damköhler numbers (ratio of transport and chemical timescales) suggest a strong flux divergence for the NO-NO 2 -O 3 triad within the canopy during daytime. During that time, the timescale of NO 2 uptake by plants ranged from 90 to 160 s and was the fastest relevant timescale, i.e. faster than the reaction of NO and O 3 . Thus, our results reveal that grassland canopies of similar structure exhibit a strong potential to retain soilemitted NO due to oxidation and subsequent uptake of NO 2 by plants. Furthermore, photo-chemical O 3 production was observed above the canopy, which was attributed to a deviation from the NO-NO 2 -O 3 photostationary state by a surplus of NO 2 due to oxidation of NO, by e.g. peroxy radicals. The O 3 production was one order of magnitude higher during high NO x than during low NO x periods and resulted in an underestimation of the O 3 deposition flux measured with the EC method.Published by Copernicus Publications on behalf of the European Geosciences ...

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Section: Thermal Stratificationsupporting

confidence: 78%

Section: Thermal Stratificationmentioning

confidence: 92%

Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO2–O3 triad above and within a natural grassland canopy

et al. 2015

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“…To do this, we introduce an index similar to the free convective velocity scale as introduced by Tennekes and Lumley (1972) and Jacobs et al (1994). We call this the 'advection velocity' (u adv ), defined as:…”

Section: Storagementioning

confidence: 99%

Comparing CO2 Storage and Advection Conditions at Night at Different Carboeuroflux Sites

Aubinet

Berbigier

Bernhofer

et al. 2005

Boundary-Layer Meteorol

179

147

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Abstract. Anemometer and CO 2 concentration data from temporary campaigns performed at six CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulent fluxes in nighttime conditions. While storage was observed to be significant only during periods of both low turbulence and low advection, the advective fluxes strongly influence the nocturnal CO 2 balance, with the exception of almost flat and highly homogeneous sites. On the basis of the main factors determining the onset of advective fluxes, the 'advection velocity', which takes net radiation and local topography into account, was introduced as a criterion to characterise the conditions of storage enrichment/depletion. Comparative analyses of the six sites showed several common features of the advective fluxes but also some substantial differences. In particular, all sites where advection occurs show the onset of a boundary layer characterised by a downslope flow, negative vertical velocities and negative vertical CO 2 concentration gradients during nighttime. As a consequence, vertical advection was observed to be positive at all sites, which corresponds to a removal of CO 2 from the ecosystem. The main differences between sites are the distance from the ridge, which influences the boundarylayer depth, and the sign of the mean horizontal CO 2 concentration gradients, which is probably determined by the source/sink distribution. As a consequence, both positive and negative horizontal advective fluxes (corresponding respectively to CO 2 removal from the ecosystem and to CO 2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal CO 2 exchange rates.

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“…However, the calculated thermal stratification of the canopy may serve as a good indicator of model consistency. In the real world, the lower part of dense canopies often shows a typical diel pattern, which is the reverse compared to the atmospheric boundary-layer above (Jacobs et al, 1994;Bosveld et al, 1999, specifically for Amazon rain forest see Kruijt et al, 2000;Simon et al, 2005b). For further validation, direct eddy covariance fluxes of sensible heat, latent heat, CO 2 and O 3 measured above the canopy are used.…”

Section: Diagnostic Model Applicationmentioning

confidence: 99%

“…However, within dense canopies, the stratification is reversed, because the maximum cooling effect occurs in the upper canopy where biomass is most dense. In combination with soil heat storage, a weak but efficient convective energy flux is generated in the lower canopy (see Jacobs et al, 1994;Kruijt et al, 2000;Simon et al, 2005b). E. Simon et al: Modeling coupled carbon-water exchange of the Amazon rain forest For both seasonal periods, 50-80% of the available energy at the canopy surfaces is converted into latent heat (LE), especially later during the day.…”

Section: Canopy Thermal Stratificationmentioning

confidence: 99%

Coupled carbon-water exchange of the Amazon rain forest, II. Comparison of predicted and observed seasonal exchange of energy, CO2, isoprene and ozone at a remote site in Rondônia

et al. 2005

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Abstract.A one-dimensional multi-layer scheme describing the coupled exchange of energy and CO 2 , the emission of isoprene and the dry deposition of ozone is applied to a rain forest canopy in southwest Amazonia. The model was constrained using mean diel cycles of micrometeorological quantities observed during two periods in the wet and dry season 1999. Calculated net fluxes and concentration profiles for both seasonal periods are compared to observations made at two nearby towers.The modeled day-and nighttime thermal stratification of the canopy layer is consistent with observations in dense canopies. The observed and modeled net fluxes above and H 2 O and CO 2 concentration profiles within the canopy show a good agreement. The predicted net carbon sink decreases from 2.5 t C ha −1 yr −1 for wet season conditions to 1 t C ha −1 yr −1 for dry season conditions, whereas observed and modeled midday Bowen ratio increases from 0.5 to 0.8. The evaluation results confirmed a seasonal variability of leaf physiological parameters, as already suggested in a companion study. The calculated midday canopy net flux of isoprene increased from 7.1 mg C m −2 h −1 during the wet season to 11.4 mg C m −2 h −1 during the late dry season. Applying a constant emission capacity in all canopy layers, resulted in a disagreement between observed and simulated profiles of isoprene concentrations, suggesting a smaller emission capacity of shade adapted leaves and deposition to the soil or leaf surfaces. Assuming a strong light acclimation of emission capacity, equivalent to a 66% reduction of the standard emission factor for leaves in the lower canopy, resulted in a better agreement of observed and modeled concentration profiles and a 30% reduction of the canopy net flux compared to model calculations with a constant emisCorrespondence to: E. Simon (simon@mpch-mainz.mpg.de) sion factor. The mean calculated ozone flux for dry season conditions at noontime was ≈12 nmol m −2 s −1 , agreeing well with observed values. The corresponding deposition velocity increased from 0.8 cm s −1 to >1.6 cm s −1 in the wet season, which can not be explained by increased stomatal uptake. Considering reasonable physiological changes in stomatal regulation, the modeled value was not larger than 1.05 cm s −1 . Instead, the observed fluxes could be explained with the model by decreasing the cuticular resistance to ozone deposition from 5000 to 1000 s m −1 .

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Nighttime free convection characteristics within a plant canopy

Cited by 49 publications

References 12 publications

Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO<sub>2</sub>–O<sub>3</sub> triad above and within a natural grassland canopy

Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO<sub>2</sub>–O<sub>3</sub> triad above and within a natural grassland canopy

Comparing CO2 Storage and Advection Conditions at Night at Different Carboeuroflux Sites

Coupled carbon-water exchange of the Amazon rain forest, II. Comparison of predicted and observed seasonal exchange of energy, CO<sub>2</sub>, isoprene and ozone at a remote site in Rondônia

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Nighttime free convection characteristics within a plant canopy

Cited by 49 publications

References 12 publications

Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO&lt;sub&gt;2&lt;/sub&gt;–O&lt;sub&gt;3&lt;/sub&gt; triad above and within a natural grassland canopy

Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO&lt;sub&gt;2&lt;/sub&gt;–O&lt;sub&gt;3&lt;/sub&gt; triad above and within a natural grassland canopy

Comparing CO2 Storage and Advection Conditions at Night at Different Carboeuroflux Sites

Coupled carbon-water exchange of the Amazon rain forest, II. Comparison of predicted and observed seasonal exchange of energy, CO&lt;sub&gt;2&lt;/sub&gt;, isoprene and ozone at a remote site in Rondônia

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Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO<sub>2</sub>–O<sub>3</sub> triad above and within a natural grassland canopy

Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO<sub>2</sub>–O<sub>3</sub> triad above and within a natural grassland canopy

Coupled carbon-water exchange of the Amazon rain forest, II. Comparison of predicted and observed seasonal exchange of energy, CO<sub>2</sub>, isoprene and ozone at a remote site in Rondônia