Eddy covariance

Improving Parameterization of Scalar Transport through Vegetation in a Coupled Ecosystem-Atmosphere Model

Link P.A., Improving Parameterization of Scalar Transport through Vegetation in a Coupled Ecosystem-Atmosphere Model. PhD Thesis, VU University, Amsterdam, The Netherlands.

Several regional-scale ecosystem models currently parameterize subcanopy scalar transport using a rough-wall boundary eddy diffusivity formulation. This formulation predicts unreasonably high soil evaporation beneath tall, dense forests and low soil evaporation beneath short, sparse grass. This study investigates alternative formulations by reviewing literature on flow and scalar transport in canopies, taking field measurements of subcanopy latent heat flux, and testing alternative model formulations in constrained numerical experiments. A field campaign was conducted in a dense rainforest in Luquillo National Forest, Puerto Rico, to measure wind and fluxes with eddy covariance devices. Wind velocities and fluxes of latent heat, sensible heat, and momentum were found to be much smaller below the canopy than above it. Modeling experiments tested a mixing-layer-based formulation of eddy diffusivity and a soil evaporation cutoff based on vortex penetration depth. The vortex penetration cutoff was found to be the most physically accurate and computationally simple option, and this study recommends that ecosystem and land-surface models adopt this formulation for subcanopy scalar transport.

Characteristics of fog and fogwater fluxes in a Puerto Rican elfin cloud forest

Eugster, Werner ; Burkard, Reto; Holwerda, Friso; Scatena, Frederick N.; Bruijnzeel, L.A.(Sampurno) 2006. Characteristics of fog and fogwater fluxes in a Puerto Rican elfin cloud forest.. Agricultural and Forest Meteorology 139 :288-306.

The Luquillo Mountains of northeastern Puerto Rico harbours important fractions of tropical montane cloud forests. Although it is well known that the frequent occurrence of dense fog is a common climatic characteristic of cloud forests around the world, it is poorly understood how fog processes shape and influence these ecosystems. Our study focuses on the physical characteristics of fog and quantifies the fogwater input to elfin cloud forest using direct eddy covariance net flux measurements during a 43-day period in 2002.We used an ultrasonic anemometer–thermometer in combination with a size-resolving cloud droplet spectrometer capable of providing number counts in 40 droplet size classes at a rate of 12.5 times per second. Fog occurred during 85% of the time, and dense fog with a visibility <200 m persisted during 74% of the period. Fog droplet size depended linearly on liquid water content(r2 ¼ 0:89) with a volume-weighted mean diameter of 13.8 mm. Due to the high frequency of occurrence of fog the total fogwater deposition measured with the eddy covariance method and corrected for condensation and advection effects in the persistent upslope air flow, averaged 4.36 mm day1, rainfall during the same period was 28 mm day1. Thus, our estimates of the contribution of fogwater to the hydrological budget of elfin cloud forests is considerable and higher than in any other location for which comparable data exist but still not a very large component in the hydrological budget. For estimating fogwater fluxes for locations without detailed information about fog droplet distributions we provide simple empirical relationships using visibility data.

Estimating fog deposition at a Puerto Rican elfin cloud forest site: comparison of the water budget and eddy covariance methods

Holwerda, F., R. Burkard, W. Eugster, F. N. Scatena, A. G. C. A. Meesters,
and L. A. Bruijnzeel (2006), Estimating fog deposition at a Puerto
Rican elfin cloud forest site: Comparison of the water budget and eddy
covariance methods, Hydrol. Processes, 20, 2669– 2692.

The deposition of fog to a wind-exposed 3 m tall Puerto Rican cloud forest at 1010 m elevation was studied using the water budget and eddy covariance methods. Fog deposition was calculated from the water budget as throughfall plus stemflow plus interception loss minus rainfall corrected for wind-induced loss and effect of slope. The eddy covariance method was used to calculate the turbulent liquid cloud water flux from instantaneous turbulent deviations of the surface-normal wind component and cloud liquid water content as measured at 4 m above the forest canopy. Fog deposition rates according to the water budget under rain-free conditions (0Ð11 š 0Ð05 mm h1) and rainy conditions (0Ð24 š 0Ð13 mm h1) were about three to six times the eddy-covariance-based estimate (0Ð04 š 0Ð002 mm h1). Under rain-free conditions, water-budget-based fog deposition rates were positively correlated with horizontal fluxes of liquid cloud water (as calculated from wind speed and liquid water content data). Under rainy conditions, the correlation became very poor, presumably because of errors in the corrected rainfall amounts and very high spatial variability in throughfall. It was demonstrated that the turbulent liquid cloud water fluxes as measured at 4 m above the forest could be only ¾40% of the fluxes at the canopy level itself due to condensation of moisture in air moving upslope. Other factors, which may have contributed to the discrepancy in results obtained with the two methods, were related to effects of footprint mismatch and methodological problems with rainfall measurements under the prevailing windy conditions. Best estimates of annual fog deposition amounted to ¾770 mm year1 for the summit cloud forest just below the ridge top (according to the water budget method) and ¾785 mm year1 for the cloud forest on the lower windward slope (using the eddy-covariance-based deposition rate corrected for estimated vertical flux divergence). Copyright  2006 John Wiley & Sons, Ltd.
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