P. H. Schuepp

Footprint prediction of scalar fluxes from analytical solutions of the diffusion equation

The use of analytical solutions of the diffusion equation for ‘footprint prediction’ is explored. Quantitative information about the ‘footprint’, i.e., the upwind area most likely to affect a downwind flux measurement at a given height z, is essential when flux measurements from different platforms, particularly airborne ones, are compared. Analytical predictions are evaluated against numerical Lagrangian trajectory simulations which are detailed in a companion paper (Leclerc and Thurtell, 1990). For neutral stability, the structurally simple solutions proposed by Gash (1986) are shown to be capable of satisfactory approximation to numerical simulations over a wide range of heights, zero displacements and roughness lengths. Until more sophisticated practical solutions become available, it is suggested that apparent limitations in the validity of some assumptions underlying the Gash solutions for the case of very large surface roughness (forests) and tentative application of the solutions to cases of small thermal instability be dealt with by semi-empirical adjustment of the ratio of horizontal wind to friction velocity. An upper limit of validity of these solutions for z has yet to be established.

Scaling up flux measurements for the boreal forest using aircraft-tower combinations

Fluxes of carbon dioxide, water vapor, sensible heat, and momentum obtained over the boreal forest from the Twin Otter aircraft and six tower-based systems are compared. These measurements were collected as part of the Boreal Ecosystem-Atmosphere Study (BOREAS) during three intensive field campaigns between May 25 and September 17, 1994. The representativeness of the tower-based measurements collected during BOREAS is discussed. Even though the net radiation from aircraft- and tower-based systems agreed well, in general, the aircraft tended to observe larger latent heat and smaller sensible heat fluxes than the towers. The CO2 fluxes from the aircraft were substantially less than from the tower, while the differences were relatively small for the momentum fluxes. The relationships between aircraft and tower-based flux measurements obtained by making repeated runs past various towers are used to scale up tower-based fluxes to a 16×16 km2 area near Prince Albert, Saskatchewan. It is demonstrated that except for a couple of cases primarily due to rapidly changing radiation conditions, this combination of measurements provides regional flux estimates of momentum, CO2, and sensible and latent heat similar to those obtained by flying a grid pattern over the area.

An evaluation of aircraft flux measurements of CO2, water vapor and sensible heat

Ground-based flux measurements of carbon dioxide and water vapor integrate physiological processes taking place on a field scale. Aircraft flux measurements have recently been undertaken to attempt to widen the scope of applicability of such measurements. However, because of the intermittency of turbulent transfer, flux measurements must be averaged over long periods of time or long distances to give reproducible results. This requirement makes it difficult to relate aircraft flux measurements to local surface processes. Flux measurements of CO2, latent and sensible heat obtained from repeated passes in four directions and at three elevations over a homogeneous wheat-growing area are compared with ground-based measurements. Averages based on four runs of 4 km in length gave results consistent with ground-based measurements. The largest percentage differences were in the sensible heat flux. Cospectral analyses showed no significant high frequency losses for the data from flight levels of 25 and 50 m, but an underestimation of approximately 10% resulted at 10 m. Flight direction with respect to wind direction was relatively unimportant at 10 and 25 m but some effects were observed at 50 m. It was also shown that at 25 m, over a relatively smooth and homogeneous surface, the means of either three or four runs 4 km in length were similar to the means of 12–16 km runs. This confirms that at this altitude, most of the flux contribution is contained at wavelengths less than 4 km and that the mean of 3 to 4 passes accounts for most of the intermittency of turbulent transfer.

Spatial and temporal variations of the fluxes of carbon dioxide and sensible and latent heat over the FIFE site

Airborne measurements of flux densities of carbon dioxide CO2, sensible heat, and latent heat (H2O) obtained over the First ISLSCP Field Experiment (FIFE) site during three intensive field campaigns in 1987 and one in 1989 are examined to characterize the spatial and temporal variations of CO2 and energy transfer processes. These data were collected by the National Research Council Twin Otter using low-level flight patterns, all flown at constant pressure altitude during relatively clear days. The spatial variations are larger in 1989 than in 1987 and a higher correlation is observed between the fluxes and the surface features. The temporal patterns are easier to characterize with the relatively homogeneous situation of 1987. Functional relationships obtained between fluxes of CO2 and latent heat, CO2 fluxes and greenness index, latent heat fluxes and greenness index, and between sensible heat fluxes and surface air temperature differences are presented for one day in 1987 and one in 1989 as an example of the kind of information that can be obtained from grid flights at constant pressure altitude. 20 refs.

Footprint considerations in BOREAS

Comparisons of observations of concentration or flux from platforms at various heights, such as tower and aircraft, must take into account differences in the location and extent of upwind surface source or sink areas which affect the individual observations, with their physical and biological characteristics. Such “footprint” estimates are based on solutions of the diffusion/advection equation which have not previously been evaluated over a boreal ecosystem. In order to adjust an analytical footprint model within the surface layer above forest canopies typical for the Boreal Ecosystem-Atmosphere Study (BOREAS) sites, 29 tracer gas release experiments were carried out between August 30 and September 9, 1994, at three tower sites in the northern study area (NSA). Sulphur hexafluoride (SF6) was released from point sources at various upwind distances from the towers under various meteorological, terrain, and release conditions. Wind, temperature, and stability parameters, during each release period, were used as input into calculations of vertical concentration profiles sampled at the towers, based on a three-dimensional diffusion model. Predictions of concentration profiles, or back calculation of source strength from observed profiles, were in good agreement with observed concentrations or actual release rates, respectively. The diffusion model was then used to compute footprint estimates for stable to unstable conditions, for tower and aircraft-based observation platforms. They show spatially constrained footprints in the surface layer, due to effective vertical coupling, so observations from towers and low-flying aircraft must be expected to be very site specific, and scaling up to larger areas will have to be done with careful consideration of surface mosaics. Our study also included footprint estimates made for airborne observations above the surface layer, based on large-eddy simulations over “BOREAS-like” terrain, for boundary layer structures comparable to those observed in BOREAS. They document the progressive decoupling of airborne observations from the surface features at these heights.

Studies of forced-convection heat and mass transfer of fluttering realistic leaf models

The forced-convection mass transfer - and by analogy, heat transfer - of various realistic leaf models at Reynolds numbers 2 x 103<Re<4 x 104 was studied with an electrochemical method. The results are compared with similar measurements on plates and with transfer coefficients calculated from the laminar boundary-layer theory. In this way the validity of the commonly-used analytical expressions which represent the leaf by a rigid plate and neglect the effects of leaf curvature, fluttering, surface roughness and fluid turbulence, can be tested.The measurements show that for fluttering single leaves, the convective mass-transfer coefficients must be expected to be higher by a factor of 1.4 ± 0.1 than the ones calculated for rigid plates of equal size and shape. For a leaf in a crop, the increase might be as high as a factor of 2. The high transfer coefficients measured for elements of cedar foliage are also discussed.

California ozone deposition experiment: Methods, results, and opportunities

Abstract The California Ozone Deposition Experiment (CODE) is a program of observations and modeling to improve estimates of the rate of removal of tropospheric ozone at the earths surface used in grid-based photochemical models of ozone production, transport, and removal. The purpose of CODE is to test, diagnose and improve treatment of dry deposition of ozone and other gaseous species. CODE supports a larger air quality measurement and modeling effort comprised of the San Joaquin Valley Air Quality Study (SJVAQS) and Atmospheric-Utilities Signatures: Predictions and Experiments (AUSPEX) joined as SJVAQS/AUSPEX Regional Model Adaptation Project (SARMAP). However, the CODE data are also applicable to a variety of boundary layer and turbulence problems. This paper describes the field methods and data collected during summer (10 July through 6 August) of 1991 in the San Joaquin Valley (SJV) of California and introduces several related papers. General comparisons and conclusions from all the participants are summarized. The core elements of the CODE field effort consisted of a research aircraft for spatial coverage and three ground sites located in a cotton field, grape vineyard, and very dry (senescent) annual grassland. A major portion of the SJV is represented by these three vegetation types. The eddy covariance method is used to compute the vertical fluxes of ozone, carbon dioxide, water vapor, sensible heat and momentum. For the first half of the study period, flights were made mainly for comparison with tower-based fluxes. Subsequent flights were over other vegetation types and to conduct special studies. In addition to the vertical fluxes, the ground-site data include individual leaf measurements of stomatal conductance, radiative leaf temperature, wetness of surrogate leaves, soil temperature profiles and heat flux, soil composition and water content, mean nitrogen oxide and ozone concentrations, solar and net radiation, photosynthetically active radiation, and vertical profiles of wind, temperature, ozone and water vapor. Aircraft data also include reflected short-wave radiation, surface greenness index and radiative surface temperature. Several factors simplify analyses: a nearly constant synoptic situation, lack of cloud cover, low-level (30 m) flights and land use characterized by extensive homogeneous areas with well defined interfaces. Repeated five-km aircraft runs, necessary for a representative flux calculation, were commonly made over a single crop type. In addition, a partial (60%) solar eclipse on 11 July provides an opportunity to examine the influence of light intensity upon the plant-atmosphere exchange of carbon dioxide and ozone via stomatal activity.

Analysis of flux maps versus surface characteristics from Twin Otter grid flights in BOREAS 1994

In BOREAS 1994 the Canadian Twin Otter research aircraft was flown at a fixed altitude of about 30 m above ground level (agl) over two 16×16 km heterogeneous grid sites, in an attempt to document the spatial distributions of fluxes of sensible heat, latent heat, and CO2 over typical boreal systems and to relate them to spatial distributions of surface characteristics. The acquired data were used to construct maps of surface temperature excess over air temperature (Ts-Ta) and greenness index (GI) as well as flux maps of sensible heat, latent heat, and CO2. Fluxes were estimated by the eddy correlation method and the scalar and vertical wind variables detrended by a method that takes into account the physical nature of transport during convective daytime conditions. The (Ts-Ta) maps showed that surface temperatures were relatively cooler over the forests than over the disturbed, regenerating, and burn areas. However, their juxtaposition with sensible heat flux maps showed highest heat flux predominantly over the forest areas. Close correspondence was observed between maps of CO2 flux and greenness. Quantitative comparison of the flux maps for the three intensive field campaigns (IFCs) showed that the CO2 flux was the most conservative of the three fluxes, while latent heat flux showed the highest variations. Areas with persistent patterns of flux distribution across the IFCs were also identified.

Aircraft measurements of CO2 exchange over various ecosystems

The National Aeronautical Establishments Twin Otter Atmospheric Research Aircraft has been equipped with open-path CO2 analyzers in order to obtain estimates of CO2 exchange over a corn field, a forest and a lake using the eddy correlation technique. On the 18th of August 1980, mean uptakes obtained over corn and forest were 12 and 8 kg CO2 ha-1 hr-1, respectively. On the 28th of August, mean uptakes obtained over corn, forest and the lake were 36, 14, and 1 kg CO2 ha-1 hr-1, respectively. The data are discussed in the light of general conditions on the two days.

Methods of estimating CO2, latent heat and sensible heat fluxes from estimates of land cover fractions in the flux footprint

We present a description of the process of estimating surface fluxes of CO 2 , latent heat and sensible heat from estimates of fractions of satellite-based land cover types in the flux footprint. The study is conducted at two heterogeneous sites in the boreal forest of Central Canada. Using a Twin Otter aircraft, fluxes were measured in a grid pattern during three Intensive Field Campaigns (IFCs) and Landsat thematic mapper data were used for land cover classification. Using a footprint function developed from tracer gas release experiments in the boreal forest, the fractions of cover types within the footprint were determined, and used in a regression analysis against observed fluxes. The results showed that the surface cover types within the flux footprint accounted for about 90% of the variations in the measured airborne fluxes of CO 2 , sensible heat and latent heat, at two different study sites. The attempted validation of the regression models, by comparing flux estimates over regional transects outside the grid area for which the regression model had been developed or over site-specific runs within the grid area against observed fluxes, based on fractional distributions of surface cover types, were encouraging. They indicate the potential for extrapolating models developed for a given location to another location, based simply on the fractions of cover types, at least for similar land cover types.