J.I. Macpherson

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.

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.

Observations of fluxes over heterogeneous surfaces

This study analyzes data collected from repeated aircraft runs 30 m over alternating regions of irrigated and dry nonirrigated surfaces, each region on the order of 10 km across, during the California Ozone Deposition Experiment (CODE). After studying the scale dependence of the flow, the variables and their fluxes are decomposed into means for sublegs defined in terms of irrigated and nonirrigated regions and deviations from such subleg means. Since the repeated runs were flown over the same track, compositing the eight flight legs for each of the two days allows partial isolation of the influences of surface heterogeneity and transient mesoscale motions.A variance analysis is carried out to quantify the relative importance of surface heterogeneity and transient mesoscale motions on the variability of the turbulence fluxes. The momentum and ozone fluxes are more influenced by transient mesoscale motions while fluxes of heat, moisture and carbon dioxide are more influenced by surface heterogeneity. The momentum field is also influenced by a quasi-stationary mesoscale front and larger scale velocity gradients.For the present case, the mesoscale modulation of the turbulent flux is numerically more important than the direct mesoscale flux. This spatial modulation of the turbulent fluxes leads to extra Reynolds terms which act to reduce the area-averaged turbulent momentum flux and enhance the area-averaged turbulent heat flux.

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.

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.

Flux estimates of latent and sensible heat, carbon dioxide, and ozone using an aircraft-tower combination

Abstract Several recent large-scale experiments in land-surface climatology have used the combination of aircraft- and tower-based flux measurements. Within these experiments, much effort has been made to assess the potential of aircraft to serve as extended observation platforms for the scaling up from local (tower-based) to regional estimates of surface-atmosphere exchange. Data collected as part of the California Ozone Deposition Experiment (CODE), during a one-month period of consistent day-to-day wind and radiation conditions, were particularly conducive to such a study. Aircraft-based flux estimates obtained at 30 and 60 m above two vineyard sites of approximately 9 km2 (site A, 6 km x 1.5 km and site B, 4.5 km x 2 krn) were compared to tower-based measurements made at 9.4 m above a section of site A. This paper shows that over the measuring period the sum of sensible and latent heat flux at 30 m, as measured with the aircraft-based system, was about 11 % less than that measured with a tower-based system at 9.4 m. It also shows that fluxes at 30 m were on the average about 10% larger than those at 60 m. It documents the flux variability of CO2, sensible and latent heat and ozone observed over relatively homogeneous surfaces. Although aircraft-tower comparisons must be interpreted with caution, due to the dissimilar surface areas sampled by the stationary and moving sensors located at different elevations, the consistent relationship obtained between these two sampling systems demonstrates the complementary nature of such measurements. It confirms that very repeatable measurements can be obtained with aircraft-based systems over distances as short as a few kilometers. It also demonstrates that compositing data for several runs and for several days !From aircraft- and tower-based measurements can provide diurnal patterns of fluxes of sensible and latent heat as well as CO2 and O3 on a regional scale.

Relative photosynthetic activity of agricultural lands from airborne carbon dioxide and satellite data.

Abstract The relative net photosynthetic activity of crop, grass and woodland areas was estimated from airborne eddy correlation carbon dioxide flux density measurements for various transects within a 100 km by 300 km agricultural region in southern Manitoba. Landsat Multispectral Scanner (MSS) data were used to estimate the density and vegetation class boundaries associated with these measurements. The airborne carbon dioxide flux measurements were made during good growing conditions, namely with soil moisture reserves (> 50 per cent field capacity), high radiation (950 W/m2), cloudless skies and warm air temperature (> 25°C). The carbon dioxide flux ranged from (a) -20 to -35 kg CO2 ha−1 h−1 for the heavy crop canopies of the cereal and oilseed growing areas located on the clay and clay loam soils, (b) -12 to -25kg CO2 ha−1 h−1 for the mixed crop grassland and dense woodlands on loam to sandy loam soils and (c) -8 to - 15 kg CO2 ha−1 h−1 for the mixed grassland areas on sandy loam to sandy soils. Indice...

Surface conductances for ozone uptake derived from aircraft eddy correlation data

Abstract Plants and soils act as major sinks for tropospheric ozone, especially during daylight hours when plant stomata are thought to provide the dominant pathway for ozone uptake. The present study, as part of the larger California Ozone Deposition Experiment, uses aircraft eddy covariance measurements taken during the summer of 1991 in the San Joaquin Valley of California to estimate the surface conductance for ozone uptake. To explore for possible sources of discrepancies between the aircraft-derived and tower-based surface conductances a comparison is first made between tower-based fluxes and aircraft fluxes at three tower-based sites. On the average the momentum and surface energy fluxes (sensible and latent heat) observed between 30 and 33 m altitude with an aircraft agreed to within ± 10% with simultaneously measured tower-based fluxes (observed between 4 and 10 m at a vineyard, a cotton and a grassland site). However, comparisons of the aircraft- and tower-based ozone fluxes indicate that between about 4 and 33 m there is an average loss of ozone flux with height of about 18%. It is suggested that either (or both) soil NO emissions or entrainment of ozone free air at the top of the mixed layer may be responsible for this relatively larger discrepancy in the ozone fluxes. Nevertheless, in spite of any relatively larger uncertainties associated with the ozone flux, the tower-based and aircraft-based conductances are in good agreement. The aircraft-derived conductances display a similar magnitude and range of variation as the tower-based conductances and the regression coefficient between the two sets of conductances is 0.9 ± 0.08. Therefore, results from this study suggest that the aircraft can be used to estimate surface conductances of ozone deposition; however, these conductances are subject to large uncertainties.

A simple scheme for partitioning aircraft-easured ozone fluxes into surface-uptake and chemical transformation

Abstract The interplay between surface uptake and atmospheric chemistry makes aircraft-measured ozone fluxes near the surface complex to interpret over such surfaces as recently, cultivated bare soil, newly cut hay, cities and near highways in the San Joaquin Valley of California (the California Ozone Deposition Experiment, 1991). In this study, a simple partitioning scheme is proposed, based on the estimation of the ozone surface;-uptake rate by a regressional fit of ozone flux vs latent heat flux and a vegetation index, over well-irrigated vegetative surfaces, by assuming that ozone flux residuals are due to atmospheric chemistry. The contributions to ozone fluxes by atmospheric chemistry over surfaces other than well-irrigated vegetative surfaces can then be estimated from the difference between the estimated ozone surface-uptake rate and the measured ozone flux. The estimates indicate that chemical contributions are more significant than surface-uptake and that the chemical contributions are dominated primarily by ozone destruction over recently cultivated bare soil, newly cut hay, city and near highway. The dominant sink for ozone destruction is analytically shown to be caused by NO concentrations in excess of the photostationary state, which are believed to be linked to the strong NO emissions from the surfaces.

The correspondence of aircraft-measured fluxes of sensible heat, latent heat, CO2 and ozone to the surface characteristics in the San Joaquin Valley of California

Abstract Fluxes of sensible heat, latent heat, CO2 and ozone over a variety of land surfaces, based on 5 km-averaged near-surface airborne data, are examined within the 1991 California Ozone Deposition Experiment (CODE) in the San Joaquin Valley of California. It is found that the fluxes of sensible heat, latent heat and CO2 are well correlated and correspond significantly to the surface greenness index over all selected surface types, which indicates that the three fluxes are primarily driven by the surface biological (physiological) processes. However, a significant correspondence between ozone flux and the surface greenness index is found only over the vegetative surfaces. The nonvegetative surfaces are composed of recently-cultivated bare soil and newly-cut hay, where the rapid chemical reactions of ozone with its precursors originating from soil microbial activities might become an important controlling factor. This suggests that ozone flux may be controlled by a combination of the surface uptake process dominant over the vegetative surfaces and the atmospheric chemical process dominant over the nonvegetative surfaces. The two kinds of processes may mask each others effects, and lead to the poor and confusing correspondence between ozone flux and the surface greenness index.