Ian MacPherson

Transport of carbon dioxide, water vapor, and ozone by turbulence and local circulations

Nocturnal land breezes and daytime lake breezes are studied using data collected by the Canadian Twin Otter aircraft and a deck boat which traversed Candle Lake during the Boreal Ecosystem-Atmosphere Study (BOREAS). The nocturnal vertical transport of CO2, water vapor, and ozone over the lake consists of two parts: (1) mesoscale rising motion associated with land breeze convergence and (2) significant turbulence and vertical mixing driven by buoyancy in the lower part of the internal boundary layer and shear generation in the top part of the internal boundary layer. For comparison, the role of the lake in the daytime is examined in terms of formation of a stable internal boundary layer due to advection of warm air from land with small CO2 concentration over the cooler lake surface. Analysis of the aircraft and boat data indicates that the nocturnal land breeze plays an important role in the regional CO2 budget in the lake region. In the present study, CO2 is advected horizontally by a nocturnal land breeze circulation and vented vertically over Candle Lake (“chimney effect”). Such near-surface horizontal transport implies that part of the respirated CO2 never reaches the tower observational level, particularly under light wind conditions. This study speculates that preferred locations of vertical venting of CO2 may also occur due to convergence of nocturnal drainage circulations or flow meandering, although probably weaker than that associated with the land breeze. These circulations partly explain recent findings that tower-measured nocturnal turbulent fluxes of CO2 above the canopy and the subcanopy storage of CO2 frequently sum to less than the total respiration of CO2, leading to “missing CO2.” Unfortunately, the present study does not allow evaluation of all of the terms in the carbon dioxide budget.

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.

Correcting airborne flux measurements for aircraft speed variation

Airplane aerodynamic characteristics correlate aircraft speed with vertical wind velocity, making the time average inappropriate for estimating the ensemble average in airborne eddy-correlation flux computations. The space average, the proper form, is implemented as a time integral by a transformation of variables, which can be interpreted as a ground-speed correction to the time average. The mathematical forms are presented, and the importance of the speed correction is illustrated with airborne data. The computed correction is found to be highly variable, depending on both the turbulent flow encountered and the aircraft used. In general, the speed connection becomes more important as airplane size is reduced. For a small, single-engine Long-EZ airplane, used as an example, the straight time average erred, half the time, by 12%, 10%, 20%, and 15%, respectively, for computed fluxes of momentum, heat, moisture, and CO2. For a much heavier Twin Otter airplane, also used as an example, the straight time average erred, half the time by only 1%. These errors increased with decreasing altitude for the Long-EZ and with increasing altitude for the Twin Otter.

Structural Analysis of Airborne Flux Estimates over a Region

Abstract Aircraft-based observations of turbulence fields of velocity, moisture, and temperature are used to study coherent turbulent structures that dominate turbulent transfer of moisture and heat above three different eco-systems. Flux traces are defragmented, to reconstruct the presumed full size (along the sampled transect) of these structures, and flux traces are simplified by elimination of those that contribute negligibly to the flux estimate. Structures are analyzed in terms of size, spatial distribution, and contribution to the flux, in the four “quadrant” modes of eddy-covariance transfer (excess up/down and deficit up/down). The effect of nonlinear detrending of moisture and temperature data on this “structural analysis,” over surfaces with heterogeneous surface wetness, is also examined. Results over grassland, wetland, and moist and dry agricultural land, show that nonlinear detrending may provide a more physically realistic description of structures. Significant differences are observed bet...

Spatial distribution and co-occurrence of surface-atmosphere energy and gas exchange processes over the code grid site

Grid-type flight patterns at an altitude of 30 m were executed in the summer of 1991 by the Canadian Twin Otter flux research aircraft over a 15 km x 16.1 km agricultural area, as part of the California ozone deposition experiment (CODE). This permitted the mapping, by eddy covariance techniques, of surface-atmosphere exchange for sensible heat, moisture, CO 2 and ozone, in the form of GIS-interpolated flux maps and in the discrete form of those coherent structures of the turbulent transfer process that dominate these exchange processes. The magnitude of surface-related mesoscale contributions to the flux was also quantified. Flux observations were compared against radiometrically observed surface temperatures and vegetation indices (NDVI and VI), observed from aircraft and satellite, and surface characteristics obtained from ground surveys. Flux maps showed the expected close correspondence between greenness, evapotranspiration (ET) and CO 2 exchange, but a weaker correspondence between these and ozone flux maps than would be expected if ozone uptake could consistently be scaled to stomatal conductance for moisture or CO 2 . Examination of the spatial coincidence between transporting structures for the various scalars (heat, moisture, CO 2 and ozone), through the Jaccard coefficient of co-location, J, showed a lower overall value (0.45<J<0.55) for coincidence in transfer between ozone and moisture than between CO 2 and moisture (0.6<J<0.7), and analysis of coincidence for the various crop types within the grid permits quantitative assessment of the potential error made when scaling ozone uptake to stomatal conductance. In general, the findings suggest the existence of two sinks for ozone only one of which is tied to stomatal conductance, and indicates a possible relative advantage for estimates of ozone uptake based on CO 2 rather than moisture exchange.

Atmospheric Response to a Partial Solar Eclipse over a Cotton Field in Central California

Abstract The partial solar eclipse on 11 July 1991 in central California, with 58.3% maximum coverage, provided an exceptional opportunity to study the temporal response of processes in the atmospheric boundary layer to an abrupt change in solar radiation. Almost laboratory-like conditions were met over a cotton field, since no clouds disturbed the course of the eclipse. Tower-based and complementing aircraft-based systems monitored the micrometeorological conditions over the site. Temperature profile measurements indicated neutral stratification during the maximum eclipse in contrast to the unstable conditions before and after the eclipse. Accordingly, the sensible heat exchange completely stopped, as a wavelet analysis of the tower measurements and airborne eddy-covariance measurements showed. Turbulent fluxes of water vapor, carbon dioxide, and ozone were reduced by approximately ⅔ at the peak of the eclipse. Wavelet analysis further indicated that the same eddies contributed to the turbulent transport...

Comparison of the spatial and temporal distribution of fluxes of sensible heat, latent heat and CO2from grid flights in BOREAS 1994 and 1996

Analysis of airborne eddy correlation flux measurements of heat (H), moisture (LE) and CO2 (C) over two 16 km × 16 km heterogeneous grid sites in BOREAS 1994 (IFC-2) and 1996 are compared in order to examine persistence and variability in the distributions of surface characteristics and fluxes between the two years. The data used were obtained in grid patterns flown at 30 m above ground level, under generally clear sky and thermally unstable conditions. Maps of fluxes and surface characteristics were constructed by block averaging over 2 km windows along the flight lines, analyzed for similarities, and used to quantify spatial variability of the fluxes. Sensitivity analysis suggested minor effects of boundary layer variability and window size on the main features of the source/sink distributions. Incident radiation was more highly correlated with grid-averaged values of C than with H and LE. The dominant role of surface inhomogeneity, as opposed to local variations in solar energy input, on spatial variation of flux distributions was confirmed, and mesoscale motion was found negligible, probably because of the small sizes of homogeneous subareas with sufficient surface contrast to induce thermally generated motion. CO2 flux and greenness index were highly correlated, but correlation was site- and time-specific. The previously observed low correlation between sensible heat flux and surface minus air temperature difference (Ts-Ta), primarily over old black spruce, was confirmed. The high Bowen ratio over the forest contributed to the growth and development of the observed deep boundary layers over the sites, but no clear correlation emerged between boundary layer depth and observed near-surface fluxes.

Flux association in coherent structures transporting CO2, H2O, heat and ozone over the code grid site

Abstract Aircraft-based eddy correlation flux data obtained at a height of 30 m above irrigated and non-irrigated agricultural land in southern California have been analyzed in terms of the coherent structures that dominate the turbulent exchange of energy and gases during daytime conditions. The analysis focused on transport of sensible heat, moisture, carbon dioxide and ozone in the gradient modes, i.e. excess up or deficit down for heat and moisture, and deficit up or excess down for carbon dioxide and ozone. Results are presented for composition and size of the dominant structures, over water-stressed and non-water-stressed surfaces, and on the relative frequency with which structures carrying only a single scalar, or given combinations of scalars, were encountered along the flight paths. Interpretation of results provides further evidence for the existence of a second (non-physiological) sink for ozone. The relative preponderance of structures that carry moisture, carbon dioxide and ozone simultaneously, particularly in the gradient up mode, reflects the importance of vegetation as co-located source/sink for these scalars. Surface characteristics resulting in thermal buoyancy and water vapour density gradients appears to be responsible for about 85% of gradient up transport. Finally, the detrending procedures described here may help to define more effective separation between local and mesoscale events in biosphere-atmosphere interactions.