Influence of ground surface characteristics on the mean radiant temperature in urban areas

The effect of variations in land cover on mean radiant surface temperature (Tmrt) is explored through a simple scheme developed within the radiation model SOLWEIG. Outgoing longwave radiation is parameterised using surface temperature observations on a grass and an asphalt surface, whereas outgoing shortwave radiation is modelled through variations in albedo for the different surfaces. The influence of surface materials on Tmrt is small compared to the effects of shadowing. Nevertheless, altering ground surface materials could contribute to a reduction on Tmrt to reduce the radiant load during heat-wave episodes in locations where shadowing is not an option. Evaluation of the new scheme suggests that despite its simplicity it can simulate the outgoing fluxes well, especially during sunny conditions. However, it underestimates at night and in shadowed locations. One grass surface used to develop the parameterisation, with very different characteristics compared to an evaluation grass site, caused Tmrt to be underestimated. The implications of using high resolution (e.g. 15 minutes) temporal forcing data under partly cloudy conditions are demonstrated even for fairly proximal sites

Spatial variability of energy fluxes in suburban terrain

Energy fluxes over an area of “homogeneous” suburban residential land-use in Vancouver, B.C., Canada are shown to vary by up to 25–40% within horizontal scales on the order of 102–103 m. Previously, variability of this magnitude has been expected to occur only at larger scales, between land-use zones or as urban-rural differences. In view of these findings, it is recognized that microadvective interaction between surface types at small scales may be important and can affect the energy balance even at larger scales. The present study discusses the small-scale spatial variability of energy fluxes and shows that it varies greatly for each term in the surface energy balance. Net radiation shows a relatively conservative behaviour (via albedo-surface temperature feedback) with little spatial variability. The turbulent fluxes (measured by eddy correlation at 28 m height), on the other hand, show a link between their temporal and spatial variability as the result of a temporally shifting source area which contains varying combinations of surface cover (using the dynamical source area concept of Schmid and Oke, 1990). As a result, part of the measured temporal variation is attributable to spatial differences in surface cover. Anthropogenic heat flux and storage heat flux (both modelled using a high resolution surface data-base) exhibit temporally varying spatial distributions. Their spatial pattern, however, is governed by nested scales of urban morphology (blocks, streets, properties, etc.). These differences in the source of variability between each component flux suggest a difficulty in the interpretation of the energy balance over urban areas, unless each term is spatially-averaged over the principal morphological units occurring in the area

Flow divergence and density flows above and below a deciduous forest

Abstract Ecosystem-atmosphere exchange flux measurements above tall vegetation in hilly terrain are well known to suffer from systematic underestimates of nighttime fluxes of CO 2 and other scalars with significant sources in or below the canopy. This bias is commonly attributed to advection driven by thermotopographic density flows and the resulting horizontal flow divergence. Flux correction methods have been proposed based on this notion. To examine the structure and dynamics of horizontal flow divergence and vertical convergence mean vertical velocities are analyzed. These are derived from sonic anemometers at 1.8 and 1.3 times canopy height for 3 years above a deciduous forest in hilly terrain at the Morgan-Monroe State Forest (Indiana, USA) Fluxnet site. Measured vertical velocities are linked to forcing parameters represented in the equations of motion and heat for sloping terrain. In the leaf-off season, the data suggest that the dynamics and daily patterns of horizontal flow divergence (implied from vertical convergence through continuity) are entirely consistent with the hypothesis that the divergence is driven by thermotopographic density flows. However, in the vegetative season with a full canopy, a more complex picture emerges, suggesting strong dynamic and thermal decoupling of the horizontal divergence below canopy from flow conditions above. Thus we conclude that flux correction methods based on above-canopy conditions alone may significantly misrepresent scalar transport below canopy during the vegetative season and should be avoided.

Uncertainty of annual net ecosystem productivity estimated using eddy covariance flux measurements

Eddy-covariance-based estimates of net ecosystem exchange are subject to various sources of systematic bias and random measurement uncertainty. Here we concentrate on the cumulative effect of random uncertainty on annual estimates of net ecosystem productivity of carbon (NEP). A 8-year data set of eddy covariance measurements over a mixed deciduous forest at the Morgan-Monroe State Forest (MMSF, Indiana, USA) was used, in conjunction with a 6-day period of paired observations with the AmeriFlux portable system, to evaluate two different approaches to estimate measurement system uncertainty, and an analogous method to estimate the uncertainty in a standard parametric model used to fill data gaps in the annual time series. The cumulative annual uncertainty was obtained by Monte Carlo simulation, separately for the observations and the model estimates. Our results indicate that the overall uncertainty of annual NEP is dominated by the contribution of the gap-filling model, even at relatively small gap fractions of 20%. The magnitude of random uncertainty in NEP varied between ±10–12 gC m−2 yr−1 (i.e., 3–4% of annual NEP at MMSF for years 1999–2006). Thus it must be expected that random uncertainty of eddy-covariance-based NEP is small compared to other potential sources of systematic bias, but we note that very little is known about the long-term cumulative covariate effects of systematic bias in the measured flux

The Urban Boundary-layer Field Campaign in Marseille (ubl/clu-escompte): Set-up and First Results

The UBL/CLU (urban boundary layer/couche limite urbaine) observation and modelling campaign is a side-project of the regional photochemistry campaign ESCOMPTE. UBL/CLU focuses on the dynamics and thermodynamics of the urban boundary layer of Marseille, on the Mediterranean coast of France. The objective of UBL/CLU is to document the four-dimensional structure of the urban boundary layer and its relation to the heat and moisture exchanges between the urban canopy and the atmosphere during periods of low wind conditions, from June 4 to July 16, 2001. The project took advantage of the comprehensive observational set-up of the ESCOMPTE campaign over the Berre–Marseille area, especially the ground-based remote sensing, airborne measurements, and the intensive documentation of the regional meteorology. Additional instrumentation was installed as part of UBL/CLU. Analysis objectives focus on (i) validation of several energy balance computational schemes such as LUMPS, TEB and SM2-U, (ii) ground truth and urban canopy signatures suitable for the estimation of urban albedos and aerodynamic surface temperatures from satellite data, (iii) high resolution mapping of urban land cover, land-use and aerodynamic parameters used in UBL models, and (iv) testing the ability of high resolution atmospheric models to simulate the structure of the UBL during land and sea breezes, and the related transport and diffusion of pollutants over different districts of the city. This paper presents initial results from such analyses and details of the overall experimental set-up