Surface Energy Balance Closure Research Articles

Soil Heat Flux Plates: Heat Flow Distortion and Thermal Contact Resistance

Persistent concern regarding surface energy balance closure encourages increased scrutiny of potential sources of error. Laboratory and field experiments addressed heat flow distortion and thermal contact resistance errors during measurement of soil heat flux (G) using the flux plate technique. Steady‐state, one‐dimensional heat flow experiments determined flux plate thermal conductivities (λm) and measured the effect of air gaps and thermal heat sink coatings on plate performance. Use of measured instead of manufacturer‐specified λm and plate dimensions in a heat flow distortion correction improved the consistency but not the average disagreement between imposed sand G and corrected plate heat flux density (Gm). Consistent underestimates of G in dry sand by 20 to 25% after heat flow distortion correction was attributed to thermal contact resistance effects. A convex air gap 0.1 to 1.32 mm thick across 5.9% of the plate face area reduced Gm by up to 9.7%. A thin layer of a thermal heat sink compound with λ 0.18 W m−1 K−1 greater than the plate λm (1.0 W m−1 K−1) did not increase Gm in a clay soil but increased Gm by ∼6% in quartz sand. A 6.5% increase in Gm was also observed for plates treated with the same heat sink compound in a silt loam soil under field conditions. Thermal contact resistance errors are probably <10% in moist, medium‐textured soils and can be minimized by careful plate installation. Relatively greater errors in Gm may occur due to thermal contact resistance in dry sand and due to heat flow distortion when soil λ >> λm

Surface energy balance closure by the eddy-covariance method above three boreal forest stands and implications for the measurement of the CO 2 flux

Closure of the surface energy balance provides an objective criterion for evaluating eddy-covariance (EC) flux measurements. This study analyses 5 years of EC carbon dioxide, water vapor, and sensible heat flux measurements from three mature boreal forest stands in central Saskatchewan, Canada. The EC sensible and latent heat fluxes, H and λE, underestimated the surface available energy by 11% (aspen), 15% (black spruce), and 14% (jack pine). At all sites, the energy-closure fraction CF responded similarly to the friction velocity u *, atmospheric stability, and time of day. At night, CF increased from ∼0.3 at very low- u * to an asymptotic maximum of ∼0.9 at u * above 0.35 m s −1. During unstable-daytime periods, CF varied linearly from ∼0.7 at low- u * to ∼1.0 at high- u *. The energy imbalance pattern was similar among sites and may be characteristic of the continental, boreal forest. EC measurements of net ecosystem exchange F NEE have no objective, diagnostic parameter that is equivalent to CF. We therefore derived an analogous F NEE “closure fraction” CF NEE by normalizing measured F NEE against estimates from an empirical model that was tuned to the high- u * data. CF and CF NEE responded similarly to u *, atmospheric stability, and time of day. We discuss two implications for EC flux data post-processing. The results uphold the common practice of rejecting EC measurements during low- u * periods. They also lend support to the application of energy-closure adjustments to H, λE, and F NEE.

A case study of short-term closure of the surface energy balance

A case study of short-term closure of the surface energy balance

Seasonal cycles of Antarctic surface energy balance from automatic weather stations

AbstractWe present the seasonal cycle of the Antarctic surface energy balance (SEB) using 4 years (1998–2001) of automatic weather station (AWS) data. The four AWSs are situated on an ice shelf, in the coastal and inland katabatic wind zone and the interior plateau of Dronning Maud Land. To calculate surface temperature we use a SEB closure assumption for a surface skin layer. Modelled and observed surface have a root-mean-square difference of 1.8 K at the plateau AWS (corresponding to an uncertainty in the SEB of 5Wm–2) and <1 K (3Wm–2) at the other sites. The effect of wind-speed sensor freezing on the calculated SEB is discussed. At all sites the annual mean net radiation is negative and the near-surface air is on average stably stratified. Differences in the seasonal cycle of the SEB are mainly caused by the different wind climates at the AWS sites. In the katabatic wind zone, a combination of clear skies and strong winds forces a large wintertime turbulent transport of sensible heat towards the surface, which in turn enhances the longwave radiative heat loss. On the coastal ice shelf and on the plateau, strong winds are associated with overcast conditions, limiting the radiative heat loss and sensible heat exchange. During the short Antarctic summer, the net radiation becomes slightly positive at all sites. Away from the cold interior, the main compensating heat loss at the surface is sublimation. In the interior, summer temperatures are too low to allow significant sublimation to occur; here, surface heat loss is mainly due to convection.

Surface energy balance closure in an arid region: role of soil heat flux

The large soil heat fluxes in hot desert regions are very important in energy balance studies. Surface energy balance (SEB) observations, however, reveal that there is an imbalance in surface flux measurements and that it is difficult to isolate those flux measurements causing the imbalance errors. In this paper a new approach was tested focusing on a high temporal resolution of soil heat flux measurements at the surface. To determine if improved soil heat flux measurements could be obtained for a sandy desert, a standard soil heat flux sensor was buried at the surface of a playa and covered with less than 1 mm of playa soil, as opposed to burial several centimetres deep. It was anticipated that this would permit direct surface soil heat flux measurements. A field campaign was carried out in September and October in 1997 and for the same period in 2000, in a sandy desert belt situated in Nizzana, NW Negev, Israel. The research was designed to examine the separate components of the SEB. The location was considered ideal since only the soil heat flux and sensible heat flux are balanced by the net radiation. This new approach was compared to traditional soil heat flux measurements. The resulting energy balance closure was found to be very good. The results suggest that this method could be used as a reference surface soil heat flux measurement. Further examination of the energy balance closure is needed over a range of land surface types.

TURBULENCE FLUX ESTIMATES OF SENSIBLE AND LATENT HEAT NEAR SHELTERBELTS DURING LOW WIND CONDITIONS

A study was conducted to evaluate sensible and latent heat fluxes near shelterbelts under low wind conditionsusing eddy correlation. The study site was located at the Agricultural Meteorological Research Center in Mead, Nebraskaduring the summer of 1995. A wheat stubble and alfalfa field surrounded by a shelterbelt 12 m in height were instrumentedwith surface energy balance systems using eddy correlation that were located in the windward and leeward side of theshelterbelts for both fields. Estimates of sensible and latent heat fluxes were compared for the windward and leeward sidesof the shelterbelt for both surfaces. Daily sensible heat fluxes tended to be larger in the open location relative to thesheltered while latent heat fluxes tended to be higher behind the shelterbelt. Overall surface energy balance closure duringthe study averaged 0.85 indicating reliable consistent estimates of turbulence flux using eddy correlation near shelterbelts.