Abstract
In complex, sloping terrain, horizontal measurements of net radiation are not reflective of the radiative energy available for the conductive and convective heat exchange of the underlying surface. Using data from a grassland site on a mountain slope characterised by spatial heterogeneity in inclination and aspect, we tested the hypothesis that a correction of the horizontal net radiation measurements which accounts for the individual footprint contributions of the various surfaces to the measured sensible and latent heat eddy covariance fluxes will yield more realistic slope-parallel net radiation estimates compared to a correction based on the average inclination and aspect of the footprint. Our main result is that both approaches led to clear, but very similar improvements in the phase between available energy and the sum of the latent and sensible heat fluxes. As a consequence the variance in the sum of latent and sensible heat flux explained by available radiation improved by>10%, while energy balance closure improved only slightly. This is shown to be mainly due to the average inclination and aspect corresponding largely with the inclination and aspect of the main flux source area in combination with a limited sensitivity of the slope correction to small angular differences in, particularly, inclination and aspect. We conclude with a discussion of limitations of the present approach and future research directions.
Highlights
The widespread observation that the sum of the latent and sensible (H) heat exchange measured by the eddy covariance method systematically underestimates the available energy, i.e. net radiation (Rn) minus the energy storage in the system (S), by 10-40 % (Stoy et al, 2013), has been puzzling the micrometeorological community for several decades (Eq 1): Eq (1)where we adopt a sign convention by which positive fluxes are directed towards the atmosphere and vice versa.As the lack of energy balance closure violates the first law of thermodynamics and may be indicative of systematic measurement errors in the individual terms of Eq (1), a large body of literature has accumulated on this topic
Using data from a grassland site on a mountain slope characterised by spatial heterogeneity in inclination and aspect, we tested the hypothesis that a correction of the horizontal net radiation measurements which accounts for the individual footprint contributions of the various surfaces to the measured sensible and latent heat eddy covariance fluxes will yield more realistic slope-parallel net radiation estimates compared to a correction based on the average inclination and aspect of the footprint
Characterisation of the study site Wind directions at the site were on average down-slope from northeast during nighttime and up-slope from southwest during daytime (Fig. 2c), with a rapid and a more gradual transition between the two main wind directions in the morning and afternoon, respectively, reflecting the interaction between valley and slope winds (Whiteman, 2000)
Summary
The widespread observation that the sum of the latent (λE) and sensible (H) heat exchange measured by the eddy covariance method systematically underestimates the available energy, i.e. net radiation (Rn) minus the energy storage in the system (S), by 10-40 % (Stoy et al, 2013), has been puzzling the micrometeorological community for several decades (Eq 1): Eq (1)where we adopt a sign convention by which positive fluxes are directed towards the atmosphere and vice versa.As the lack of energy balance closure violates the first law of thermodynamics and may be indicative of systematic measurement errors in the individual terms of Eq (1), a large body of literature has accumulated on this topic. Doing so is complicated by the mismatch in footprint between net radiation and eddy covariance flux measurements (Schmid, 1997), in particular if, as frequently the case under real-world conditions, the eddy covariance flux footprint is spatially heterogeneous in terms of inclination and aspect. In such a case, the correction of horizontal net radiation should be weighted by the individual contributions of the underlying surfaces of various inclinations and aspect to the total flux measured at the eddy covariance tower. To the best of our knowledge, this has not been attempted and researchers instead have used average inclination and aspect of the slope in the correction (Hammerle et al, 2007; Hiller et al, 2008)
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