Sparse canopy parameterizations for meteorological models

B.J.J.M. van den Hurk

Research output: Thesisinternal PhD, WU

Abstract

Meteorological models for numerical weather prediction or climate simulation require a description of land surface exchange processes. The degree of complexity of these land-surface parameterization schemes - or SVAT's - that is necessary for accurate model predictions, is yet unclear. Also, the calibration of these SVAT's for relatively complex terrain, such as sparse canopies, is not completely resolved. This thesis pays attention to the sensitivity of the atmospheric boundary layer to the parameterization of surface exchange processes for a sparse canopy surface.<p>During two experimental campaigns carried out in a sparsely vegetated vineyard surface in La Mancha, Spain, detailed measurements were collected, including the flux densities of sensible, soil and latent heat, radiative fluxes, aerodynamic properties, and soil and vegetation characteristics. These measurements were used for calibration and validation of various SVAT-models and their components.<p>In a theoretical analysis the traditional treatment of aerodynamic transport of heat and moisture between a sparse canopy surface and the atmosphere was considered, and compared by an alternative formulation based on Lagrangian diffusion theory. An analysis of field observations was carried out to quantify the spatial and temporal variability of the surface albedo of a sparsely vegetated surface. Furthermore, a model for the stomatal conductance, based on the calculation of leaf photosynthesis and its relations with stomatal water vapour transport, was tested and scaled- up to the canopy level.<p>Various existing SVAT's, designed for sparse canopies, were described and compared to field measurements in a zero-dimensional mode, that is, with forcings measured at reference height close above the surface. These models were all based on different physical treatment of soil heat flux, aerodynamic exchange and canopy resistance. None of the included models gave an optimum description of the observed fluxes, but a model could be constructed that combined the best parts of each of these SVAT's.<p>In an additional model study, this new description has been coupled to a onedimensional planetary boundary-layer (PBL) model. Parts of the SVAT were replaced by other components, and the impact on simulated PBL-dynamics has been evaluated. Large effects are found when (a) the reference two-layer model was replaced with a single layer ('big leaf') model, (b) soil heat flux was simulated with a resistance scheme rather than a diffusion or force-restore scheme, and (c) the aerodynamic resistance between the reference level and the bare soil was chosen too low. Since vegetation cover was small, smaller effects resulted from an alteration of the canopy resistance formulation. Also, it was found that the simulated entrainment of heat at the top of the boundary layer is low compared to entrainment ratios cited in literature.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Wieringa, J., Promotor, External person
  • de Bruin, H.A.R., Co-promotor
Award date22 Jan 1996
Place of PublicationS.l.
Publisher
Print ISBNs9789054854913
Publication statusPublished - 1996

Keywords

  • microclimate
  • soil
  • landscape
  • models
  • theory
  • climatic zones
  • climatology

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