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Universidade de Sao Paulo (2006)

Aerodynamic roughness parameters over sparse vegetation

Bastos Lyra Gustavo

Titre : Aerodynamic roughness parameters over sparse vegetation / Parâmetros de rugosidade aerodinâmica sobre vegetação esparsa

Auteur : Gustavo Bastos Lyra

Université de soutenance : Universidade de Sao Paulo

Grade : Doutorado 2006

For sparse and tall vegetation the estimate of roughness parameters is compromised by the difficulty in observing conditions that satisfy the windspeed logarithmic law. The roughness length (z0) and the zero-plane displacement (d) were estimated by some methods with micrometeorological measurements and the physical structure of sparse shrubs in semi-arid region, during the HAPEX-Sahel experiment. The wind speed was measured at four heights above of surface (3.0, 4.1, 5.3 and 8.5 m), and the turbulent flows determined by eddy correlations at the height of 9m. Methods based on the logarithmic profile have been applied in neutral atmosphere conditions. The average height of the vegetation was h = 2.06 ± 0.47 m. The conventional method (statistical fit) resulted in good estimates of d and z0 only under conditions of validity of the logarithmic law. Only one height of measurement located above of the inertial sublayer is enough to result in physically inconsistent values. When the friction velocity, given by eddy correlation, was used in the logarithmic law, the estimates improved. The combination of the logarithmic law with z0 = λ (h - d) provided satisfactory estimates of the surface roughness for λ = 0.188 and 0.190 determined in function of the physical structure of the vegetation ; but for λ = 0.166, the average value of literature, the estimates where not good. Relationships between the physical structure of the vegetation and the momentum transfer estimated appropriately d and z0. The area roughness was better described by d = 0.95 m = 0.46 h and z0 = 0.204 m = 0.1 h, being λ = 0.185. Wind speed and friction velocity were more sensible to variations in z0 than in d.

Assunto(s) : atmospheric turbulence, environmental physics, física ambiental, micrometeorologia, micrometeorology, rugosidade da superfície, surface roughness, turbulência atmosférica

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