Caracterização do Escoamento e Fluxo Atmosférico de Calor Latente em Montado de Sobro

 

Abel Martins Rodrigues*¹ e Gabriel Paulo Alcântara Pita**

*Investigador Auxiliar

Estação Florestal Nacional. Departamento de Silvicultura e Produtos Florestais,
Av. da República, Quinta do Marquês, 2780-149 OEIRAS

**Professor Auxiliar

Instituto Superior Técnico. Departamento de Engenharia Mecânica, Av. Rovisco Pais, 1049-001 LISBOA

 

Sumário. A partir de medição, em torre de observação agronómica em montado de sobro, das variáveis micrometerológicas usuais e do regime turbulento de flutuações de componentes de velocidade do vento e temperatura do ar, procedeu-se a uma caracterização dos parâmetros aerodinâmicos e, por inspecção da equação de Penman-Monteith, da respectiva influência no regime de evapotranspiração, bem como a uma análise do escoamento turbulento nos domínios do tempo e frequência. O valor médio diário total da razão entre os resultados do fluxo turbulento de calor latente e de evapotranspiração de equilíbrio, foi de 0,44, característico de um coberto florestal seco. O factor de acoplamento médio de 0,18 foi indicativo do predomínio de condições do regime de evaporação imposta, com forte dependência ao défice de pressão de vapor da atmosfera e à resistência de coberto. Os valores de resistência de coberto aumentaram ao longo dos períodos da tarde, com o acréscimo do défice de pressão de vapor, indiciando um encerramento parcial dos estomas para controlo da transpiração. Os espectros das componentes da velocidade e temperatura do ar, aproximaram-se razoavelmente das funções empíricas normais. Os declives das curvas espectrais na subgama inercial, obedeceram à lei de potência de -2/3. Os valores calculados das escalas médias eulerianas integrais de comprimento a partir da função de autocorrelação, adimensionalizados à altura das árvores, foram de 8,65 e 0,69 para as componentes horizontal e vertical da velocidade do vento. Os valores calculados para as escalas médias de tempo correspondentes são de 12 e 0,96 s. A aplicação do método dos quadrantes confirmou a importância dos fenómenos intermitentes de ejecção e rajada no transporte descendente de momento.

Palavras chave: montado; potência espectral; acoplamento; escalas de tempo e comprimento

 

Abstract. At an observation tower in a cork oak stand, experimental measurements were made of the usual micrometeorological variables and of the turbulent fluctuations of wind velocity components and air temperature, in order to characterize their influence on the evapotranspiration regime and, by inspection of the Penman-Monteith equation, as well as time and frequency domain analysis of turbulent flow. The total mean daily ratio between the latent heat flux and equilibrium evapotranspiration was 0.44, typical of a dry canopy. The mean daily value of the coupling coefficient was 0.18, representative of an imposed evaporation, with a strong dependence on atmospheric humidity and canopy resistance. Canopy resistance increased during the afternoon, with an increasung atmospheric vapor pressure deficit, indicating a partial stomatal closure. The spectra of air temperature and velocity components fitted well with the usual empirical functions. The slopes of spectral curves followed the –2/3 power law in the inertial subrange. The calculated mean integral eulerian length scales, relating to tree heigths, calculated from autocorrelation functions, for the horizontal and vertical velocity components of wind velocity, were 8.65 and 0.69. Their corresponding time scales were 12 and 0.96 s. An application of the quadrant method confirmed the importance of intermittent sweeps and ejections, in the descendent transport of horizontal momentum.

Key words: cork oak stand; power spectra; coupling; time and length scales

 

Résumé. À partir du mesurage, dans une tour d'observation agronomique, dans un peuplement de chêne-liège, des variables micrométéorologiques usuelles et du régime turbulent de fluctuations des composants de vélocité du vent et température de l'air, on a fait une caractérisation des paramètres d'aérodynamique, et par l'analyse de l'équation Penman-Monteith de la respective influence sur le régime d'évapotranspiration, ainsi qu'une analyse de l’écoulement turbulent dans les domaines du temps et de fréquence. La valeur moyenne quotidienne totale de la raison entre les résultats du flux turbulent de chaleur latente et de l'évapotranspiration de bilan a été 0.44, caractéristique d'un peuplement forestier sec. Le facteur de couplage moyen de 0.18 a été indicatif de la prédominance de conditions de régime d'évaporation imposée, ayant une grande dépendance du déficit de pression de vapeur de l'atmosphère et de la résistance du couvert. Les valeurs de résistance du couvert ont augmenté au cours des périodes de l'après-midi, avec l'accroissement du déficit de pression de vapeur, en indiquant une fermeture partielle des stomates pour le contrôle de la transpiration.

Les spectres des composants de la vitesse et température de l'air s'approchent raisonnablement des fonctions empiriques normales. Les déclives des courbes spectrales dans la sous-gamme inertielle ont obéi à la loi de la puissance de -2/3. Les valeurs calculées des échelles moyennes de eulériennes intégrales de longueur à partir de la fonction d'autocorrélation, dimensionnés à la hauteur des arbres ont été de 8.65 et 0.69 pour les composants horizontal et vertical de la vélocité du vent. Les valeurs calculées pour les échelles moyennes de temps correspondantes sont de 12 et 0.96 s. L'application de la méthode des quadrants a confirmé l'importance des phénomènes intermittents d'éjection et de rafale dans le transport descendant de moment.

Mots clés: peuplements de chêne-liège; puissance spectrale; couplage; échelles de temps et longueur 

 

 

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Entregue para publicação em Setembro de 2003

Aceite para publicação em Outubro de 2003

 

¹1º Autor E-mail: abel.rodrigues@efn.com.pt