Turbulent transport characteristics of momentum, sensible heat, and water vapor above a forest canopy

Abstract

This paper describes the measurements of turbulent transport characteristics above a forest canopy in relation to the calculated eddy correlation fluxes. An observation tower was placed in the center of a 17.5 a hinoki (Chamaecyparis obtusa (SIEB. et ZUCC.) ENDL.) stand with an average height of 8.5m. An ultrasonic anemometer-thermometer and an infrared humidity fluctuation meter were used to detect wind velocity, temperature, and humidity fluctuations, respectively, at a height of 13.5m. The turbulent transport characteristics of momentum, sensible heat, and water vapor were analyzed for three different atmospheric stabilities. For unstable conditions, both temperature and humidity initially increased slowly and then declined rapidly, approximating a triangular pattern. This suggests the existence of vertical transport by convective plumes. During upward transport of sensible heat and water vapor from the forest canopy, cospectra of sensible heat and water vapor have secondary peaks which are not seen in the momentum cospectra. This also provides evidence of vertical transport by convective plumes. The nondimensional frequency which contributes to the transport ranges from 0.001 to 10 for sensible heat and from 0.001 to 5 for momentum and water vapor. Thus, a sampling speed of 10 Hz and an averaging time of 10min give reliable fluxes from unstable through stable air conditions.