Investigating the Flux Patterns within the Forest Subcanopy over a Hilly Terrain

The tower-based eddy- covariance (EC) flux measurement could provide a reliable way to quantify the exchange of scalars, energy and momentum across the interface between the atmosphere and the terrestrial ecosystem. However, the ideal assumptions for EC measurement hardly meet the real situation, and it has been well documented that the complex terrain and tall forest canopy could significantly cause complicate situations to underestimate flux measurement. In addition, slope-induced drainage flow and horizontal/vertical mean transport have been suggested to misestimate nocturnal fluxes of the carbon dioxide and energy fluxes through deep forest canopy in hilly terrain due to the local complex wind field. In this study, the data collected from Chi-Lan Mountain flux site, a typical subtropical cloud forest located in north-eastern Taiwan, is used to characterize the flow pattern and its influences on the flux measurement. At this site, a long-term flux tower (height is 24 m) is operating under continuous basis. In this research, two 2-m-high towers were established since September of 2009 trying to characterize flux patterns below canopy and provide sufficient information to adjust the fluxes measurement over a complex terrain. The result showed that long term mean vertical wind speed obtained from planar-fit (PF) method may be very sensitive to the PF coefficients. The PF coefficients calculated from wind speed data from the shorter time period may not be applicable to estimate annual fluxes. The daily pattern of sensible heat flux is significantly different between the above- and below-canopy due to the influences of canopy structure. The horizontal momentum advection within the subcanopy directly related to the vertical mean wind speed. During the nighttime when the atmosphere is relatively stable with negative vertical mean wind speed, the downslope horizontal momentum advection becomes obvious. This strongly implies that the nighttime drainage flow within the subcanopy plays an important role to affect flux estimation at this site.