Abstract:

Micrometeorological measurements depend on knowing the spatial resolution of the heterogeneous surface, defined as the source area. Based on 2002-2003 flux data at the Mid-Subtropical forest area (Qianyanzhou Station), the flux source area was examined using the FSAM analytical model. Evidence suggested that the source area changed with measurement height, time and atmospheric stability, i.e., under the same conditions, a higher measurement and more stable atmospheric layer created a larger flux source area and farther minimum horizontal distance (a) away from the tower. Seasonal variation of wind direction influenced the distribution direction of the area. A higher measurement height and more stable atmospheric layer caused a stronger pertinence of d/Z 0 and S v/u *, which was independent of wind direction. Because of the big fetch, the measurements were representative of the specific characteristics of the instrument location, i.e., the surface of the underlying forest. Moreover, plotting the wind direction and flux data (including the CO 2 flux, the sensible heat flux and the latent heat flux) on a synchronous basis indicated that when the Z/L data satisfy the model requirements, the flux data almost converge at the same wind direction. The results were obtained under conditions of horizontally homogeneous turbulence, which meets the basic need of the model. In Qianyanzhou Station, the FSAM model can only simulate the flux source area under conditions of day-and-night stable atmospheric layer and inactive turbulence. Future work needs to study the turbulent characteristics within and above the forest and improve the model so that it can be applied more widely in the station.

Key words: Flux source area, Forest, Eddy-covariance technique, Monin-Obukhov length