NIGHTTIME FREE-CONVECTION CHARACTERISTICS WITHIN A PLANT CANOPY

An intensive measurement campaign within and above a maize row canopy was carried out to investigate flow characteristics within this vegetation. Attention was given to finding adequate scaling parameters of the within-canopy windspeed and air temperature profiles under above-canopy stable stratification. During clear and calm nights the within-canopy condition differs considerably from the above-canopy state. In contrast to the daytime, the windspeed and temperature profiles do not scale with the above-canopy friction velocity, u*, and the scaling temperature, T*, respectively. A free convection flow regime is generated, forced by the soil heat flux at the canopy floor and by cooling at the top of the canopy. However, the windspeed and temperature profiles appear to scale well with the free convective velocity scale, w*, and the free convective temperature scale, T(f), respectively. The free convective state within the canopy agrees well with the free convection criterion Gr > 16Re2(u*), where Gr is the Grashof number and Re(u*) the Reynolds number, a criterion often used in technical flow problems. Also it is shown that under within-canopy free convection, there is a unique relation between the Grashof number, Gr, and the Reynolds number if the latter is based on the free convective velocity scale. Under within-canopy free convective conditions, it appears that within the canopy the fluxes of heat and water vapour can be estimated well with the relatively simple variance technique. Under these conditions, the Grashof, or Rayleigh number, represents a measure for the kinetic energy of the turbulence within the canopy.