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 vegeta tion. Attention was given to finding adequate scaling parameters of th e within-canopy windspeed and air temperature profiles under above-can opy stable stratification. During clear and calm nights the within-can opy condition differs considerably from the above-canopy state. In con trast to the daytime, the windspeed and temperature profiles do not sc ale with the above-canopy friction velocity, u, and the scaling tempe rature, 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 ap pear to scale well with the free convective velocity scale, w, and th e free convective temperature scale, T(f), respectively. The free conv ective state within the canopy agrees well with the free convection cr iterion Gr > 16Re2(u), where Gr is the Grashof number and Re(u*) the Reynolds number, a criterion often used in technical flow problems. Al so it is shown that under within-canopy free convection, there is a un ique relation between the Grashof number, Gr, and the Reynolds number if the latter is based on the free convective velocity scale. Under wi thin-canopy free convective conditions, it appears that within the can opy the fluxes of heat and water vapour can be estimated well with the relatively simple variance technique. Under these conditions, the Gra shof, or Rayleigh number, represents a measure for the kinetic energy of the turbulence within the canopy.