Wind speed and air temperature scaling within and just above a crop canopy

ABSTRACT Mean profiles of temperature and wind speed as well as their variance are analyzed within and just above a maize canopy for a daily cycle. During daytime the within-canopy turbulence is ruled by the above canopy turbulence. Consequently, all profiles scale well with the above-canopy temperature scale, T*, and velocity scale, u*. During wind still nights however, a free convection state develops in the lower region of the canopy, while in the upper and above-canopy region the thermal stratification is stable. During this above-canopy stable stratified period, the well-mixed lower layer of the canopy is more or less decoupled from the stratified upper layer. Convective heat at the soil's surface due to the soil heat flux and the radiative cooling at the top of the canopy is responsible for this separation. During these decoupled states, the above canopy scaling parameters, T* and u*, are not appropriate anymore for the within-canopy processes. During the decoupled periods the within canopy convective temperature and velocity scales, Tf and wf, respectively appear to be the ruling scaling parameters for the within-canopy processes.