Estimation of scalar source/sink distributions in plant canopies using Lagrangian dispersion analysis: Corrections for atmospheric stability and comparison with a multilayer canopy model

Source/sink distributions of heat, water vapour and CO2 within a rice canop y were inferred using an inverse Lagrangian dispersion analysis and measure d mean profiles of temperature, specific humidity and CO2 mixing ratio. Mon in-Obukhov similarity theory was used to account for the effects of atmosph eric stability on sigma(w)(z), the standard deviation of vertical velocity and tau(L)(z), the Lagrangian time scale of the turbulence. Classical surfa ce layer scaling was applied in the inertial sublayer (z > z(r)uf) using th e similarity parameter zeta = (z - d)/L, where z is height above ground, d is the zero plane displacement height for momentum, L is the Obukhov length , and z(r)uf approximate to 2.3h(c), where h(c) is canopy height. A single length scale h(c), was used for the stability parameter 3 = h(c)/L in the h eight range 0.25 < z/h(c) < 2.5. This choice is justified by mixing layer t heory, which shows that within the roughness sublayer there is one dominant turbulence length scale determined by the degree of inflection in the wind profile at the canopy top. In the absence of theoretical or experimental e vidence for guidance, standard Monin-Obukhov similarity functions, with zet a = h(c)/L, were used to calculate the stability dependence of sigma(w)(z) and tau(L)(z) in the roughness sublayer. For z/h(c) < 0.25 the turbulence l ength and time scales are influenced by the presence of the lower surface, and stability effects are minimal. With these assumptions there was excelle nt agreement between eddy covariance flux measurements and deductions from the inverse Lagrangian analysis. Stability corrections were particularly ne cessary for night time fluxes when the atmosphere was stably stratified. The inverse Lagrangian analysis provides a useful tool for testing and refi ning multilayer canopy models used to predict radiation absorption, energy partitioning and CO2 exchanges within the canopy and at the soil surface. C omparison of model predictions with source strengths deduced from the inver se analysis gave good results. Observed discrepancies may be due to incorre ct specification of the turbulent time scales and vertical velocity fluctua tions close to the ground. Further investigation of turbulence characterist ics within plant canopies is required to resolve these issues.