Estimating the turbulent eddy difussion and aerodynamic resistance for heat under unstable conditions using air temperature measurements as input

Visual examination of the scalar traces has revealed ramp like patterns in scalars (called coherent or organized structures) over and within plant canopies and over bare soil. Field studies suggest that the ramp-like patterns are responsible for the majority of scalar exchange in the canopy-atmosphere interface. Each ramp-like event represents an injection of flux of the scalar across a surface located at the measurement height, so the sum of all injections over a given time period may permit estimates of the mean flux. In our paradigm, we neglect advection and abstract the scalar trace as a sequence of N identical ramp events (defined by an amplitude A and frequency L), and in addition, carry out our analysis for the quiescent portion of the coherent structures. If during this quiescent period, the mixing is driven by quasi-homogeneous smaller scale motions, the turbulent eddy diffusion coefficient for heat (Kh) can be assumed constant. Such an assumption permits (using the one-dimensional turbulent diffusion equation) the linkage of the vertical gradient with the air temperature history. This ultimately allows one to derive an expression for estimating Kh and the aerodynamic resistance for heat using temperature measurements as input under unstable conditions. It will be shown that the expressions performed well under unstable conditions during an experiment carried out during year 2002 over rangeland grass (0.25 m tall at Ione, Ca) and measuring air temperature at 10 Hz at a level of 2 m height with a fine-wire thermocouple (0.0127 mm diameter).