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ARS Home » Pacific West Area » Davis, California » Crops Pathology and Genetics Research » Research » Publications at this Location » Publication #297069

Title: Thermocouple frequency response compensation leads to convergence of the surface renewal alpha calibration

item SHAPLAND, THOMAS - University Of California
item SNYDER, RICHARD - University Of California
item PAW U, KYAW THA - University Of California
item McElrone, Andrew

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2013
Publication Date: 1/7/2014
Citation: Shapland, T., Snyder, R.L., Paw U, K., Mcelrone, A.J. 2014. Thermocouple frequency response compensation leads to convergence of the surface renewal alpha calibration. Agricultural and Forest Meteorology. 189-190:36-47.

Interpretive Summary: In this manuscript, we describe methods for compensating the frequency response of a fine wire thermocouple are presented that are used in the surface renewal method. Our findings show that the surface renewal alpha calibration converges with thermocouple compensation, and improves estimates without eddy covariance calibration. Compensation allows for the use of more rugged thermocouples in field experiments, and enables the development of surface renewal as an economically viable way to directly measure site specific water use for most agricultural crop surfaces.

Technical Abstract: Sensible heat flux measurements are used in conjunction with net radiation and ground heat flux measurements to determine the latent heat flux as the energy balance residual. Surface renewal is a relatively inexpensive technique for sensible heat flux estimation because it requires only a fast-response air temperature sensor. Ramp-like shapes in the turbulent scalar trace are the signature of coherent structures, and their characteristics (i.e., amplitude and duration) are resolved via a structure-function model for use in the surface renewal flux calculation. The potential for surface renewal to provide inexpensive sensible heat flux measurements has not been fully realized because this method has required calibration against eddy covariance or other more expensive flux measurement techniques. The calibration factor alpha is theoretically predicted as 0.5, but a broad range of values have been reported in the surface renewal literature. Although it has been hypothesized that the sensor size, and hence sensor frequency response characteristics, influence alpha, no effort has been previously made to compensate the thermocouple signal in surface renewal measurements. We present methods for compensating the frequency response of a thermocouple in the time domain, the lag domain (i.e., compensating the structure function directly), and the frequency domain. We evaluate the efficacy of the compensation procedure in resolving ramp characteristics at both the smallest and the second smallest scales of ramp-like turbulent shapes. The surface renewal sensible heat flux estimates from the compensated robust thermocouples (76 micrometer diameter wire) agree well with the estimates from the compensated fragile thermocouples (13 micrometer diameter). Using both the data collected for the present experiment and a meta-analysis of data in the surface renewal literature, we correct the surface renewal estimates for thermocouple frequency response characteristics to obtain alpha calibrations that converge to close to the predicted value of 0.5. We conclude that the frequency response characteristics of the thermocouple are the prevailing influence on the alpha calibrations reported in the literature.