Thermocouple Frequency Response Compensation for Surface Renewal Sensible Heat Flux Estimates

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 sensible heat flux estimation technique because it does not require a fast-response velocity sensor, but only a fast-response scalar sensor. Ramp-like shapes in the turbulent scalar trace are the signature of coherent structures and their characteristics (i.e., amplitude and duration) are used in the surface renewal flux calculation. Inexpensive fine-wire thermocouples are used to record high frequency temperature data, and the ramp characteristics are resolved using a model based on structure functions. The robust thermocouples (76 micron diameter wire) commonly deployed in field experiments are known to underestimate sensible heat flux, yielding results that are less than half of the sensible heat flux estimated with a finer wire sensor (13 micron diameter wire). In this poster, we present methods for compensating the frequency response of a thermocouple in the time domain, the time lag domain (i.e., compensating the structure functions 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 compensated robust thermocouples (76 micron diameter wire) produce surface renewal sensible heat flux estimates within 20% of the estimates from 13 micron diameter thermocouples.