Triple-hot-film anemometer performance in CASES-99 and a comparison with sonic anemometer measurements

Two levels of triple-hot-film and sonic anemometers were deployed on a 5.5-m tower during the Cooperative Atmospheric Surface Exchange Study (CASES-99) in October 1999. Each triple-hot-film probe was collocated 50 mm from the sonic sensing path on a common boom. Various problems with using triple-hot-films in the atmosphere to resolve wind components are addressed including the derivation of a yaw angle correction using the collocated sensors. It was found that output voltage drift due to changes in environmental temperature could be monitored and corrected using an automated system. Non-unique solutions to heat transfer equations can be resolved using a collocated sonic anemometer. Multi-resolution decomposition of the hot-film data was used to estimate appropriate day and night averaging periods for turbulent flux measurements in and near the roughness sub-layer. Finally, triple-hot-film measurements of mean wind magnitude (M), turbulent kinetic energy (TKE), sensible heat flux (H), and local friction velocity (u(*)) are compared to those of the collocated CSAT3 sonic anemometers. Overall, the mean wind magnitudes measured by the triple-hot-film and the collocated sonic sensors were close, consistent and independent of stability or proximity to the ground. The turbulent statistics, TKE, u(*), and H, measured by the two sensor systems were reasonably close together at z = 5 m. However, the ratio of sonic measurement/hot-film measurement decreased toward the ground surface, especially during stable conditions.