The OASIS experience: Comparison of aircraft and tower-based flux measurements
The OASIS experiment, undertaken in October 1995 in New South Wales, Australia, employed aircraft to estimate the spatial variability of surface-atmosphere exchanges of heat, moisture and CO2 and to interpolate these variables between a small number of surface micro-meteorological stations. This study describes a comparison between tower-based measurements at one of these sites and aircraft measurements from a number of dedicated low-level flights. Confidence in the surface measurements is assured by the closure of the surface energy budget. Good agreement between the independent measures of mean temperature, wind speed and wind direction was achieved. Aircraft measurements of the standard deviation of the vertical wind speed and absolute humidity show no bias but some scatter compared to the tower measurements while the aircraft standard deviation of air temperature is less than the tower measurement by, on average, a factor of 1.6. Latent heat fluxes from the aircraft appear to be slightly larger than tower values but somewhat scattered whereas the aircraft sensible heat flux clearly underestimates tower values by a factor of 2. Net radiation measured from the aircraft was approximately 20% higher than the tower data. These results are consistent with previous aircraft-surface comparisons. If wind tunnel tests confirm our suspicions that the underestimation of temperature variance and sensible heat flux is due to the poor high frequency response of the aircraft temperature sensor, then the results will demonstrate that aircraft and tower based data are directly comparable provided care is taken with instrument calibrations, response times, and footprint differences.