Comparison of aircraft and ground-based flux measurements during OASIS95

Aircraft and ground-based measurements made during the 1995 Australian OASIS field campaign are compared. The aircraft data were recorded during low-level flights at 6 m above ground level and grid flights at altitudes of between 15 and 65 m, all in unstable atmospheric conditions. The low-level flights revealed an inadequate temperature sensor response time, a correction for which was determined from subsequent work in a wind tunnel. Aircraft and ground-based measurements of mean wind speed, wind direction and air temperature agree to within 0.2 m s^-1, 4° and 0.9 °C respectively. Comparisons between aircraft and ground-based observations of the standard deviations of vertical velocity, horizontal wind speed, air temperature and specific humidity have slopes of 0.96, 0.97, 0.92 and 0.99 respectively but the observed scatter is roughly twice the random error expected due to the averaging length of the aircraft data and the averaging period of the ground-based data. For the low-level flights, the ground-based and aircraft measurements of sensible and latent heat flux show mean differences of 27 and -25 W m^-2 respectively, which reduce to 11 and -4 W m^-2 respectively when analysis of aircraft data is limited to areas immediately adjacent to the flux towers. For the flights at 15 to 65 m above ground level, the mean differences between the ground-based and aircraft measurements of sensible and latent heat flux are -22 and -1 W m^-2 respectively and these change to -1 and -7 W m^-2 respectively once the effect of surface heterogeneity is included. Aircraft and ground-based measurements of net radiation agree to within 6% at one ground-based site but differ by 20% at a second. Aircraft measurements of friction velocity at 6 m above the ground agree well with ground-based data, but those from flights between 15 and 65 m above ground level do not. This is because at these heights the aircraft measurements provide the local shear stress, not the surface shear stress. Overall, the level of agreement allows confidence in the aircraft data provided due care is taken of instrument response times and differences in the surfaces sampled by aircraft and ground-based systems.