STATIONARITY OF TURBULENCE IN LIGHT WINDS DURING THE MARITIME CONTINENT THUNDERSTORM EXPERIMENT

摘要

Non-stationary turbulence can invalidate eddy flux calculations. Two-hour long records of wind velocity, temperature and humidity are classified as stationary or non-stationary based on the behaviour of the flux as a function of Reynolds averaging period; a number of indicators of station-arity are investigated. The two-hour Maritime Continent Thunderstorm Experiment wind datasets are not completely stationary, as indicated by the lack of a spectral gap, but can be classified as approximately stationary, when the mean wind speed is greater than 3.8 m s~(-1), or the standard deviation of true wind direction is <= 10° or the ratio of horizontal wind variance to wind speed is < 0.25. In the stationary case the calculated friction velocity exhibits a 7% decrease on average when the Reynolds averaging period is doubled, while data classified as non-stationary exhibit an increase of 32%, There is little non-stationary behaviour in the kinematic heat fluxes, and is independent of the non-stationarity ofthe friction velocity. The kinematic heat fluxes show small decreases (around 3%) when the Reynolds averaging period is doubled.