On the determination of stability conditions over forested areas from velocity measurements

Two proxies able to determine the sign of the atmospheric stability in the absence of temperature measurements were investigated using data from four forested sites in Sweden. The results indicate that the simple proxy based on the time of the day when the measurement was taken was sufficient to identify approximately 75% of the stable occurrences.

However, also during daytime, stable stratification was not infrequent. A criterion based on a combination of the local turbulence intensity and shear exponent was therefore also investigated, and the combination of the two criteria was able to estimate approximately 90% of the overall stable data. When selecting data with mean wind speeds over 5 m/s at 80 m height, the presented data sets include mostly data where temperature effects are small (near-neutral), followed by stable data.

Only few occurrences of unstable conditions were anyway observed. The investigated data sets show large variation of shear and turbulence intensity with increasing stability, indicating the need for proxies that also can be used to characterize the degree of atmospheric stability.