The temperature fluctuations in stable stratification

Abstract

Abstract The intensity and spectrum of temperature fluctuations recorded at a height of 2 m over flat grassland in stable conditions are examined. Thr results show a demarcation between two distinct régimes, ‘ turbulent ’ for Ri less than a critical value of about 0.2 and ‘ quiet ’ for larger Ri. The magnitude of the temperature fluctuations, scaled relative to z δ z , is comparatively large in the turbulent régime, decreases as Ri increases towards the critical value, and remains small in the quiet régime. The temperature, while always of a typically turbulent character in the turbulent régime, is of variable behaviour in the quiet régime ‐ intermittently quiescent with occasional isolated smooth pulses (usually negative), or wave‐like with assorted frequencies, or turbulent. In the turbulent régime, the spectrum approximates to the − 5/3 power form for normalized frequencies nz/U greater than 0.6. In the quiet régime, the form of the spectrum varies unpredictably from one occasion to another.