SURFACE-LAYER TURBULENCE PROCESSES IN LOW WIND SPEEDS OVER LAND

The atmospheric diffusion in the tropics is generally associated with low wind speeds, typically of magnitudes less than 3 ms(-1). Low wind speeds would cause significant free convection in the daytime and stro ng stable conditions in the nighttime. Hence, the atmospheric surface layer turbulence associated with low wind speeds could be different fr om that of moderate to high wind speeds. The purpose of this paper is to present and discuss the variation of surface layer turbulence param eters and their dependence on atmospheric stability. The turbulence da ta collected from micrometeorological tower at Indian Institute of Tec hnology (IIT) campus, New Delhi, India, using a sonic anemometer at a height of 4 m as part of the SF6 tracer diffusion experiments have bee n analyzed to achieve these objectives. The turbulence statistics comp uted include averages, variances and covariances of the fluctuating wi nd components and temperature. In all, 38 hourly test runs were analyz ed to compute various parameters such as surface turbulent fluxes of h eat and momentum and the variances of velocity fluctuations. Results r eveal that the turbulence parameters vary differently with atmospheric stability and wind speed for the wind speeds more than 1 ms(-1) as ag ainst wind speeds less than 1 m s(-1). A spectral analysis was carried out on the turbulence data. Results indicate that larger eddies domin ate during nighttime as compared to the daytime convective conditions.