Surface layer turbulence processes in low wind speeds over land

Abstract

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 strong stable conditions in the nighttime. Hence, the atmospheric surface layer turbulence associated with low wind speeds could be different from that of moderate to high wind speeds. The purpose of this paper is to present and discuss the variation of surface layer turbulence parameters and their dependence on atmospheric stability. The turbulence data collected from micrometeorological tower at Indian Institute of Technology (IIT) campus, New Delhi, India, using a sonic anemometer at a height of 4 m as part of the SF 6 tracer diffusion experiments have been analyzed to achieve these objectives. The turbulence statistics computed include averages, variances and covariances of the fluctuating wind components and temperature. In all, 38 hourly test runs were analyzed to compute various parameters such as surface turbulent fluxes of heat and momentum and the variances of velocity fluctuations. Results reveal that the turbulence parameters vary differently with atmospheric stability and wind speed for the wind speeds more than 1 ms −1 as against wind speeds less than 1 ms −1. A spectral analysis was carried out on the turbulence data. Results indicate that larger eddies dominate during nighttime as compared to the daytime convective conditions.