Variance, skewness and kurtosis characteristics within and just above a crop canopy

ABSTRACT The daily cycle of the variance, skewness and kurtosis of the temperature as well as horizontal and vertical wind speed components are analyzed within and just above a maize canopy. At first sight the within canopy statistical characteristics are similar to the above canopy statistics, which means that the within canopy turbulence is mainly ruled by the large scale above canopy turbulence. However, there are important differences. When during daytime forced convection dominates, the skewness of the within canopy temperature is opposite to the above canopy temperature skewness. This behavior is easily to be explained by the coherent structure of gusts penetrating the canopy from above. During nighttime a decoupling exists between the within and above canopy processes. Above the canopy the flow stabilizes due to radiative cooling while within the canopy a free convective state develops due to above canopy cooling and due to the soil heating at the bottom of the canopy. Then the within canopy temperature skewness is positive as well but by a different mechanism. During nighttime, the above as well as within vertical velocity skewness change sign and are both positive. Then it appears that the ratio between above canopy friction velocity and within canopy free convective velocity is an appropriate discrimination parameter between forced and free convection.