The Measurement of Vertical Transports of Sensible Heat within and above a Sorghum Canopy

Abstract

The observations of sensible heat flux were made at the 3 heights (260cm, 160cm, 100cm) in the sorghum field with a mean plant height of 160cm in August, 1982. The fluctuations of air temperature were measured at the 5 heights (260cm, 160cm, 100cm, 65cm, 20cm). The results are summarized as follows.1) The fluxes of sensible heat in the daytime were upward above and within plant canopies. The sensible heat flux above plant canopies were provided about 60% of flux from the upper layer between 160cm and 100 cm and about 40% from the layer below 100cm.2) The non-dimensional standard deviations (σT/|T*|) of air temperature were maximum at the top of the plant canopies, where T*=wT/u*, w and T are fluctuations of vertical velocity and temperature, and u* the friction velocity. The skewness factors (ST) of air temperature tended to become positive above plant canopies and negative within plant canopies. The flatness factors (FT) scatters around 3 through the air layer above and within plant canopies.3) The non-dimensional standard deviations (σw/u*) of vertical velocity had smaller values within plant canopies, compared with those above plant canopies. Within plant canopies, skewness factors (Sw) were negative and flatness factors Fw were greater than 4. The height dependency was in good agreement with that of previous results (Wilson et al. (1982) and Maitani and Seo (1983)).4) The correlation coefficients (rwT) between vertical velocity and air temperature were about 0.3 at the three heights of 260cm, 160cm and 100cm. On the other hand, correlation coefficients (rw·wT) between vertical velocity and instantaneous sensible heat flux were negative within plant canopies. The negative correlation coefficients rw·wT and joint probability densities (w-wT) within plant canopies implys that downdrafts were much efficient for a upward heat flux in the daytime.5) The peak frequencies of power spectrum (nST(n)) of T and cospectrum of (nCowT(n)) at the top of the plant canopies were higher than those within plant canopies. The shapes of nST(n) and nCowT(n) become flatter with decreasing heights within plant canopies.