Turbulent Transports of Sensible Heat within and above a Wheat Field
1984 Volume 39 Issue 4 Pages 297-304
Details
1984 Volume 39 Issue 4 Pages 297-304
In order to study turbulence characteristics related to sensible heat transports within and above plant canopies, the observations were made within and above wheat crops. The mean height of canopy was about 100cm. The results obtained at two heights (110cm and 35cm) during 1200-1700 JST on June 1, 1982 are summarized as follows:
1) Vertical fluxes of sensible heat at 100cm and 35cm were upward. They showed remarkably large variations, reflecting variabilities of solar radiation. Sensible heat fluxes at 35cm were smaller than one fifth of those at 110cm.
2) The standard deviations (σT) of air temperature at 110cm and 35cm were almost same and varied in the range of 0.5 and 2.0°C, corresponding to variations of solar radiation.
3) The skewness factors (ST) of air temperature at 110cm were all positive but 4/5 of those at 35cm were negative. The flatness factors (FT) at 110cm and 35cm fluctuated around the value 3 expected in the normal distribution.
4) The standard deviations (σw) of vertical wind velocity at 35cm were about 1/4 of those at 110cm. The skewness factors (Sw) were negative at both the heights, with remarkably large negative values at 35cm. The flatness factors (Fw) at 110cm were 3-5 but those at 35cm had greater values of 5-13. These results illustrate as shown in a previous paper (Maitani and Seo, 1983) that downdrafts penetrate intermittently in the lowest parts of wheat field.
5) The correlation coefficients rwT at 35cm were about 0.2 and halves of those at 110cm. The correlation coefficients rw110⋅w35 were almost constant (=0.4) but the correlation coefficients rT110⋅T35 varied in the range of 0.2 and 0.7.
6) Joint probability densities (w-T or w-wT) at both heights had similar distributions. Downdrafts were more efficient for upward heat flux than updrafts at the heights of 110cm and 35cm.
7) The vertical fluxes wT2 of temperature variance at 35cm were downward and the fluxes were divegent from the air layer between 110cm and 35cm.
8) The vertical fluxes wwT of covariance wT at both heights were downward and the fluxes were convergent into the air layer between 110cm and 35cm.
9) The peak frequency of the cospectrum nCowT (n) between w and T within plant canopies was lower than that above plant canopy. This trend was also seen in the power spectra of w and T.
