干旱区生态环境知识资源中心

For the first time, the exchange coefficient of heat C-H has been estimated from eddy correlation of velocity and virtual temperature fluctuations using sonic anemometer measurements made at low wind speeds over the monsoon land at Jodhpur (26degrees18’ N, 73degrees04’ E), a semi arid station. It shows strong dependence on wind speed, increasing rapidly with decreasing wind speed, and scales according to a power law C-H = 0.025 (U-10) over bar (-0.7) (where (U-10) over bar is the mean wind speed at 10-m height). A similar but more rapid increase in the drag coefficient C-D has already been reported in an earlier study. Low winds (<4 m s(-1)) are associated with both near neutral and strong unstable situations. It is noted that C-H increases with increasing instability. The present observations best describe a low wind convective regime as revealed in the scaling behaviour of drag, sensible heat flux and the non-dimensional temperature gradient. Neutral drag and heat cofficients, corrected using Monin-Obukhov (M-O) theory, show a more uniform behaviour at low wind speeds in convective conditions, when compared with the observed coefficients discussed in a coming paper.

At low wind convective conditions, M-O theory is unable to capture the observed linear dependence of drag on wind speed, unlike during forced convections. The non-dimensional shear inferred from the present data shows noticeable deviations from Businger’s formulation, a forced convection similarity. Heat flux is insensitive to drag associated with weak winds superposed on true free convection. With heat flux as the primary variable, definition of new velocity scales leads to a new drag parameterization scheme at low wind speeds during convective conditions discussed in a coming paper.