Drag and Bulk Transfer Coefficients Over Water Surfaces in Light Winds

The drag coefficient (CD), experimentally determined from observed wind speed and surface stress, has been reported to increase in the low wind-speed range (<3 m s−1) as wind speed becomes smaller. However, until now, the exact causes for its occurrence have not been determined. Here, possible causes for increased CD values in near-calm conditions are examined using high quality datasets selected from three-year continuous measurements obtained from the centre of Lake Kasumigaura, the second largest lake in Japan. Based on our analysis, suggested causes including (i) measurement errors, (ii) lake currents, (iii) capillary waves, (iv) the possibility of a measurement height within the interfacial/transition sublayer, and (v) a possible mismatch in the representative time scale used for mean and covariance averaging, are not considered major factors. The use of vector-averaged, instead of scalar-averaged, wind speeds and the presence of waves only partially explain the increase in CD under light winds. A small increase in turbulent kinetic energy due to buoyant production at low wind speeds is identified as the likely major cause for this increase in CD in the unstable atmosphere dominant over inland water surfaces