Some evaluations of drag and bulk transfer coefficients over water bodies of different sizes

Abstract

Three recent experiments allow evaluation of the bulk transfer coefficients for momentum, water vapour and sensible heat over water bodies of different sizes. As part of a study of evaporation rates from a swamp, measurements of latent and sensible heat fluxes were made over Lake Wyangan in southern N.S.W., Australia. This lake is of several kilometers diameter. In a later experiment, Reynolds stress and sensible heat transfer were measured from a natural-gas platform standing in Bass Strait, south of mainland Australia. The most recent experiment involved the direct measurement of each of these turbulent fluxes from a fixed tower erected in Lake Michigan, U.S.A. Perhaps the most important of the results is the finding that drag coefficients measured over Bass Strait are not significantly different from those over Lake Michigan, despite the obvious differences in depth, fetch, and hence surface wave structure. At both locations, drag coefficients are found to increase slightly with increasing wind speed, while at low wind speeds they are not significantly different from those corresponding to aerodynamic smoothness. Near-neutral bulk transfer coefficients for sensible heat and for water vapour are found to be similar. An average value of about 1.4 × 10⁻³ is obtained. It is emphasized that stability effects should be considered in any discussion of drag coefficients or bulk transfer coefficients. Large errors can result if near-neutrality is incorrectly assumed.