Bulk Transfer Coefficients Estimated from Eddy-Covariance Measurements
over Lakes and Reservoirs
Abstract
The drag coefficient (CDN), Stanton number (CHN) and Dalton number (CEN)
are of particular importance for the bulk estimation of the surface
turbulent fluxes of momentum, heat and water vapor at water surfaces.
Although these bulk transfer coefficients have been extensively studied
over the past several decades mainly in marine and large-lake
environments, there are no studies focusing on their synthesis for many
lakes. Here, we evaluated these coefficients through directly measured
surface fluxes using the eddy-covariance technique over more than 30
lakes and reservoirs of different sizes and depths. Our analysis showed
that generally CDN, CHN, CEN (adjusted to neutral atmospheric stability)
were within the range reported in previous studies for large lakes and
oceans. CHN was found to be on average a factor of 1.4 higher than CEN
for all wind speeds, therefore, likely affecting the Bowen ratio method
used for lake evaporation measurements. All bulk transfer coefficients
exhibit substantial increase at low wind speeds (< 3 m s-1),
which could not be explained by any of the existing physical approaches.
However, the wind gustiness could partially explain this increase. At
high wind speeds CDN, CHN, CEN remained relatively constant at values of
2 10-3, 1.5 10-3, 1.1 10 -3, respectively. We found that the variability
of the transfer coefficients among the lakes could be associated with
lake surface area or wind fetch. The empirical formula C=b1[1+b2exp(b3
U10)] described the dependence of CDN, CHN, CEN on wind speed well and
it could be beneficial for modeling when coupling atmosphere and lakes.