From AMS Glossary
The sum of molecular and turbulent fluxes within the bottom few millimeters of the earth's surface.
Immediately at the surface of the solid earth, turbulent fluxes must be zero. At this interface, the flux of sensible or latent heat must be totally transported by molecular motions (conduction and diffusion). Within a few millimeters above the surface, turbulence takes over to transport the fluxes, while the molecular effects become negligible. For simplicity, the sum of the molecular and turbulent fluxes is sometimes called the effective turbulent flux; frequently, "effective" is left out entirely. Thus, the expression "surface turbulent flux" really means "surface effective flux."