Why does turbulence transport momentum, heat, and moisture differently?

Turbulent surface fluxes of momentum and scalars are of critical importance to atmospheric dynamics at all scales, to hydrological studies, and to various other geophysical processes. The assumption that momentum and all scalars are transported similarly (i.e. in the same “way” and with the same efficiency), also know as the Reynolds analogy, has been shown to be invalid under most conditions. Yet few studies have examined the physical basis for the failure of the Reynolds analogy and for the dissimilar transport of various scalars such as heat (active scalar) and humidity (passive scalar).

Using data sets collected over water, snow, and urban surfaces, we revisit this problem with a focus on the ties between coherent structures, atmospheric stability, and turbulent transport. We first assess various definitions of transport efficiency based on ejection/sweep motions and on similarity relations. Then, by examining the characteristics of sweeps and ejections (time fractions, flux contribution, event duration, ..) along with atmospheric co-spectra, we seek to explain and predict conditions that lead to similar or dissimilar turbulent transports in the atmospheric surface layer. Atmospheric stability is found to be central to our inquiry.