Why sensible and latent heat fluxes vary differently with height: horizontal advection and multiscale turbulent transport over a heterogeneous surface

Eddy-covariance fluxes measured over heterogeneous surfaces may not represent lower-level exchange when sensible heat (SH) and latent heat (LE) vary with height in the atmospheric surface layer. This problem is especially relevant over irrigated land adjacent to warmer, drier terrain, where horizontal advection can modify turbulent transport. Using two-level EC measurements over an irrigated cotton field, we examine why SH and LE differ between 8.7 m and 2.7 m during daytime. Four recurring SH–LE convergence/divergence classes were identified and analyzed using complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) and complementary turbulence diagnostics. Middle- and large-scale motions distinguished the classes, whereas small-scale turbulence provided mainly a background contribution. The clearest mechanism was reduced vertical mixing aloft: vertical-velocity variance and transport efficiency weakened more at 8.7 m, especially at middle scales. Altered downward scalar transport provided a secondary, more interpretive explanation for the contrasting SH and LE responses. Late-afternoon negative SH and stable near-surface conditions were consistent with oasis-effect behavior in this advection-favorable setting. Together, the results show that recurring SH–LE flux-gradient states reflect systematic differences in multiscale, height-dependent transport and caution against assuming that a single-height EC flux represents exchange below the measurement level.