The Diurnal Behavior of Evaporative Fraction in the Soil–Vegetation–Atmospheric Boundary Layer Continuum

Abstract

The components of the land surface energy balance respond to periodic incoming radiation forcing with different amplitude and phase characteristics. Evaporative fraction (EF), the ratio of latent heat to available energy at the land surface, supposedly isolates surface control (soil moisture and vegetation) from radiation and turbulent factors. EF is thus supposed to be a diagnostic of the surface energy balance that is constant or self-preserved during daytime. If this holds, EF can be an effective way to estimate surface characteristics from temperature and energy flux measurements. Evidence for EF diurnal self-preservation is based on limited-duration field measurements. The daytime EF self-preservation using both long-term measurements and a model of the soil–vegetation–atmosphere continuum is reexamined here. It is demonstrated that EF is rarely constant and that its temporal power spectrum is wide; thus emphasizing the role of all diurnal frequencies associated with reduced predictability in its daylight response. Oppositely, surface turbulent heat fluxes are characterized by a strong response to the principal daily frequencies (daily and semi-daily) of the solar radiative forcing. It is shown that the phase lag and bias between the turbulent flux components of the surface energy balance are key to the shape of the daytime EF. Therefore, an understanding of the physical factors that affect the phase lag and bias in the response of the components of the surface energy balance to periodic radiative forcing is needed. A linearized model of the soil–vegetation–atmosphere continuum is used that can be solved in terms of harmonics to explore the physical factors that determine the phase characteristics. The dependency of these phase and offsets on environmental parameters—friction velocity, water availability, solar radiation intensity, relative humidity, and boundary layer entrainment—is then analyzed using the model that solves the dynamics of subsurface and atmospheric boundary layer temperatures and heat fluxes in a continuum. Additionally, the asymptotical diurnal lower limit of EF is derived as a function of these surface parameters and shown to be an important indicator of the self-preservation value when the conditions (also identified) for such behavior are present.