Understanding the diurnal cycle of land-atmosphere interactions from flux site observations

Abstract. Land–atmosphere interactions have been investigated at daily or longer time scales due to limited data availability and large sensor errors for measuring high-frequency signals. Yet coupling at the sub-daily time scale is characterized by the diurnal cycle of incoming solar radiation and soil wetness. Based on flux tower observations, this study investigates the climatology of the observed land–atmosphere interactions on a sub-daily time scale during the warm season. Process-based multivariate metrics are employed to quantitatively measure the segmented coupling processes and the mixing diagram is adopted to demonstrate the integrative moist and thermal energy budget evolution in the atmospheric mixed layer. The land, atmosphere, and combined couplings for the entire daily mean, midday, and midnight show the different situations to which surface latent and sensible heat fluxes are relevant, and they also reveal the climate sensitivity to soil moisture and surface air temperature. The coevolution of the diurnal moisture and thermal energy within the boundary layer traces a particular path on mixing diagrams, exhibiting different degrees of hysteresis in water– and energy–limited locations. Water– and energy–limited processes also show opposing long tails during the daytime and night-time related to the impact on the land and atmospheric couplings via latent heat flux and other diabatic processes like radiative cooling. This study illustrates the necessity of considering the whole diurnal cycle to understand land-atmosphere coupling processes comprehensively in observations and modelling.