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

[摘要] Land–atmosphere interactions have been investigated atdaily or longer timescales due to limited data availability and largeerrors for measuring high-frequency variations. Yet coupling at thesubdaily timescale is characterized by the diurnal cycle of incoming solarradiation and surface fluxes. Based on flux tower observations, this studyinvestigates the climatology of observed land–atmosphere interactions onsubdaily timescales during the warm season. Process-based multivariatemetrics are employed to quantitatively measure segmented coupling processes,and mixing diagrams are adopted to demonstrate the integrative moist andthermal energy budget evolution in the atmospheric mixed layer. The land,atmosphere, and combined couplings for the entire daily mean, midday, andmidnight periods show different situations to which surface latent and sensible heatfluxes are relevant, and they also reveal the climate sensitivity to soilmoisture and surface air temperature. The 24 h coevolution of the moistand thermal energy within the boundary layer traces a particular path onmixing diagrams, exhibiting different degrees of asymmetry (time shifts) inwater- and energy-limited locations. Water- and energy-limited processesalso show opposing long tails of low humidity during the daytime andnighttime, related to the impact on land and atmospheric couplings oflatent heat flux and other diabatic processes like radiative cooling. Thisstudy illustrates the necessity of considering the entire diurnal cycle tounderstand land–atmosphere coupling processes comprehensively inobservations and models.

[学科分类] 妇产科学