Diurnal Variations of the Flux Imbalance Over Homogeneous and Heterogeneous Landscapes
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It is well known that the sum of the turbulent sensible and latent heat fluxes as measured by the eddy-covariance method is systematically lower than the available energy (i.e., the net radiation minus the ground heat flux). We examine the separate and joint effects of diurnal and spatial variations of surface temperature on this flux imbalance in a dry convective boundary layer using the Weather Research and Forecasting model. Results show that, over homogeneous surfaces, the flux due to turbulent-organized structures is responsible for the imbalance, whereas over heterogeneous surfaces, the flux due to mesoscale or secondary circulations is the main contributor to the imbalance. Over homogeneous surfaces, the flux imbalance in free convective conditions exhibits a clear diurnal cycle, showing that the flux-imbalance magnitude slowly decreases during the morning period and rapidly increases during the afternoon period. However, in shear convective conditions, the flux-imbalance magnitude is much smaller, but slightly increases with time. The flux imbalance over heterogeneous surfaces exhibits a diurnal cycle under both free and shear convective conditions, which is similar to that over homogeneous surfaces in free convective conditions, and is also consistent with the general trend in the global observations. The rapid increase in the flux-imbalance magnitude during the afternoon period is mainly caused by the afternoon decay of the turbulent kinetic energy (TKE). Interestingly, over heterogeneous surfaces, the flux imbalance is linearly related to the TKE and the difference between the potential temperature and surface temperature, ΔT; the larger the TKE and ΔT values, the smaller the flux-imbalance magnitude.
KeywordsConvective boundary layer Diurnal variations Flux imbalance Large-eddy simulation Spatial heterogeneity
This work was jointly supported by the National Natural Science Foundation of China (Grant: 91425303 and 41630856) and the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant: XDA19070100. H.L. acknowledges support by National Science Foundation AGS under Grants: 1419614. The major part of this work was conducted when the first author visited Boston University in 2017. We thank Professor Guido Salvucci and Dr. Angela Rigden at Boston University for their constructive comments and suggestions.
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