Diurnal Variations of the Flux Imbalance Over Homogeneous and Heterogeneous Landscapes |
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| Authors: | " target="_blank">Yanzhao Zhou Dan Li Heping Liu Xin Li |
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| Institution: | 1.Key Laboratory of Remote Sensing of Gansu Province, Northwest Institute of Eco-Environment and Resources,Chinese Academy of Sciences,Lanzhou,China;2.University of Chinese Academy of Sciences,Beijing,China;3.Department of Earth and Environment,Boston University,Boston,USA;4.Laboratory for Atmospheric Research, Department of Civil and Environmental Engineering,Washington State University,Pullman,USA;5.Institute of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing,China;6.CAS Center for Excellence in Tibetan Plateau Earth Sciences,Chinese Academy of Sciences,Beijing,China |
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| Abstract: | 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. |
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