In-depth understanding of component temperatures over sparsely vegetated surfaces

Ground-based component temperatures are important inputs to model the energy exchange between the heterogeneous land surface and atmosphere. Although some excellent researches have been done, factors affecting the ground-based component temperatures over sparsely vegetated surface are still needed for further understanding. In order to obtain an in-depth understanding of component temperatures, a MUlti-Scale Observation Experiment on land Surface temperature (MUSOES) was designed and performed in a nature oasis with highly heterogeneous vegetated surface in the downstream of Heihe River basin, northwest China. The MUSOES was under the framework of the Heihe Watershed Applied Telemetry Experimental Research (HiWATER). During the intensive observation period, between July 20th and August 20th of 2014, surface temperatures were obtained at two scales around two hydrometeorological observatory stations: the mixed forest station (MFS) and the superstation (SUP). At MFS, six and four component temperatures were identified from the measurements of scale I (scale of thermal infrared (TIR) imagers) and scale II (scale of TIR radiometers), respectively. Due to the dense shrubs and absence of trees at SUP, two component temperatures were identified under each scale. Results show that intrinsic characteristics influence the variability of temperature between components and within a component. For the component temperatures measured by TIR imager, the mean differences between sunlit soil, shrub, and tree canopy were 17.9 K and 7.0 K at MFS; the mean difference between soil and shrub was 17.7 K at SUP. At the direction-level and pixel-level, the components exhibit different internal temperature differences. Furthermore, the component temperatures significantly affected by the illumination condition, viewing direction, and instrument type. Deviation exists between the component temperatures measured by the TIR radiometer and the TIR imager and depends on the components (e.g. 6.7 K for soil and -7.9 K for shrub at SUP). The TIR radiometer faces great challenge to measure the component temperature with good spatial representativeness over highly heterogeneous surface. Findings from this study can benefit our understandings of the attribute and ground observation of component temperatures, and promote the further study of the energy exchange between the heterogeneous land surface and atmosphere.