Search

In temperate forests, autumn leaf phenology signals the end of leaf growing season and shows large variability across tree-crowns, which importantly mediates photosynthetic seasonality, hydrological regulation, and nutrient cycling of forest ecosystems. However, critical challenges remain with the monitoring of autumn leaf phenology at the tree-crown scale due to the lack of spatially explicit information for individual tree-crowns and high (spatial and temporal) resolution observations with nadir view. Recent availability of the PlanetScope constellation with a 3 m spatial resolution and near-daily nadir view coverage might help address these observational challenges, but remains underexplored. Here we developed an integration of PlanetScope with drone observations for improved monitoring of crown-scale autumn leaf phenology in a temperate forest in Northeast China. This integration includes: 1) visual identification of individual tree-crowns (and species) from drone observations; 2) extraction of time series of PlanetScope vegetation indices (VIs) for each identified tree-crown; 3) derivation of three metrics of autumn leaf phenology from the extracted VI time series, including the start of fall (SOF), middle of fall (MOF), and end of fall (EOF); and 4) accuracy assessments of the PlanetScope-derived phenology metrics with reference from local phenocams. Our results show that (1) the PlanetScope-drone integration captures large inter-crown phenological variations, with a range of 28 days, 25 days, and 30 days for SOF, MOF, and EOF, respectively, (2) the extracted crown-level phenology metrics strongly agree with those derived from local phenocams, with a root-mean-square-error (RMSE) of 4.1 days, 3.0 days and 5.4 days for SOF, MOF, and EOF, respectively, and (3) PlanetScope maps large variations in autumn leaf phenology over the entire forest landscape with spatially explicit information. These results demonstrate the ability of our proposed method in monitoring the large spatial heterogeneity of crown-scale autumn leaf phenology in the temperate forest, suggesting the potential of using high-resolution satellites to advance crown-scale phenology studies over large geographical areas.

Direct numerical simulations were performed to investigate crossflow transition control by the three-dimensional steady upstream flow deformation technique under the influence of low-frequency fluc...

Industrialization is criticized for posing environmental concerns despite its contribution to local economic growth and modern urbanization. Accurately mapping industrial zones is a necessity for the strategic planning of modern industrialization. Unfortunately, industrial zones mapping by remotely sensed imagery over large areas remains methodologically challenging, as the spectral and texture features of industrial zones resemble those of other man-made surfaces. To fill the gap, this paper presents the first attempt at industrial zones mapping through identifying the ‘blue steel tile’-roofed buildings (BSTBs). Specifically, we develop a BSTB index (BSTBI) for extracting the BSTBs in Landsat images. The new index combines the BSTB feature of ‘V-shaped valley’ over visible-near infrared reflection (NIR) span and the most separable band of Landsat sensors (shortwave infrared-2 band). Through the experiments conducted in Linyi city, China, the new index is demonstrated a good performance in detecti...

It has been shown recently by direct numerical simulations that plasma actuators can be used to delay laminar-turbulent transition caused by steady crossflow vortices (CFVs) in three-dimensional boundary layers on swept aerodynamic surfaces. In the current work the applicability of such actuators to control transition caused by traveling CFVs is explored by two techniques. In the first technique, named upstream flow deformation, the actuators are used to excite steady CFV control modes. The resulting narrow spaced control CFVs induce a beneficial mean-flow distortion and weaken the primary crossflow instability, yielding delayed transition. In the second technique, the direct attenuation of nonlinear traveling CFVs, the actuators are positioned more downstream, where the traveling CFVs have already established. The localized unsteady forcing against the direction of the crossflow is then aimed at attenuating the amplitude of the traveling CFVs by directly tackling the three-dimensional nonlinear disturbance state. With both techniques transition can be delayed, however with a significantly higher efficiency for the method of upstream flow deformation.

Most vegetation indices (VIs) of remote sensing were designed based on the concept of soil-line, which represents a linear correlation between bare soil reflectance at the red and near-infrared (NIR) bands. Unfortunately, the soil-line can only suppress brightness variation, not color differences of bare soil. Consequently, soil variation has a considerable impact on vegetation indices, although significant efforts have been devoted to this issue. In this study, a new soil-line is established in a new feature space of the NIR band and a virtual band that combines the red and shortwave-infrared (SWIR) bands (0.74ρred+0.26ρswir). Then, plus versions of vegetation indices (VI+), i.e., normalized difference vegetation index plus (NDVI+), enhanced vegetation index plus (EVI+), soil-adjusted vegetation index plus (SAVI+), and modified soil-adjusted vegetation index plus (MSAVI+), are proposed based on the new soil-line, which replaces the red band with the red-SWIR band in the vegetation indices. Soil spectral data from several spectral libraries confirm that bare soil has much less variation for VI+ than the original VI. Simulation experiments show that VI+ correlates better with fractional vegetation coverage (FVC) and leaf area index (LAI) than original VI. Ground measured LAI data collected from BigFoot, VALERI, and other previous references also confirm that VI+ derived from Moderate Resolution Imaging Spectroradiometer (MODIS) data correlates better with ground measured LAI than original VI. These data analyses suggest that replacing the red band with the red-SWIR band can reduce the sensitivity of VIs to soil background. We recommend employing the proposed NDVI+, EVI+, SAVI+, and MSAVI+ in applications of large area, sparse vegetation, or when soil color variation cannot be neglected, although sensitivity to soil moisture and clay content might cause slight side effects for the proposed VI+s.