Yousuke Noumi

Discrepancy Between ASTER- and MODIS- Derived Land Surface Temperatures: Terrain Effects

The MODerate resolution Imaging Spectroradiometer (MODIS) and the Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) are onboard the same satellite platform NASA TERRA. Both MODIS and ASTER offer routine retrieval of land surface temperatures (LSTs), and the ASTER- and MODIS-retrieved LST products have been used worldwide. Because a large fraction of the earth surface consists of mountainous areas, variations in elevation, terrain slope and aspect angles can cause biases in the retrieved LSTs. However, terrain-induced effects are generally neglected in most satellite retrievals, which may generate discrepancy between ASTER and MODIS LSTs. In this paper, we reported the terrain effects on the LST discrepancy with a case examination over a relief area at the Loess Plateau of China. Results showed that the terrain-induced effects were not major, but nevertheless important for the total LST discrepancy. A large local slope did not necessarily lead to a large LST discrepancy. The angle of emitted radiance was more important than the angle of local slope in generating the LST discrepancy. Specifically, the conventional terrain correction may be unsuitable for densely vegetated areas. The distribution of ASTER-to-MODIS emissivity suggested that the terrain correction was included in the generalized split window (GSW) based approach used to rectify MODIS LSTs. Further study should include the classification-induced uncertainty in emissivity for reliable use of satellite-retrieved LSTs over relief areas.

Quantifying variability of satellite data in the reflective band for long-term monitoring of the Earth's surface: inference from a multi-temporal relationship between remotely sensed pixels

Reliable long-term monitoring of the Earths surface is urgently needed for a comprehensive understanding of our environment. However, remotely sensed data is generally affected by a number of temporal factors such as lifetime sensor degradation, Sun–target–satellite geometry and atmospheric conditions. The induced inconsistencies weaken the reliability of satellite-based change studies. A direct method is to remove the inconsistencies through converting the satellite digital number (DN) into a physical quantity using a physically or statistically based model. The associated errors in the conversion are generally hard to trace in the converted quantity, and this leaves questions unanswered. In this study, we propose an alternative approach to quantifying the influences on DN values, based on a multi-temporal relationship in the visible bands. First, we make use of the spectral dependency of aerosol optical thickness on wavelength to expand the validity of the multi-temporal relationship for reflective bands. As an inference of the relationship, a satellite DN value is determined analytically with the temporal influences in terms of a multiplicative and an additive component. In the case of the Landsat-5 Thematic Mapper (TM), we illustrate the variability in DN value in a spatio-temporal context. Lifetime sensor degradation (long-term effect) leads to an increase in the multiplicative effect and a minor change in the additive effect on the DN value. The combined Sun–target–satellite geometry and atmospheric variation induce periodic oscillations in both the multiplicative and additive effects on the DN value. The variation is generally larger for surfaces with a high reflectance than those with a low reflectance. The proposed approach combines sensor calibration and atmospheric correction into one equation, which offers the potential for tracing associated uncertainties propagated into a quantity converted or derived from satellite data, for long-term monitoring of changing surfaces.

Temporal Influences on Normalized Difference Vegetation Index

Normalized difference vegetation index (NDVI) is defined as a ratio of the difference of the infrared and red bands to the sum of the two bands. It can be estimated directly from satellite data, and has been widely used in numerous environmental studies. Yet the satellite-based NDVI was criticized for its variations with temporal factors (e.g. sun-surface-satellite geometry, atmospheric variations). Such variations may result in false change of vegetation over surface. However, the uncertainties relevant to the false change are generally unquantified in the studies. It is therefore unclear to what extent the satellite-based NDVI would be reliable. In this study, we used a derived relationship between the digital number (DN) with and without temporal influences for the same area. Using the derived relationship, NDVI can be expressed as a function of atmospheric optical thickness (AOT), view angle, and DN without temporal influences. As a result, the uncertainties relevant to the temporal factors were quantified with a mathematical expression. We found that satellite-based NDVI was a function of AOT, day of year, latitude, and NDVI without temporal influences. We made simulations in the case of Landsat TM data. Simulations showed that atmospheric effect was most influential to a satellite-based NDVI, and the NDVI would suffer more serious influences at higher latitude than at lower latitude. In general, the temporal influences on NDVI cannot be ignored for a reliable monitoring of surface phenological processes.

Cathodoluminescence of synthetic zircon implanted by He+ ion

Abstract He+ ion implantation at 4.0 MeV, equivalent to energy of α particles from natural radioactive nuclei 238U and 232Th, has been conducted for undoped synthetic zircon. The cathodoluminescence (CL) of implanted samples was measured to clarify the radiation-induced effects. Unimplanted synthetic zircon shows pronounced and multiple blue emission bands between 310 nm and 380 nm, whereas the implanted samples have an intense yellow band at ~550 nm. The blue emission bands can be assigned to intrinsic defect centers formed during crystal growth. The yellow band should be derived from induced-defect centers by He+ ion implantation, which might be related to the metamicitization originated from a self-induced radiation in natural zircon. The yellow band may be separated into two emission components at 1.96 eV and 2.16 eV. The emission component at 2.16 eV is recognized in both unimplanted and implanted samples, and its intensity increases with an increase in the implantation dose. The CL of zircon can be used as the geodosimeter.