Generally, soil moisture plays an important role in water cycle, water resources and other diverse applications over land.
Passive microwave remote sensors (e.g., ASCAT, AMSR-E, SMOS, and SMAP) have successfully used for estimating
the amount of soil moisture irrespective of their low temporal and special resolutions. In this study, we present a TVDI
(temperature-vegetation dryness index)-based soil moisture retrieval algorithm based on visible and infrared remote
sensors. The TERRA/MODIS products such LST (MOD11A2) and NDVI (MOD13A2) data were used. Far-East Asia
area including the Korean peninsula were investigated for the case study. In particular, we found the elevation
dependence on the soil moisture retrieval. We developed a correction method for this elevation effect. The proposed
TVDI-based soil moisture algorithm in visible and infrared bands were compared and validated with soil moisture
contents estimated from GCOM-W1/AMSR-2 observations in microwave bands.
Sea ice is a good indicator to monitor the global climate change. Many of previous studies using the satellite
observations show a steady decline in Arctic sea ice. The study investigates the characteristics of the averaged
surface roughness, and refractive index from March 2003 to July 2011 using the AMSR-E daily data. The surface
roughness and refractive index of the sea ice is retrieved using a unique inversion algorithm based on the
characteristics of the polarized reflectivities, the Hong approximation, and the incidence angles of the many current
passive microwave satellite sensors. The averaged roughness and refractive index show the downward trend and
opposite signature with an acceleration, respectively. From the seasonal variations, the averaged roughness and
refractive index show the minimum and maximum values in the summer period, respectively. In addition, the annual
peaks of two physical parameters exhibit the phase difference of a month. In conclusion, this research provides a
physical explanation that the sea ice is melting increasingly using the satellite observation.
Cirrus is important in the energy balance of the earth-atmosphere system. Previous studies are based on the forward
models using the single scattering in the cirrus. In this study, we provide a unique method to detect the cirrus using the
polarized reflectivities, single-scattering, and optical properties of water and ice with Moderate Resolution Imaging
Spectrometer (MODIS) observations. Consequently, the polarized reflectivities at MODIS 1.375 μm channel show the
reasonable characteristics of the cirrus. The effective refractive indexes are retrieved approximately from 1.1 to 1.4. This
investigation provides an effective inversion procedure for detecting the cirrus using the physical characteristics in the
cirrus on the basis of the polarization and refractive index.
Polarization knowledge is important to detect object characteristics. An approximate relationship between vertically and horizontally polarized reflectivities of specular surfaces is developed and validated using the refractive index datasets of water in the various wavelength ranges at various incidence angles. This study proposes a unique technique to estimate the refractive indexes of a specular surface at a given view angle by the direct inversion of the Fresnel equation and the decomposition of the unpolarized emissivity. The unpolarized emissivity is calculated using the Fresnel equation and the refractive index of water at wavelengths ranging from ultraviolet (200 nm) to microwave (18.75 cm). Consequently, the differences of reflectivity between the Fresnel equations and the Hong approximation are approximately less than 0.001 within the Brewster angles of a material. In addition, the results for refractive index show a reasonable range of retrievals within the Brewster's angle. The imaginary parts of the refractive indexes have larger errors than the real parts, due to the uncertainty of the direct inversion of the Fresnel equation.