Shusun Li

Applications of SAR Interferometry in Earth and Environmental Science Research

This paper provides a review of the progress in regard to the InSAR remote sensing technique and its applications in earth and environmental sciences, especially in the past decade. Basic principles, factors, limits, InSAR sensors, available software packages for the generation of InSAR interferograms were summarized to support future applications. Emphasis was placed on the applications of InSAR in seismology, volcanology, land subsidence/uplift, landslide, glaciology, hydrology, and forestry sciences. It ends with a discussion of future research directions.

Geometrical-optics code for computing the optical properties of large dielectric spheres.

Absorption of electromagnetic radiation by absorptive dielectric spheres such as snow grains in the near-infrared part of the solar spectrum cannot be neglected when radiative properties of snow are computed. Thus a new, to our knowledge, geometrical-optics code is developed to compute scattering and absorption cross sections of large dielectric particles of arbitrary complex refractive index. The number of internal reflections and transmissions are truncated on the basis of the ratio of the irradiance incident at the nth interface to the irradiance incident at the first interface for a specific optical ray. Thus the truncation number is a function of the angle of incidence. Phase functions for both near- and far-field absorption and scattering of electromagnetic radiation are calculated directly at any desired scattering angle by using a hybrid algorithm based on the bisection and Newton-Raphson methods. With these methods a large spheres absorption and scattering properties of light can be calculated for any wavelength from the ultraviolet to the microwave regions. Assuming that large snow meltclusters (1-cm order), observed ubiquitously in the snow cover during summer, can be characterized as spheres, one may compute absorption and scattering efficiencies and the scattering phase function on the basis of this geometrical-optics method. A geometrical-optics method for sphere (GOMsphere) code is developed and tested against Wiscombes Mie scattering code (MIE0) and a Monte Carlo code for a range of size parameters. GOMsphere can be combined with MIE0 to calculate the single-scattering properties of dielectric spheres of any size.

Measurement of all-wave and spectral albedos of snow-covered summer sea ice in the Ross Sea, Antarctica

Abstract All-wave albedo and spectral albedo data were collected over snow-covered pack-ice floes during summer 1999 (January and February) in the Ross Sea, Antarctica. Temporal variation of the all-wave albedo and spectral albedo was measured from the northern edge to the southern edge of the pack ice along three lines of longitude: 165° W, 150° Wand 135° W. Snow depth, snow-cover stratification, snow-temperature profiles, and grain-size and morphology were also documented. It was observed that daily-averaged albedos were 0.70−0.86 for cloudy conditions over the pack ice. Only two sets of daily-averaged albedo were collected for clear-sky conditions (0.788 and 0:825). Albedo was lower at the marginal edges of the pack ice than in the central pack ice. Albedo was higher over the southern part of the central pack ice than over the northern part. Clear- and cloudy-sky albedos measured on the same site indicate that the average increase in albedo due to clouds is 1.4% (maximum 4.0%). Spectral albedos of the pack-ice floes were similar under clear-sky and cloudy conditions. Both are mainly controlled by the snow grain-size, especially in the top snow layer. All-wave albedos derived from measured visible and near-infrared spectral albedos, with extrapolation to ultraviolet and shortwave infrared regions for both clear- and cloudy-sky conditions, agreed well with all-wave albedos from direct measurements.

Comparison between in situ and MODIS-derived spectral reflectances of snow and sea ice in the Amundsen Sea, Antarctica

The spectral albedo and directional reflectance of snow and sea ice were measured on sea ice of various types, including nilas, grey ice, pancake ice, multi-year pack ice, and land-fast ice in the Ross, Amundsen and Bellingshausen seas during a summer cruise in February through March 2000. Measurements were made using a spectroradiometer that has 512 channels in the visible and near-infrared (VNIR) region in which 16 of the 36 bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) are covered. Directional reflectance is also retrieved from the MODIS radiometrically calibrated data (Level 1B) concurrently acquired from the first National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) satellite, Terra. The locations of the ground ice stations are identified accurately on the MODIS images, and the spectral albedo and directional reflectance values at the 16 VNIR MODIS bands are extracted for those pixel locations. MODIS-derived reflectance is then corrected for the intervening atmosphere whose parameters are retrieved from the MODIS atmospheric profiles product (MOD07_L2) for the same granule. The corresponding spectral albedo and directional reflectance with the same viewing geometry as MODIS are derived from our ground-based spectroradiometer measurements. Because the footprint of the ground spectroradiometer is much smaller than the pixel sizes of MODIS images, the averaged spectral reflectance and albedo in the vicinity of each ice station are simulated for the corresponding MODIS pixel from the ground spectral measurements by weighting over different surface types (various ice types and open water). An accurate determination of ice concentration is important in deriving ground reflectance of a simulated pixel from in situ measurements. The best agreement between the in situ and MODIS measurements was found when the ground had 10/10 ice concentration (discrepancy range 0.2–11.69%, average 4.8%) or was oneice-type dominant (discrepancy range 0.8–16.9%, average 6.2%). The more homogeneous the ground surface and the less variable the ground topography, the more comparable between the in situ and satellite-derived reflectance is expected.

Measurement of snow and sea ice surface temperature and emissivity in the Ross Sea

The snow and sea ice surface temperature in the Ross Sea was measured aboard the NSF Research Vessel Nathaniel B. Palmer during two cruises in 1998 and 1999, including a winter cruise in May and June 1998, and a summer cruise in January and February 1999. The main objectives of the cruises were to collect sea ice surface temperature data sets using a variety of instruments, and to devise schemes for future validation of the satellite data sets to be derived from the Moderate Resolution Imaging Spectroradiometer (MODIS). Attempts for estimation of emissivity of snow surface in the thermal infrared region were also made. The authors collected surface temperatures on 37 individual ice floes. Three sets of instruments were available, including 20 Optic StowAway thermistors, a Heintronics Model KT 19.82 infrared radiation pyrometer, and an Exergen Model DJOI-RS infrared microscanner. Each thermister has its own automatic data logger, and has been calibrated separately at the factory. The Heintronics thermal infrared pyrometer has an 8-14 /spl mu/m spectral response. The brightness temperature measurements can be read manually off the instrument screen, although a data logger with voltage output is also connected to the pyrometer for recording time series automatically and continuously. The Exergen infrared microscanner has a spectral response between 2-20 /spl mu/m. It has an automatic emissivity compensation mechanism for most real surfaces of unknown emissivity, except for metal surfaces.

Assessment of the accuracy of snow surface direct beam spectral albedo under a variety of overcast skies derived by a reciprocal approach through radiative transfer simulation

With radiative transfer simulations it is suggested that stable estimates of the highly anisotropic direct beam spectral albedo of snow surface can be derived reciprocally under a variety of overcast skies. An accuracy of +/- 0.008 is achieved over a solar zenith angle range of theta0 < or = 74 degrees for visible wavelengths and up to theta0 < or = 63 degrees at the near-infrared wavelength lambda = 862 nm. This new method helps expand the database of snow surface albedo for the polar regions where direct measurement of clear-sky surface albedo is limited to large theta0s only. The enhancement will assist in the validation of snow surface albedo models and improve the representation of polar surface albedo in global circulation models.

The spatial variability of summer sea ice in the Amundsen Sea seen from MODIS, RADARSAT SCANSAR and LANDSAT 7 ETM+ images

The summer sea ice extent, concentration and zonation in the Southern Ocean is investigated using three new satellite sensors. They are the MODerate resolution Imaging Spectroradiometer (MODIS) on the first NASA Earth Observing System (EOS) satellite, Terra, the synthetic aperture radar (SAR) on the first Canadian Space Agency Radarsat satellite, and the Enhanced Thematic Mapper Plus (ETM+) on the NASA Landsat 7 satellite. A large number of images of these satellite sensors were acquired in February and March 2000, when a sea ice field campaign was conducted aboard the U.S. research vessel Nathaniel B. Palmer. MODIS images acquired under clear sky conditions provide regional information of the spatial variability of the austral sea ice cover. Mosaics of Radarsat images present complemental information under various weather conditions and fill the MODIS information gap caused by cloudiness. ETM+ images give detailed information of the characteristics of individual ice zones, including the landfast, coastal polynyas, pack ice, and pancake and cake ice zones.

Measurement of BRDF of snow and sea ice in the Ross and Amundsen Seas by the visible and near-infrared channels of MODIS

To validate snow and ice products of MODIS, data sets of BRDF of snow and sea ice surface were collected in January to February 1999 in the Ross Sea, and in February to March 2000 in the Ross Sea and Amundsen Sea. Clear sky BRDF patterns of snow surface on sea ice floes show that for moderate to large viewing zenith angles, maximum reflectance occurs around the principal plane, and a minimum generally occurs between 100/spl deg/ and 140/spl deg/ of azimuth angle. BRDF patterns for a ridged ice floe with a refrozen snow surface full of ice clusters show much stronger anisotropy than an unridged flat ice floe with a pure and more homogeneous snow cover.

Deriving reciprocal kernel functional expression of Antarctic sea ice surface BRDF from field measurements

We investigate derivation of kernel functional expression of Antarctic sea ice surface BRDF using field measurements. Two new methods we developed recently are used to expand the scope and enhance the use of field albedo and reflectance measurements. The first normalizes, among ice stations of the same ice type, the BRDF difference caused by slight differences in the physical properties of the ice cover. The second method derives direct beam spectral albedo from hemispherical directional spectral reflectance measured under overcast skies using the reciprocity between them. The derived direct beam spectral albedo estimates cover the full range of solar zenith angles. We combine the resulting data sets together to determine the coefficients of the BRDF kernel functions through stepwise linear regression. The derived BRDF kernel functional expression is representative of the ice type of interest, and is optimized in the sense that the ordinary limitation of narrow solar zenith angle range no longer exists. The resulting BRDF expression will contribute to improving derivation of sea ice surface albedo from remote sensing measurements.

Phase functions of large snow meltclusters calculated using the geometrical optics method

Large snow meltclusters of 1 cm-order size are ubiquitously observed in summer snow cover on sea ice in the Southern Ocean. To study their effect on remotely sensed signals, we have to understand the absorption and single scattering properties of electromagnetic waves by these meltclusters. In this study, the meltclusters are characterized as spheres and scattering phase function and asymmetry factor are calculated on the basis of geometrical optics methods. The number of internal reflections and transmissions are truncated based on the ratio of incident irradiance at the n-th interface to the initial incident irradiance. The effect of finite size in absorption efficiency calculation using GOM method is corrected using Mie calculation. We use a combination of bracketing and a hybrid algorithm of bisection and Newton-Raphson methods to find the incident angles for a given scattering angle. The absorption efficiency, scattering efficiency, and phase function of a large single snow grain are calculated for the whole solar spectrum.