Electromagnetic signatures of terrain exhibit significant spatial heterogeneity on a range of scales as well as considerable
temporal variability. A statistical characterization of the spatial heterogeneity and spatial scaling algorithms of terrain
electromagnetic signatures are required to extrapolate measurements to larger scales. Basic terrain elements including
bare soil, grass, deciduous, and coniferous trees were studied in a quasi-laboratory setting using instrumented test sites in
Hanover, NH and Yuma, AZ. Observations were made using a visible and near infrared spectroradiometer (350 - 2500
nm) and hyperspectral camera (400 - 1100 nm). Results are reported illustrating: i) several difference scenes; ii) a terrain
scene time series sampled over an annual cycle; and iii) the detection of artifacts in scenes. A principal component
analysis indicated that the first three principal components typically explained between 90 and 99% of the variance of
the 30 to 40-channel hyperspectral images. Higher order principal components of hyperspectral images are useful for
detecting artifacts in scenes.
The thickness of Arctic sea ice plays a critical role in Earth's climate and ocean circulation. An accurate measurement of this parameter on synoptic scales at regular intervals would enable characterization of this important component for the understanding of ocean circulation and the global heat balance. Presented in this paper is a low frequency VHF interferometer technique and associated radar instrument design to measure sea ice thickness based on the use of backscatter correlation functions. The sea ice medium is represented as a multi-layered medium consisting of snow, sea-ice and sea water, with the interfaces between layers characterized as rough surfaces. This technique utilizes the correlation of two radar waves of different frequencies and incident and observation angles, scattered from the sea ice medium. The correlation functions relate information about the sea ice thickness. Inversion techniques such as the genetic algorithm, gradient descent, and least square methods, are used to derive sea ice thickness from the phase information related by the correlation functions. The radar instrument is designed to be implemented on a spacecraft and the initial test-bed will be on a Twin Otter aircraft. Radar system and instrument design and development parameters as well as some measurement requirements are reviewed. The ability to obtain reliable phase information for successful ice thickness retrieval for various thickness and surface interface geometries is examined.
KEYWORDS: Microwave radiation, Data modeling, Atmospheric modeling, Radiometry, Synthetic aperture radar, Temperature metrology, Lead, Atmospheric sensing, In situ metrology, Global Positioning System
A multidisciplinary, multi-institution team of scientists has been working for over three years to evaluate the performance of sea ice parameter algorithms applied to data from the AMSR-E (Advanced Microwave Scanning Radiometer - EOS) carried aboard NASA's Aqua platform. The AMSR-E data and derived sea ice geophysical products have been compared against a variety of measurements, including ground truth data from an ice field camp, imagery from aerosondes and an aircraft-borne microwave radiometer, and imagery from RADARSAT, MODIS, and AVHRR. Arctic ice environments examined include first-year and multiyear pack ice in the Beaufort and Chukchi Seas, polynyas and flaw leads in the Bering Sea, and the ice edge. This paper will outline the AMSRIce03 project, cover the validation methodology in detail, and discuss the results and their implications for use of sea ice products derived from the AMSR-E.
A knowledge of the reflection of light from a sea ice cover is important for both the interpretation of remote sensing imagery at visible and near-infrared wavelengths and for climatological studies involving the energy balance of the polar regions. Spectral measurements of albedo, bidirectional reflectance function (BDRF), and polarized reflectance were made for sea ice conditions found during the onset of melt in the Canadian Arctic. The wavelength region studied was from the ultraviolet to the near infrared (370 - 1000 nm). Results for five surface types are presented: (1) dry snow, (2) dry snow with a glazed surface, (3) bare ice, (4) blue ice, and (5) a melt pond. Results indicate that spectral albedos decrease at all wavelengths as the melt season progresses and the surface conditions evolve from (1) through (5), and that the decrease is most pronounced at longer wavelengths. Reflectance data suggest that (1) at most angles reflectance has the same spectral shape as albedo, (2) at 30 degree(s) elevation reflectance is for the most part azimuthally isotropic and (3) at 60 degree(s) elevation a significant specular component was evident at 0 degree(s) azimuth, especially for the bare ice case.
An understanding of the physical properties of sea ice and their variability is critical both to interpret observations of the optical properties and to develop models of radiative transfer. Sea ice has an intricate structure consisting of platelets of fresh ice with inclusions of brine and air. The total volume and the distribution of these inclusions strongly affect the optical properties. The physical properties of the ice are highly dependent on the growth conditions and the seasonal evolution of the ice. Consequently, the state and structure of the ice exhibit large spatial and temporal variability. For example, the crystal texture can be granular or columnar, while crystal sizes can vary from millimeters to a few centimeters. Observed brine volumes can vary from 0% in the surface layer of multi-year ice to as much as 50% in the skeletal layer at the bottom of a growing ice sheet. Densities show a similar variability ranging from 0.60 to 0.92 g/cm3. Because of this variability there is a need to use the large body of ice property observations to develop ice properties models, either of an empirical or physical nature.
During the seasonal transition from summer to winter conditions a profound
transformation occurs in a sea ice cover. As air temperatures drop, the ice cools
causing a reduction in the brine volume, melt ponds freeze, new ice forms in areas
of open water, and the surface becomes snow-covered. There is a corresponding
evolution in the optical properties ofthe ice cover with albedos increasing and
transmittances decreasing. As part of the drift phase of the Coordinated Eastern
Arctic Experiment (CEAREX), spectral albedos and reflectances in the visible and
near-infrared (400-1100 rim) were measured during fall freeze-up. Observed albedos
are presented for first-year ice, multiyear ice, and new-ice cases. In general,
albedos increased as freeze-up progressed, with the increase being most pronounced
at shorter wavelengths. There was a sharp increase in albedo associated with the
surface becoming snow-covered. The greatest temporal changes occurred in a freezing
lead where albedos increased from 0.1 for open water to 0.9 for snow-covered young
ice in only a few days.
The evolution of the transmitted radiation field under the ice was estimated
using a simple two-stream radiative transfer model in conjunction with observations
of ice morphology and thickness. Light transmission decreased dramatically due to
ice cooling, snowfall, and declining incident solar irradiances.
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