Below are highlights of a few of the research projects in the Radar Science & Engineering section:
Cryosphere – Arctic/Antarctic Sea Ice
Understanding the natural variability of sea ice thickness is critical for improving global climate models. Sea ice regulates the energy exchange between ocean and atmosphere. Not only does it play a very important role in Earth's climate system, its anomalous variability is an early indicator of the magnitude and impact of climate change.
Recent studies indicate that the sea ice cover is undergoing significant climate-induced changes, affecting both its extent and thickness. While the extent of the sea ice cover is effectively monitored from satellite platforms, using mostly passive microwave imagery, systematic information about ice thickness is much harder to extract from currently available remote sensing measurements. Ron Kwok and his subgroup have recently succeeded in estimating sea ice thickness distributions from the IceSat/GLAS data set.
Shear deformation showing the development of fractures in the Arctic sea ice cover over a period of 18 days.
Section 334's sea ice subgroup is using data from radar and passive microwave instruments to measure the export of sea ice (area and volume fluxes) through eight gates out of the Arctic basin, from 1996 to the present, to help characterize the mass balance of the polar ocean regions. Subgroup members are also involved in many other aspects of this research, including high-resolution measurement of the motion and deformation of the Arctic sea ice pack, understanding linkages between sea ice behavior and atmospheric oscillations, and modeling energy exchanges at the surface.
For more information, please contact Ron Kwok and visit http://www-radar.jpl.nasa.gov/rgps.
Cryosphere – Antarctic Ice Shelves
"Will there be catastrophic collapse of the major ice sheets, including Greenland and West Antarctic and, if so, how rapidly will this occur? What will be the time patterns of sea level rise as a result?" These are the first two questions in the National Research Council's Decadal Survey list of National Imperatives for the Next Decade and Beyond. The collapse of ice shelves, which fringe the major ice sheets of Antarctica, results in the acceleration of their tributary glaciers and ice streams, thus increasing the discharge of Antarctic ice to the ocean and contributing to the rise in sea level. Current forecasting models of ice-shelf flow and deformation rely on ad-hoc relations between the stress and the resulting strain rate of the ice. Ala Khazendar and Eric Rignot are combining interferometric radar measurements of ice velocity with numerical models to retrieve the spatial distribution of the flow parameter. They applied their method to the Larsen B ice shelf, a 10,000-year-old structure extending over 3,000 km2 and 200 m of thickness, which disintegrated over a 6-week period between January and March 2002. Section 334's ice-sheet subgroup is also making in situ measurements to validate and improve ice thickness estimates using ground penetrating radars.
The black-and-white MODIS images (courtesy of NSDIC) captured the disintegration of the Larsen B Ice Shelf in 2002. The leftmost color panel shows the velocity field as estimated from interferometric processing of Radarsat-1 data collected in September/October 2000, with a precision of 10 m/a from Radarsat-1 tracks. The color panel to the right shows the spatial distribution of the depth-averaged flow parameter obtained from the velocity data and a finite element flow model. The black line, which traces the front along which the 2002 disintegration occurred, shows how closely the ice shelf collapse line followed the zones of weakness in the ice shelf revealed by the data-assimilation method.
For more information, please contact Ala Khazendar, Eric Rignot or Ali Safaeinili.
Inland Coastal Ecology
The main objective of Marc Simard's inland coastal ecology project is to map coastal vegetation in 3D and estimate above-ground biomass and ecosystem productivity, using radar interferometry, lidar and field data. Among the different coastal vegetation classes, mangroves are particularly interesting because they contribute about 11% of the global carbon export to the ocean (though they amount to less than 1% of all global coastal vegetation), they are responsible for 15% of modern coastal sediment accumulation, and they are threatened – 35% of the global mangrove cover in 1950 disappeared over the last 50 years, and it is expected that one-third of the remaining mangroves will disappear over the next 50 years. While Dr. Simard's project currently focuses on the mangrove forests of the Americas, the estimation techniques are being extended to other biomes.
3D vegetation structure of mangrove forests of Colombia measured with interferometric radar and lidar data.
For more information, please contact Marc Simard and visit http://www-radar.jpl.nasa.gov/coastal.
JPL has been involved in ocean altimetry since the trailblazing Seasat mission in 1978. The work continued with the highly successful TOPEX/Poseidon, Jason-1 and Jason-2/OSTM series of altimeter missions that systematically measure global sea surface height with an accuracy approaching the 1-cm level.
This image shows a Jason-class spacecraft with downward-facing antennas for the CNES altimeter and microwave radiometer developed by JPL.
These measurements allow scientists to quantify the effects of ocean currents on global climate change, to monitor large-scale ocean features like the Rossby and Kelvin waves that characterize such phenomena as the El-Niño/Southern-Oscillation and the Pacific Decadal Oscillation, and to map year-to-year changes in the spatial distribution of heat stored in the upper ocean.
The observations also allow the tracking of smaller-scale ocean eddies that are important in the transport of heat within the ocean. Warm eddies are likely involved in hurricane intensification. In addition, the data have led to a significant improvement in global tide models. Altimeters also measure wave height and wind speed, which are useful in meteorological applications.
For more information, please contact Ernesto Rodriguez and Phil Callahan.
Interannual Variability of Precipitation
While some temperate regions in different continents experience significant amounts of precipitation, especially during the local winter season, most of the rain produced by our atmosphere falls in the tropics – over the tropical oceans where it is very difficult to measure without relying on data from spaceborne instruments such as the radar and passive radiometer on the Tropical Rainfall Measuring Mission (TRMM) satellite. Section 334's precipitation subgroup analyzed the year-on-year variability of global precipitation using the TRMM measurements, and condensed the "interannual rain anomaly" into an index (similar to the stock market indices), which objectively tracks the main component of the change in global precipitation. Comparisons with subjective indices tied to physical phenomena show that the objective rain-variability index is most highly correlated with the ENSO indices, implying that the El Niño phenomenon is the main driver behind the global variability of rainfall.
Color illustration of the index tracking the main component of the rain anomaly according to TRMM: when orange-to-red areas experience increased rain, green-to-blue areas experience a rain deficit, and vice versa. Yellow areas do not contribute as significantly to the main component of the global rainfall variability.
For more information, please contact Ziad S. Haddad and visit http://trmm.jpl.nasa.gov/global.
Electromagnetic Properties of Planetary Surfaces
Radar has become an increasingly important tool in the investigation of a wide range of objects in the solar system, including the terrestrial planets, the Moon, asteroids and comets, and the icy satellites of the outer planets. Earth-based radar investigations of these objects have spanned several decades and have recently been supplemented by orbital spacecraft investigations using synthetic aperture radar and sounding techniques.
The success of these current and future radar investigations depends strongly on our ability to model the interaction of the radar wave with the planetary surface, and its propagation in the subsurface, as a function of the composition and temperature of the medium. To date, the radiometric properties of the dusty ice- and hydrate-rich materials found in many cold planetary environments have not been fully quantified. To address this need, section 334’s dielectric-characterization subgroup established a laboratory dedicated to the measurement of the electromagnetic properties of planetary analog materials, including dusty-ice and hydrate-rich materials as they may occur in a wide range of cold planetary environments. Mixtures of dust (including volcanic, sedimentary, and meteoritic materials), water ice and hydrates are being measured in an effort to provide a better understanding of the relevant radiometric properties of new planetary environments. This capability is crucial to the design of radar imaging and sounding systems capable of providing scientific insights far exceeding the results of Earth-based analyses.
2 MHz Parametric Dielectric map of the Martian surface as modeled from laboratory measurements of Mars analog materials and TES thermal inertia data.
For more information, please contact Essam Heggy.