Our final look at satellite use in oceanographic monitoring concerns sea ice, which normally occurs year round in the polar regions but to varying extents. When one thinks of sea ice, these two scenes are typical:
These are ice flows (blocks of ice of varying sizes separated by open water. When the open water occupies narrow cracks, these are called leads (top and center images below); when the open water is present as elliptical to irregular, wider stretches, they are called polynyas (bottom image):
Sometimes the ice floes have rounded edges and are called pancake ice:
Sea ice is easy to monitor from space as long as the scene is largely cloudfree when a satellite passes overhead. Polar orbiting satellites repeatedly pass near the poles on a daily cycle. Visible and radar imagery is effective in observing sea ice on a continual basis. Radar at bands that penetrate clouds (e.g., L-band) is now operational (Canada's Radarsat, for example) to monitor shipping lanes subject to ice hazards. Below is a SIR-C multiband color composite that shows ice in the Weddell Sea, off Antarctica, south of the Atlantic Ocean. Open water polynyas show as darker tones.
Sea ice may retreat during the summer but reform in new patterns as winter ensues, as shown in this Landsat image off the Greenland coast:
As the ice forms seasonally, the growing pack can take on wispy patterns, as seen in this Envisat image:
Offshore eddy currents can cause the forming ice to swirl:
This next image is ice swirls off the Labrador coast, as seen by SeaWIFs:
We see the nature of ice packs in the Arctic, over a large area, in this HCMM-Visible image of the Chukchi Sea in the Bering Strait between Alaska and Siberia. The ice in this scene has a network of cracks, the so-called leads, which open during summer breakup and refreeze when conditions demand.
The MODIS instrument on Terra has also observed this region:
Sea ice distribution over a large region is effectively being monitored now by the MODIS sensor(see Section 16). Here is a view of ice passing through, and blocking, the Bering Strait between Siberia and western Alaska.
The polar regions of the Northern Hemisphere are covered by sea ice, ice caps or sheets (e.g., Greenland) and snow, as depicted by this composite image constructed from Radarsat SAR images:
Below is another view of the north polar regions, showing the prevailing ice cover, made by NSCAT (the NASA Scatterometer).
Next, we show the growth and shrinkage of the ice fields surrounding Antarctica as sensed by ESMR that appeared earlier in this section. Below these images are SMRR images that show changes in the ice cover for four years, during the 1978-86 period, in the top diagram.
The ice shelf surrounding the Antarctic continent extends sometimes 100s of kilometers from shore. Periodically parts of its edge will break loose and float free in the southern ocean. In 1995 a large (greater than Rhode Island in size) raft of ice, assigned the identifier B10, separated and starting moving north. It later broke again into two sections. Small pieces continue to break off (calve) as icebergs. Here is a Landsat image of B10A showing the sheet and its offspring 'bergs:
In early 2002, another great slab of ice from the Larsen ice shelf, between the Bellingshausen and Wendell seas, broke off and is now cruising the sea off Antarctica. Here is a scene during this rupture stage, imaged by the ASAR radar on the Envisat (see Overview) launched in February 2002.
As temperatures continue to rise slightly in the Antarctic, more ice from the shelves is breaking away. Consider this image of the Wilkins Ice Shelf, as imaged by the SAR on Envisat:
While some changes in sea ice (and ice cap) cover, both in area and thickness, may be progressing towards lower overall areal extent because of natural/man-induced global warming, there are as well normal seasonal fluctuations. Here are two sets of images made by the AMSR-E sensor on Aqua (page 16-11) of Arctic sea ice (left) and Antarctic shelf ice (right) during two periods in 2002 between June and July:
Both ice and clouds appear white in some images. But using different bands, a distinction can be made. Here is an image of Antarctica. The question arises: is this all clouds or is there a solid surface within the scene. The image on the right was processed using selective bands. It brings out the few clouds present and discloses the other patterns to be ice markings on the continental ice surface:
In the northern hemisphere, the ice shrinks in extent as the summer progresses. But in the southern hemisphere, where it is winter at this time of the year, the ice shelf around the Antarctic increases.
In the U.S., winter ice on the Great Lakes becomes a major impediment to shipping and usually some or all of the lakes are closed to normal travel. The AVHRR on NOAA satellites daily monitors the status of ice cover, giving results like this thermal IR image, made on January 31, 1996 (ice is light-toned because its temperature is above that of the land that is experiencing a cold-air outbreak from a Canadian airmass that has cooled land surfaces to lower temperatures than the ice; clouds appear very dark):
The latest addition to the "stable" of ice-monitoring satellites is ICESat, which stands for Ice, Clouds, and land Elevation Satellite (check out its Home Page). Part of the ESE (Earth Sciences Enterprise; see page 16-6) series, the primary mission is to map changes in elevation of major concentrations of ice (Antarctica, the Arctic, Greenland) over time to gain precise data on melting or accumulation of ice mass which can have a major effect on changing sealevel worldwide. This simple diagram shows the ice model used:
The ICESat orbits at a 600 km altitude such as to cross the polar regions frequently over short time spans. The satellite's position is accurately established by radio linkage to Global Positioning Satellites (GPS series). The prime ICESat instrument is GLAS (Geoscience Laser Altimeter System). The laser used is the Nd:YAG type; this uses a Yttrium Aluminum Garnet (a manmade mineral with a specific crystal structure) in which some of the Yttrium is substituted for by Neodymium (Nd+3) (said to be "doped"). In the mode used, this laser produces signals at two wavelengths (green: 0.532µm, used to determine aerosols and IR: 1.064µm [twice the first], used to measure elevations. The pulses are sent in bursts of 40 seconds (traveling at the speed of light), each covering a circular surface footprint 75 meters wide and pairs spaced 175 m apart, and they require 5 nanoseconds to travel from satellite to Earth and back. Very slight differences in travel time allow surface elevation changes of 10 cm to be measured accurately.
ICESat was launched on January 13, 2003 and required a few months to refine the operation of GLAS. But now a number of laser-determined elevation profiles have been produced. Here is one from the glacier off the Ross Ice Shelf in the Antarctic:
The GLAS sensor has now obtained enough topographic measurements to produce an elevation map of all but the central part of the Antarctic:
This next illustration shows off the ability of the 0.532 µm laser pulses to obtain information about clouds and dust in the atmosphere.
Although its primary mission is to measure ice surface changes, the ability to determine heights from space has other applications, one of which is to determine canopy heights for forests, as presented at the bottom of page 3-5.