backscatter of sea-ice


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Radar backscatter of sea-ice

One fourth of the backscatter coefficients obtained from the AMI-WIND are generated over the polar oceans and are available to improve our knowledge of sea-ice. In active micro-wave, sea-ice areas appear as isotropic surfaces. Their signatures are not related to the azimuthal angle but to the incidence angle of the observation. Therefore, the forward and rear beams, observing the surface with identical incidence angles, provide very similar measurements over pure ice. However, the radar backscattering varies considerably with the incidence angle. In polar areas, 4 different curves of the backscatter, expressed in dB, as a function of the incidence angle can be related to the composition of the observed surface.

	The open water curve shows a strong decrease with the incidence angle and a strong variability with the wind strength around average values.
	The compact first-year ice shows relatively low backscatter coefficients. The backscatter coefficients decrease linearly with the incidence angles
	The multi-year ice, which has survived the melt period, is broken and less saline. High backscatter coefficients are attained on this type of ice. The low angular dependence shows the important contribution of the volume scattering. The chaotic distribution of the bubbles diminishes the effect of the incoming radiation geometry.
	The marginal ice, covering huge surfaces on the southern oceans, shows a very characteristic behavior of its backscatter curve. From that point of view, marginal ice can be distinguished from open water and from the surrounding first-year ice. Between 20° and 30°., the backscatter coefficients vary strongly as a function of the incidence angle while, at 50°. the derivative of the curve is lower and high values of the backscatter coefficients are observed.

Weekly images of key-parameters of the backscatter curves were generated at CERSAT, from august 1991 to may 1995. Two of those parameters characterize efficiently the state of the polar ocean surface.

The derivative of the backscatter as a function of the incidence angle, chosen at the steep angle of 28°, is an efficient parameter for discriminating ice and water. The highest values of this parameter being obtained for open water, marginal ice zones, in which the water concentration is significant, are colored in yellow-orange on the polar grids. As sea-ice in polynyas (regions more or less free of ice) shows similar signatures due to the water contribution to the backscatter, one can observe on the image the formation of the Weddell Sea polynya (at about 5° E, 65° S) within the body of the ice pack. In contrast to new ice, the lowest values of the derivatives are indicative of concentrated and broken ice, very often multi-year ice or fast-ice drifting away from the coast.

Southern ocean grids
from 15 to 21/08/94

Scale :
- derivative from 0 to 0.5 dB/deg
- backscattering from -23.3 to -6.7 dB

The backscatter coefficient at large incidence angle,i.e. 50°, shows a strong sensitivity to the ice surface topography and to the ice/water transitions. Marginal ice and multi-year ice are characterized by the highest values. The Weddell sea polynya area can be considered as marginal ice zone and is clearly distinguished from the consolidate first-year ice on the images of the animation.

By clicking on this image of backscatter coefficient at 50 degrees, you obtain an animation (mpeg) of the images for this parameter from may to august 1994.

The size of the polynya and its central position within the pack have considerable effects on the exchanges Ocean/Atmosphere and on the deep water renewal. On one hand the thermal contrast between water at -1.5°C and a much colder air favors the venting of large heat fluxes into the atmosphere while, on the over hand, the cold surface water sinks and enhances the convection as the warm water rises. Open-ocean polynya like the Weddell Sea polynya and coastal polynyas contribute to the formation of the Antarctic bottom water which then moves well beyond the equator.

The mechanisms leading to the formation of open-sea polynyas are not yet elucidated but they are studied as major oceanographic events which influence the general circulation of the world ocean and the world climate. Thanks to the space-borne sensors, working in active and passive micro-wave modes, observations of these highly variable phenomena are now carried on routinely.