CONCEPT

The wealth of spectral, directional and polarized POLDER measurements opens up new opportunities for determining cloud phase and altitude, and for discriminating aerosols and biosphere parameters.

Polarization

Natural light from the sun is not polarized, or rather it is ramdomly polarized. However, certain physical processes orient electric vibrations, thus generating polarized light.

As illustrated on the examples below, polarization, when combined to directionality, adds another dimension to the conventional analysis of spectral signatures of aerosols and clouds.

1 - The reflection of light by a mirror is a highly polarizing process.
Sun glitter on the surface generates a bright spot (A) on natural light images as well as a strong signal (B) on polarized images. The rougher the surface, the wider and less intense the glitter pattern.

2 - Apart from reflection phenomena, the polarized component measured at the top of the atmosphere results primarily from a single scattering mechanism characterized by scattering angle .
POLDER polarized images are thus easier to interpret by overlaying scattering angle isolines.
The backscatter area (C) is dark because polarization is close to zero when =180°.

3 - Polarization from molecular scattering in the atmosphere prevails over the polarized contribution from the surface. This explains why the color blue is predominant and why there is little contrast between land and sea on polarized images taken in clear sky conditions. Over clouds (in white because they are spectrally neutral) the molecular polarization enables to retrieve the cloud altitude.

This image, taken in the southern hemisphere on November 10, 1996, shows 2 cases of spectacular reflection: a broad sun glitter pattern over the ocean partly hidden (upper center) by a dense cirrus cloud, and a sharp, peaked glitter around =93° on the cirrus itself, which indicates that ice crystals in the cloud are mostly lying horizontally. Irisation may also be seen in the cloudbow (=140°). It can be used for an accurate estimation of the size of water droplets in this type of cloud.

4 - The rainbow is better known for its colors than for its directional and polarized properties! It is characteristic of the scattering of light by water droplets observed at a scattering angle of 140° (note that the observer has then his back to the sun). In such viewing conditions, liquid water clouds have a strong polarizing effect which clearly distinguishes them from ice clouds.

5 - Once molecular scattering and ground polarized contributions have been substracted, the residual signal provides information on aerosol load in the atmosphere.

Images of minimum and maximum polarized radiances observed between 11/16/96 and 12/18/96 showing the detection of aerosols in the Ganges Valley.