<title>Cloud altimetry and water phase retrieval from POLDER instrument during EUCREX '94</title>

Abstract

POLDER (POLarization and Directionality of the Earth's Reflectances) is a new instrument devoted to the global observation of the polarization and directionality of solar radiation reflected by the Earth surface-atmosphere system. The instrument concept has been accepted on the Japanese ADEOS platform scheduled to be launched early 1996. The original capabilities of POLDER, compared to previous current radiometers are, (1) its polarized reflectance measurements in the visible and near-infrared range of the solar spectrum, (2) its capability to measure a surface target reflectance from about 10 directions during a single pass. A method for cloud phase retrieving from POLDER measurements is tested. Indeed, liquid water clouds could be discriminated from ice clouds provided they exhibit distinct polarization signatures. In the rainbow region (scattering angles of about 140 degree(s)), water droplets strongly polarize incident sunlight while ice crystals probably do not. This feature is examined on data acquired by the airborne POLDER instrument over cirrus and stratocumulus clouds during the EUCREX'94 (EUropean Clouds and Radiation EXperiment, April 1994) experiment. Moreover, over clouds, the polarized component of the reflectance at the wavelength of 443 nm and scattering angle of 90 - 100 degree(s) is sensitive to molecular optical thickness between the cloud top and the satellite altitude and, therefore, may be used for cloud altimetry. On the other hand, a method for cloud top pressure retrieval from POLDER measurements based on a differential absorption technique is presented. It makes use of the ratio of two radiances measured in the absorption A band of the oxygen (at 763 nm). The two different methods are compared on data acquired during EUCREX'94. Considering the main limitations of the instrument and the methods, the two mean retrieved cloud-top pressures are found to be in good agreement and are close to the expected true one.