Clouds

The models used to predict potential climate changes related to the increase in greenhouse gases and anthropogenic aerosols require a realistic description of global cloudiness and its associated properties.

Polarized and bidirectional reflectances from the clouds are analyzed to improve the characterization of both their macrophysical properties (three-dimensional structure, validity of the plane-parallel hypothesis) and their microphysical properties (phase - water or ice - particle shape and size).

The POLDER derived cloud properties, such as cloudiness, cloud phase, cloud optical thickness, cloud pressure (computed with two different methods) intend to contribute to a better cloud climatology on a global scale.

	Composite of total reflectances at 443, 670 and 865 nm	Composite of polarized reflectances at 443, 670 and 865 nm
Stratocumulus banks off the Chile coast (1996 Nov. 12). On the polarized image, the iridescence inside the bright arch (140° scattering angle) may be used for high precision determination of cloud droplets size (source: LSCE).
Depression over south Indian Ocean (1996 Nov. 28). High level icy cloud (right) masks the high polarization arch due to underlaying low level broken clouds (source: LSCE).

Modified polarized radiances observed by POLDER as a function of scattering angle. Polarization signatures of liquid (red) and ice (blue) clouds display significantly different features that allow for simple and direct cloud thermodynamic phase discrimination (source: LOA)

Water Vapor

Atmospheric water vapor is a variable that fluctuates greatly over time and space. It is a vector of the energy exchanges at the surface/atmosphere interface and in the atmosphere by the liberation of latent heat.

POLDER is able to estimate the integrated water vapor content in clear sky conditions, either above continents or oceans in glitter conditions, by means of differential absorption techniques using the 910 nm and 865 nm channels. These evaluations, with an expected accuracy of around 10 %, complement the existing measurements, especially over land, which suffers from sparse sounding sampling and the physical limitations of microwave and infrared techniques.

Comparison of TCWV (Total Column water Vapor) from ADEOS 1-POLDER measurements and TCWV from radiosonde measurements for November 1996 and June 1997. The root mean square difference is 2.8 kg m-² (source: LOA).

Earth Radiation Budget

The anisotropy of the radiation measured from space, together with diurnal variability and the calibration accuracy of the sensors, is one of the major sources of uncertainty in the determination of the Earth Radiation Budget. The conversion of the measured radiances into fluxes assumes a priori knowledge of the angular behavior.

POLDER performs near-simultaneous multidirectional measurements and consequently aids to the estimation of TOA (top of the atmosphere) shortwave reflected fluxes.

Shortwave flux density (Wm-2) derived from ADEOS-1 POLDER observations for March 1997 (source: LMD)