POLDER 2 - CLOUD PROPERTIES
CLOUD PROPERTIES
The cloud properties derived from POLDER "ERB & clouds" algorithms are cloud phase and microphysical index from polarization measurements at 865 nm, cloud optical thickness from radiance measurements at 670 nm over land and 865 nm over ocean, and cloud pressures from polarization measurements at 443 nm (cloud top pressure) and from oxygen absorption measurements near 763 nm (cloud middle pressure).
Cloud phase and microphysical index
Because cloud optical properties strongly depend on shape and size of condensed water particles composing it, thermodynamic phase is a critical parameter for most clouds analysis. The new BRN algorithm now differentiates between liquid and ice clouds for optical thickness and cloud top pressure retrievals. Therefore, cloud phase determination comes more than ever as a first and compulsory step for further cloud analysis.
A short description is given here.
A more complete description can be downloaded from the "ERB, WV & clouds" algorithm documents for POLDER-2. If there are further questions regarding the cloud phase derivation, please contact Dr. J. Riedi (at LOA). For questions regarding the microphysical index, please contact Dr. L. C.-Labonnotte (at LOA)
See below the main parameters related to cloud phase and microphysical index that appear in the level-2 POLDER "ERB, WV & clouds" product.
| Phase | Cloud thermodynamic phase. Click to view image |
| Micro-Index | Microphysical index calculated only for ice clouds (not yet available). |
See below the main parameters related to cloud phase and microphysical index that appear in the level-3 POLDER "ERB, WV & clouds" product.
| FREPHASE | Frequency of occurrence of cloud phase in 4 classes (1: successful phase determination; 2, 3, 4: among successful determinations, relative frequency of liquid, ice and mixed clouds respectively)[%]. Click to view liquid image Click to view ice image Click to view mixed clouds image |
| FRECRYSTAL | Frequency of occurrence of ice crystal type (currently not available). |
Cloud optical thickness
The cloud optical thickness is derived at 670 nm over land and 865 nm over ocean from radiative transfer modeling based on the plane-parallel hypothesis.
A short description is given here.
A more complete description can be downloaded from the "ERB, WV & clouds" algorithm documents for POLDER-2. If there are further questions regarding the cloud optical thickness derivation, please contact Pr. J.C. Buriez (at LOA)
See below the main parameters related to cloud optical thickness that appears in the level-2 POLDER "ERB, WV & clouds" product.
| TAU | Linear mean cloud optical thickness at 670 nm over land and 865 nm over ocean, result from an averaging over cloudy pixels and a weighting over directions. Click to view image |
| RSD(TAU) | Relative Spatial standard Deviation of the cloud optical thickness at 670/865 nm |
| Epsilon | Indicator of the radiative variability of the cloud optical thickness distribution. It is defined by  = 1 - TAU* / (CC x TAU) where TAU* and CC x TAU are respectively the radiative and the linear mean cloud optical thickness of the whole superpixel. It is equal to 0 for a homogeneous distribution and increases towards 1 as the variability increases. |
| S | Mean cloud spherical albedo (defined over black surface with no atmosphere) at 670/865 nm, result from an averaging over cloudy pixels and a weighting over directions. Note that the mean "directional" cloud spherical albedo is also furnished for each viewing direction |
See below the main parameters related to cloud optical thickness that appears in the level-3 POLDER "ERB, WV & clouds" product.
| M(TAU) | Monthly mean of the cloud optical thickness at 670 nm over land and 865 nm over ocean |
| RTD(TAU) | Relative Temporal standard Deviation of the cloud optical thickness at 670/865 nm |
| M(TAU)liquid | Monthly mean of the liquid water cloud optical thickness at 670/865 nm. Click to view image |
| M(TAU)ice | Monthly mean of the ice cloud optical thickness at 670/865 nm. Click to view image |
| M(TAU)mixed | Monthly mean of the mixed-phase cloud optical thickness at 670/865 nm. Click to view image |
| M(S) | Monthly mean of the cloud spherical albedo at 670/865 nm |
| TD(S) | Temporal standard Deviation of the cloud spherical albedo at 670/865 nm |
CLOUD PRESSURES
Two different methods were developed to retrieve cloud pressure from POLDER data. The first one (Cloud Rayleigh pressure) is based on the analysis of polarized reflected light at 443 nm. The second one (Cloud Oxygen pressure) uses the ratio of the two POLDER radiances measured in the oxygen A-band near 763 nm.
A short description is given here.
A more complete description can be downloaded from the "ERB, WV & clouds" algorithm documents for POLDER-2. If there are further questions regarding the derivation of Rayleigh and Oxygen pressure derivation, please contact Dr C. Vanbauce (at LOA
See below the main parameters related to cloud pressures that appear in the level-2 POLDER "ERB, WV & clouds" product.
| Pray | Mean cloud Rayleigh pressure. It is expected to be close to the cloud top pressure. Click to view image |
| AD(Pray) | Angular standard Deviation of the cloud Rayleigh pressure derived by direction. |
| Poxy | Mean cloud Oxygen pressure. It is expected to be close to the cloud middle pressure. Click to view image |
| AD(Poxy) | Angular standard Deviation of the cloud Oxygen pressure derived by direction |
See below the main parameters related to cloud pressures that appear in the level-3 POLDER "ERB, WV & clouds" product.
| M(Pray) | Monthly mean of the cloud Rayleigh pressure. Click to view image |
| TD(Pray) | Temporal standard Deviation of the cloud Rayleigh pressure |
| M(Poxy) | Monthly mean of the cloud Oxygen pressure. Click to view image |
| TD(Poxy) | Temporal standard Deviation of the cloud Oxygen pressure |
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