This algorithm computes the OD retrievals at the CALIOP native horizontal resolution of 333m, called SODA
The following document describes from a high level point of view the synopsis and the theorical basis of the SODA 333m algorithm ( also known as CCSRM )
The error sources of SODA have been looked at carefully [Josset et al. GRL 2008, IEEE GRSL 2010]. However, as a new product, further evaluation is needed on a larger scale basis.
Efforts have been focused up to now on validating the 532 nm optical depth. Validation of the 1064 nm retrieval should be done soon.
SODA is a self calibrated product based on a monthly analysis of the ocean surface echo. Problems can be expected when there is a satellite maneuver or a CALIOP boresight alignements which correspond to signal variations important enough to impact the monthly calibration procedure. User should refer to the list of related actions on the NASA/LaRC website.
SODA was primarily designed to study aerosols, although it seems the domain of validity can be extended to ice and water clouds, the current version of SODA does not contain all what is needed for this kind of research. It has to be kept in mind that :
- SODA retrieves the effective attenuation of the laser light and the total column effective optical depth are then derived. For aerosols, it is shown to be close from the optical depth as measured by MODIS [Josset et al. GRL 2008, IEEE GRSL 2010]. For water and ice clouds, multiple scattering should be taken into account and this is not the case in the first version of SODA, which provides effective optical depths. For water and ice clouds, SODA optical depth will typically be lowered by a factor going up to several units [Hu et al. Optics letters 2006] with respect to the single scattering optical depth.
- Marine boundary layer aerosols will likely be present and included in all cloud optical depth retrievals.
- CloudSat beam is attenuated by liquid water clouds. Dense features are excluded in the current version of SODA but the interpretation of the optical thickness for a mixed aerosol/liquid water cloud feature is not straightforward. The expected attenuation (in dB) as a function of the liquid water path is approximatively linear and is reaching 6dB for LWPs of 0.8 kg/m2 (D. Josset, thèse UPMC, Paris, 2009).