A better understanding of cloud optical thickness derived from the passive sensors MODIS/AQUA and POLDER/PARASOL in the A-Train constellation Laboratoire Optique d'Atmosphérique, CNRS, UMR8518, Université Lille 1, Villeneuve d'Ascq, France
03 Dec 2012
Received: 06 Mar 2012 – Published in Atmos. Chem. Phys. Discuss.: 08 May 2012 Abstract. Cloud optical thickness (COT) is one of the most important parameter for the
characterization of cloud in the Earth radiative budget. Its retrieval
strongly depends on instrument characteristics and on many cloud and
environment factors. Using coincident observations from POLDER/PARASOL and
MODIS/AQUA in the A-Train constellation, geographical distributions and
seasonal changes of COT are presented, in good agreement with general cloud
climatology characteristics. Retrieval uncertainties mainly associated to
sensor spatial resolution, cloud inhomogeneity and microphysical assumptions
Revised: 30 Sep 2012 – Accepted: 02 Nov 2012 – Published: 03 Dec 2012
Comparisons of COT derived from POLDER and MODIS illustrate that as the
primary factor, the sensor spatial resolution impacts COT retrievals and
statistics through both cloud detection and sub-pixel cloud inhomogeneity
The uncertainties associated to cloud microphysics assumptions, namely cloud
phase, particle size and shape, also impact significantly COT retrievals.
For clouds with unambiguous cloud phase, strong correlations exist between
the two COTs, with MODIS values comparable to POLDER ones for liquid clouds
and MODIS values larger than POLDER ones for ice clouds. The large
differences observed in ice phase cases are due to the use of different
microphysical models in the two retrieval schemes. In cases when the two
sensors disagree on cloud phase decision, COT retrieved assuming liquid
phase is systematically larger.
The angular biases related to specific observation geometries are also
quantified and discussed in particular based on POLDER observations. Those
exhibit a clear increase of COT with decreasing sun elevation and a decrease
of COT in forward scattering directions due to sub-pixel inhomogeneities and
shadowing effects, this especially for lower sun. It also demonstrates
unrealistic COT variations in the cloudbow and backward directions due to
inappropriate cloud optical properties representation and an important
increase of COT in the sun-glint directions in case of broken cloud.
Citation: Zeng, S., Cornet, C., Parol, F., Riedi, J., and Thieuleux, F.: A better understanding of cloud optical thickness derived from the passive sensors MODIS/AQUA and POLDER/PARASOL in the A-Train constellation, Atmos. Chem. Phys., 12, 11245-11259, doi:10.5194/acp-12-11245-2012, 2012.