Abstract. Simulations of total and polarized cloud reflectance angular signatures such
as the ones measured by the multi-angular and polarized radiometer
POLDER3/PARASOL are used to evaluate cloud heterogeneity effects on cloud
parameter retrievals. Effects on optical thickness, albedo, effective radius
and variance of the cloud droplet size distribution and aerosol parameters
above cloud are analyzed. Three different clouds that have the same mean optical
thicknesses were generated: the first with a flat top, the second
with a bumpy top and the last with a fractional cloud cover. At small
scale (50 m), for oblique solar incidence, the illumination effects lead to
higher total but also polarized reflectances. The polarized reflectances even
reach values that cannot be predicted by the 1-D homogeneous cloud assumption.
At the POLDER scale (7 km × 7 km), the angular signature is modified by a
combination of the plane–parallel bias and the shadowing and illumination
effects. In order to quantify effects of cloud heterogeneity on operational
products, we ran the POLDER operational algorithms on the simulated
reflectances to retrieve the cloud optical thickness and albedo. Results show
that the cloud optical thickness is greatly affected: biases can reach up to
−70, −50 or +40 % for backward, nadir and forward viewing
directions, respectively. Concerning the albedo of the cloudy scenes, the
errors are smaller, between −4.7 % for solar incidence angle of
20∘ and up to about +8 % for solar incidence angle of
60∘. We also tested the heterogeneity effects on new algorithms
that allow retrieving cloud droplet size distribution and cloud top pressures
and also aerosol above clouds. Contrary to the bi-spectral method, the
retrieved cloud droplet size parameters are not significantly affected by the
cloud heterogeneity, which proves to be a great advantage of using polarized
measurements. However, the cloud top pressure obtained from molecular
scattering in the forward direction can be biased up to about 60 hPa (around
550 m). Concerning the aerosol optical thickness (AOT) above cloud, the
results are different depending on the available angular information. Above
the fractional cloud, when only side scattering angles between 100
and 130∘ are available, the AOT is underestimated because of the
plane–parallel bias. However, for solar zenith angle of 60∘
it is overestimated because the polarized reflectances are increased in
forward directions.