Three-dimensional radiative transfer simulations as a tool to test remote sensing algorithms

BERNHARD MAYER

Deutsches Zentrum fuer Luft- und Raumfahrt (DLR)
Oberpfaffenhofen, 82234 Wessling, Germany



The three-dimensional radiative transfer model MYSTIC (Monte Carlo
code for the physically correct tracing of photons in cloudy
atmospheres) is used to simulate observations of different satellite
sensors. In combination with the libRadtran package for radiative
transfer (http://www.libradtran.org), MYSTIC allows the
realistic simulation of satellite observations, considering all
relevant atmospheric and surface properties and taking into account 
the spectral response of specific instruments. Together with a
cloud-resolving model, this forms an ideal test bed for addressing
specific questions like biases in satellite retrieved liquid water
paths or for testing remote sensing algorithms. Examples are shown for
the MISR, AVHRR, and MSG/SEVIRI instruments, using model-generated
clouds as input to the three-dimensional radiative transfer code. The
effect of cloud inhomogeneity on the retrieved optical thickness is
demonstrated for different spatial scales, ranging from 275 m (MISR) 
to 3 km (SEVIRI). While large pixel-to-pixel uncertainties prevail at the
higher resolutions, the plane-parallel bias is obvious at all
scales: the optical thickness is often under-estimated when
the clouds are considered homogeneous over the satellite's
field-of-view. These results clearly demonstrate the potential of the
MYSTIC model to test existing and help develop new retrievals of cloud
parameters.