A polarized discrete ordinate scattering model for radiative transfer simulations in spherical atmospheres with thermal source

This work describes the development of the new
discrete ordinate scattering algorithm, which is a part of the
Atmospheric Radiative Transfer Simulator (ARTS).
Furthermore, applications of the algorithm, which was
implemented to study for example the influence of cirrus clouds on
microwave limb sounding, are presented.

The model development requires as a theoretical basis the electromagnetic
scattering theory. The basic quantities are defined and different methods
to compute single scattering properties of small particles are
discussed. The phenomenological derivation of the vector radiative
transfer equation, which is the basic equation of the model, is
outlined. In order to represent clouds as scattering media in radiative
transfer models, information about their micro-physical state is
required as an input for calculating the scattering properties.
The micro-physical state of a cloud is defined by the phase of the cloud
particles, the particle size and shape distributions, the particle
orientation, the ice mass or
the liquid water content, and the temperature.

The model uses the Discrete Ordinate
ITerative (DOIT) method to solve the vector radiative transfer equation.
The implementation of a discrete ordinate method is challenging due
to the spherical geometry of the model atmosphere, which is required
for the simulation of limb radiances. The involved numerical issues,
grid optimization and interpolation methods, are discussed.

The new scattering algorithm was compared to three other models, which
were developed during the same time period as the DOIT
algorithm. Overall, the agreement between the models was very good,
giving confidence in new models.

Scattering simulations are presented for limb- and down-looking
geometries, for one-dimensional and three-dimensional spherical
atmospheres. They were performed for the frequency bands of the
Millimeter Wave Acquisitions for Stratosphere/Troposphere Exchange
Research (MASTER) instrument, and for selected frequencies of the
Earth Observing System Microwave Limb Sounder (EOS MLS).
The simulations show the impact of cloud particle size, shape and
orientation on the brightness temperatures and on the polarization
of microwave radiation in the atmosphere. The cloud effect is much
larger for limb radiances than for nadir radiances. Particle size is
a very important parameter in all of the simulations. The polarization
signal is small for simulations with randomly
oriented particles whereas for horizontally aligned particles with
random azimuthal orientation the polarization signal is significant.
Moreover, the effect of particle shape is only relevant for oriented
cloud particles. The simulations show that it is essential to use a
three-dimensional scattering model for inhomogeneous cloud layers.