Single-footprint retrievals for AIRS using a fast TwoSlab cloud-representation model and the all-sky infrared radiative transfer algorithm

Dataset for AMT-2017-261 by DeSouza-Machado et. al.

1D-variational retrievals of temperature and moisture fields from
hyperspectral infrared satellite sounders use cloud-cleared radiances
as their observation. These derived observations allow the use of
clear-sky only radiative transfer in the inversion for geophysical
variables but at reduced spatial resolution compared to the native
sounder observations. Cloud-clearing can introduce various errors,
although scenes with large errors can be identified and
ignored. Information content studies show that when using multi-layer
cloud liquid and ice profiles in infrared hyperspectral radiative
transfer codes, there are typically only 2-4 degrees of freedom of
cloud signal. This implies a simplified cloud representation is
sufficient for some applications which need accurate radiative
transfer. Here we describe a single-footprint retrieval approach for
clear and cloudy conditions, which uses the thermodynamic and cloud
fields from Numerical Weather Prediction (NWP) models as a first
guess, together with a simple cloud representation model coupled to a
fast scattering radiative transfer algorithm (RTA). The NWP model
thermodynamic and cloud profiles are first co-located to the
observations, after which the N-level cloud profiles are
converted to two slab clouds (typically one for ice and one for water
clouds). From these, one run of our fast cloud representation model
allows an improvement of the \emph{a-priori} cloud state by comparing the
observed and model simulated radiances in the thermal window
channels. The retrieval yield is over 90\%, while the degrees of
freedom correlate with the observed window channel brightness
temperature which itself depends on the cloud optical depth. The cloud
representation/scattering package is bench-marked against radiances
computed using a Maximum Random Overlap cloud scheme. All-sky infrared
radiances measured by NASA’s Atmospheric Infrared Sounder (AIRS) and
NWP thermodynamic and cloud profiles from the European Center for
Medium Range Weather Forecasting (ECMWF) forecast model are used in
this paper.