Abstract
One-dimensional variational retrievals of
temperature and moisture fields from hyperspectral infrared (IR) satellite
sounders use cloud-cleared radiances (CCRs) 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 multilayer cloud
liquid and ice profiles in infrared hyperspectral radiative transfer codes,
there are typically only 2–4 degrees of freedom (DOFs) 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 (TwoSlab; typically one for ice and one for water clouds). From these, one run of our
fast cloud-representation model allows an improvement of the 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
(BT) which itself depends on the cloud optical depth. The
cloud-representation and scattering package is benchmarked against radiances computed
using a maximum random overlap (RMO) cloud scheme. All-sky infrared radiances
measured by NASA's Atmospheric Infrared Sounder (AIRS) and NWP
thermodynamic and cloud profiles from the European Centre for Medium-Range
Weather Forecasts (ECMWF) forecast model are used in this paper.
Citation
ID:
263976
Ref Key:
desouzamachado2018singlefootprintatmospheric