In order to simulate the upwelling radiance measured by a satellite
instrument (e.g. for a near nadir viewing sounder like ATOVS),
the electromagnetic radiation emitted or absorbed by gases along
the viewing path and from the surface and/or cloud must be computed.
For radiance assimilation in a NWP model this calculation must
be made in a few milliseconds for all the radiance channels in
order not to delay the production of the forecast model. Hence
the development of fast radiative transfer (RT) models is an important
activity of the satellite infrared sensing group to enable assimilation
of satellite radiances within a NWP model. In variational assimilation
terminology the radiative transfer model is part of the 'observation
operator' which also includes the interpolation from the NWP model
pressure levels to the RT model levels.
|Current Fast RT Models:
RTTOV model is used for rapid RT calculations for all the
meteorological satellite instruments of current interest.
The present version of RTTOV (RTTOV-7) can simulate radiances
for many different sensors. More details can be found here.
The plot on the left shows the accuracy of the RTTOV-6 and
earlier RTTOV-5 model calculations for NOAA-15 ATOVS by
comparing with an accurate line-by-line model calculations
for a diverse set of different atmospheric profiles. Click
on the plot for a higher resolution version.
The model is developed under the EUMETSAT funded NWP-satellite
application facility and the Met Office, Météo-France
and ECMWF all contribute to its development.
With the forthcoming high resolution infrared sounders (e.g.
AIRS, CrIS and IASI) upgrades to the RTTOV model need to be made
in order to ensure we can efficiently simulate a significant number
of channels accurately within a sufficiently fast time period
to be able to use for radiance assimilation. This applies not
only to the forward model calculation but also to the calculation
of the Jacobian matrix (i.e. the derivative of the radiances for
each channel with respect to the profile variables. This is a
focus of the research activities within the group at present.
The Gastropod model was developed with advanced sounders in mind.
To develop fast models an accurate but relatively slow line-by-line
radiative transfer model is required where all the radiative processes
occurring in the atmosphere can be modelled explicitly. Currently
both infrared and microwave line-by-line models are used by group
members. Follow the link below to find out more details of line-by-line
model calculations and how they can be used to develop a fast RT
about line-by-line RT models