Version 2.3: 10th June 2002
Version 3.1: 23rd March 2004
Version 3.2: 30th August 2006
This software and documentation was developed within the context of the
EUMETSAT satellite Application Facility (NWP SAF). The partners in the
NWP SAF are the Met Office, ECMWF, KNMI, and Météo-France.
If the parameters Cloud_Top_Pressure and Cloud_Fraction
are requested to be retrieved in the
Retrievals.NL namelist, the 1DVar retrieval performed is slightly different.
In this case the internal logical variable CloudyRetrieval is set to
.TRUE.. Note that this cloud retrieval is using the simple, grey cloud
approximations that have been present in all versions of RTTOV and not the later
RTTOVCLD code. RTTOVCLD has not been implemented in this 1DVar code as yet.
The differences in processing when cloudy retrievals are used are:
B-matrix elements for cloud top pressure and cloud fraction are not
required, whereas for any other parameter a corresponing B-matrix element is
mandatory. If these B-matrix elements are missing, the cloud top pressure and
cloud fraction elements are large, i.e., there is no a priori knowledge
of these parameters.
A mimimum residual cloud parameter retrieval step is used to improve the first guess cloud top
pressure and cloud fraction. This is a simple method with
no a priori information considered and no channel weighting (e.g., due
to observational error differences). The method is described in Eyre and Menzel (QJRMS 28, p267-275, 1989). The channels specified for
cloud detection are used
here (there is no cloud detection if clouds are being retrieved).
An additional cost function term is available to ensure (without
imposing hard limits) that the cloud
parameters do not go beyond their physically reasonable limits.
This is done through the
subroutine. The form of the additional cost function value is proportional to
the cube of the amount by which the retrieved value exceeds the boundary.
It is recommended that Marquardt-Levenberg minimisation is used for cloudy
[ Return to Main Manual ]