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IFS documentation Front Page
Chapter 2. Observations: Types, variables and error statistics
Chapter 3. CMA creation (MAKEMA)
Chapter 4. The FEEBACK task
Chapter 5. The TOOLS task
Chapter 6. Central-memory array (CMA) structure/format
Chapter 7. BUFR feedback data structure/format
Chapter 8. SIMULATED-observations data structure/format
Chapter 9. NAMELISTS
Chapter 10. Processing of scatterometer data
REFERENCES
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10.1 Overview
Scatterometer data belong to those kinds of observation that need to be transformed into the variables used by the analysis within OBSPROC. The transformation consists here in converting the backscatter coefficient measurements acquired by the instrument (
triplets) into the two ambiguous 
and 
wind components that will actually be assimilated
into the IFS.This task is performed within the CMA creation process by the scatterometer data-handling subroutine SCATSIN from OBSCREEN, and especially by its core subroutine ERS1IF (ERS-1 interface). Like SCATSIN, ERS1IF deals only with one scatterometer report at a time. Moreover, either all or part of the observations can be treated, according to the thinning set up in SCATSIN. This thinning, applied only in the screening, is detailed in the corresponding part of the IFS documentation; it is controlled in OBSPROC by logical switches LSCATTHI and LTOTSCTH, which respectively enable its use and make it total, i.e. it invalidates the further processing of those reports that are not to be eventually kept.
The main purpose of ERS1IF is to retrieve the wind components by inverting the transfer function relating the backscatter coefficient
to the wind speed and direction. Some quality control
is also done in the process, based on the quality information provided with
the raw data as well as on the residual from the wind retrieval, reflecting
the agreement between the measurements and their theoretical wind dependency.
Moreover, bias corrections are applied, both in terms of 
and of wind speed, particularly to compensate for any change in the
instrumental calibration and to ensure consistency between the retrieved
and model winds. The whole procedure follows closely the PRESCAT wind retrieval and ambiguity-removal scheme developed for the ERS-1 scatterometer from the transfer function CMOD4 (Stoffelen and Anderson, 1997). Some aspects have already been addressed in Gaffard and Roquet (1995). Also, the description given here applies only to the ERS scatterometer data currently used in the IFS. Different processings would be necessary for data from other instruments such as NSCAT, for example, although the general principles may remain the same. A general reminder about the ERS scatterometers and their measurement geometry can be found in Stoffelen and Anderson (1997) as well.
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18.04.2002 |
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