D38    3DVar slant delay observation operator implementation.

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This is part of work-package 6300, the purpose of which is observation operator development for slant delays.

Objectives:  To develop a slant delay observation operator for NWP.

Methodology/Work Description: The actual GPS signal delays are measured on slant-profiles of the atmosphere. The aim in the NWP variational data assimilation is to make use of the measurements close to the raw data format with little data pre-processing. The natural first step towards using slant-delay measurements in NWP is to properly evaluate them against the model counterparts. For this task an appropriate observation operator is needed. The zenith delay observation operator is simple to develop, as the observation geometry is relatively straightforward and similar to the NWP model geometry. The slant-delay observation operator, in contrast, requires a model profile along a slanted path with unknown intersects with the model levels. Once the iterative problem of interpolating the model variables on a slanted path is solved, the associated delay can be fairly easily solved.

* FMI will develop a demonstration version of observation operator in co-operation with the expertise of the KNMI
* FMI, together with KNMI, will adjust a three-dimensional variational data assimilation system of for the HIRLAM-model for assimilation of (simulated) slant delays. Special attention will be paid to the selection of sources of data that can constrain the solution(s) (e.g. Meteosat WV, radiosonde and other satellite methods).
* FMI will make the initial evaluation of the functionality of the observation operator against the slant-delay measurements

Deliverable. FMI has developed an observation operator for slant delays for the HIRLAM model. Testing of the operator has been performed against the test dataset from TUD and by comparison to the operator developed by KNMI. The operator is described in this document: d38_fmislant.pdf. The
code has been updated and is now version 2. It is  here: d38_fmislant_code_v2.tar.gz.  For further information contact Reima Eresmaa at Reima.Eresmaa@fmi.fi.