Operational implementation of RTTOV-11 in the IFS
Lupu, Cristina ; Geer, Alan J.
An interpolation algorithmis incorporated into RTTOVto map the input profiles to the RTTOVmodel
coordinate and the Jacobians from the RTTOV model coordinate to the forecast model coordinate.<br>The default in the ECMWF's IFS is the Rochon interpolator that was implemented in RTTOV-9<br>and remained unchanged in the RTTOV-10 upgrade. Experimental results indicate that the default<br>interpolator commonly used operationally may show unphysical oscillations in Jacobians that may<br>cause unphysical analysis increments.<br>The latest development of the radiative transfer code RTTOV-11 has been enhanced to provide improved<br>profile interpolation from user levels to RTTOV levels and back again. This memorandum<br>summarises the evaluation of RTTOV-11 for use in the operational assimilation system at ECMWF.<br>Two of the new interpolation options (modes 3 and 5) are evaluated through comparisons of radiative<br>transfer simulations with RTTOV-11 using default Rochon interpolator, and through an analysis of<br>departure characteristics against observations. The impact on forecasts is also investigated through a<br>series of assimilation experiments. Results show that enabling interpolation mode 5 ensures smooth<br>Jacobians profiles required by the data assimilation calculations when the input profile levels are<br>more densely-spaced than the coefficient levels. The performance in the IFS as measured by forecast<br>skill, fit to observations and model run time supports the operational use of the RTTOV-11 and<br>interpolation mode 5 with the ECMWF IFS cycle 41r1 in May 2015.
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