Proceedings - C-SRNWP Project

Figure 5: 2m temperature biases for Aladin-NORAF. Spin-up model from Arpège (magenta);
3D-VAR assimilation (blue).
3.3. Assimilation of reflectivities for the Arome version
The principle of the assimilation of radar reflectivities, which is felt as being a major
component of the future Arome data assimilation system, is based on a 1D retrieval method,
followed by a 3D-VAR analysis of the retrieved profiles of humidity and possibly
temperature. The 1D retrieval method is a Bayesian inversion technique where the pseudoobserved
column of temperature and humidity is obtained as a weighted average of a number
of neighbouring model columns. The weights are determined by a cost function which
measures the likelihood of any column to produce the observed reflectivity profile (Caumont
et al., 2005). The overall dataflow for the 1D+3D-VAR analysis is sketched in Figure 6
below. Only the retrieved temperature and humidity profiles enter the analysis, in a way
similar to radiosonde data.
Figure 6: sketch of the technical dataflow of the 1D+3D-VAR retrieval and analysis method.
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