Proceedings - C-SRNWP Project

• However inspection of T2m forecasts in deep valleys, like around Grenoble (see Figure 2),
revealed that an interaction between the above-described best ‘dynamical’ choice for SLHD
and the AROME physical package created spurious spots of too hot temperatures, these
spots disappearing when almost all horizontal diffusion is removed (a testing solution
without any operational application, of course).
SLHD=T (daily run) :
SLHD=F (& very small sp. diff.)
Problem areas in SLHD run
OK but not a solution (too
noisy elsewhere)
Figure 2: Illustration of the valley ‘hot-spots’ problem (on the left). It can be cured (on the
right) by removing nearly all kind of lateral diffusion, spectral and/or SLHD, but this is not a
viable solution for a full purpose NWP model. The solution (not shown but quasi identical to
that of the picture on the right) will surprisingly come from a study about the intrinsic SLHD
behaviour. Courtesy of Yann Seity.
• Inspection of the spectral behaviour of SLHD, visualised through the vertical divergence
(closely linked to the vertical velocity w) created a big surprise (see Figure 3). The studied
syndrome was a ‘bifurcation-type’ one. Indeed the spectra with vanishing residual diffusion
on w were radically differing from the whole effect, with the zero solution in between! Of
course, this immediately lead to a solution without hardly any sign of either the ‘chimney
syndrome’ or the presence of ‘hot-spots’, solution obtained with a division by 15 of the
intensity of the SLHD action on w.
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