Introduction of a variable Mercator map factor in the spectral part at the HARMONIE model

Abstract

For large domains, a new formulation of the spectral computation has been implemented in the HARMONIE model. The formulation makes use of the rotated/tilted Mercator projection and of its map factor represented by a linear combination of low-order Fourier harmonics. The reference model uses the maximum value of the map factor over the whole area. The improvement between using the Fourier approximation or the maximum value of the map factor increases with the size of the integration domain. In the spectral part, three coefficients of the Fourier cosine series have been introduced through a simple diagonal multiplicative operator. The first results of these modifications are presented for semi-Lagrangian integrations with long timesteps.