1. Introduction

These alternative schemes are as follows:

developed in IFS:

- LFULLIMP

developed in Aladin:

- LPC_OLD

- LPC_FULL

- LPC_TRAJ

The scheme  LPC_OLD  is the so-called partial iterative scheme which has been use for Aladin-NH since the beginning of Aladin-NH implementation (Bubnova et al, 1995).

The schemes  LFULLIMP  and  LPC_TRAJ  are based on the same concept of an iterative approximation of the trapezoidal scheme, including the calculation of SL trajectories. However, LFULLIMP  and  LPC_TRAJ  even if exploiting the same basic idea, are written (and coded) quite differently:  LFULLIMP  is written in a specific "incremental form", while  LPC_TRAJ  is written in the same "non-incremental form" as the traditional SI scheme of IFS/ARPEGE/Aladin (see Appendix A).

The   LPC_FULL  is also an iterative approximation of a trapezoidal scheme, but the trajectories are computed only from the explicit winds, as for the SI scheme. This scheme is less sophisticated (or consistent) because trajectories are not re-computed at each iteration, but it is less expensive, and could be advantageous in the cases where the trajectory iteration is not the main concern.

The new current implementations of these schemes are actually coded in "research" form not fully rationalized, and not fully integrated in a clean way into the main stream of cycles. This means in particular that:

- Aladin  LPC_FULL  and  LPC_TRAJ  schemes have been kept outside the official cycles so far, and exist only under the dangerous form of personal branches.

- IFS  LFULLIMP  scheme is inserted in the cycles, but more or less as a "foreign body". In particular it has its own specific subroutine for spectral Helmhotz computations, and similarly, the relevant grid-point part of computations is rather isolated from the normal (SI scheme) grid-point computations.

Since  LPC_FULL  and  LPC_TRAJ  schemes are similar, they should be "harmonized", i.e. merged as deeply as possible. Moreover, there is clearly a need of a better integration of these schemes in the main code. However, this requires an effort of rationalization for smooth acceptance in the official cycles by the other persons in charge of the code. This note lists the points which have been thought as necessary to achieve this objective.

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