Towards a consistent formulation of interfaces between dynamical and physical processes

1. Towards a consistent formulation of interfaces between dynamical and physical processes Bart Catry (ALADIN-2) 24-26 January 2005, Tartu

2. Interface Coding Rules The control of various options shall be done either at low-level for very specific switches or at high-level for control of the set-up sequence and/or calling of blocks of physics routines or the I/Os related to their calls The link between individual tendencies and the projection of a summed forcing on the dynamical terms obey a single set of governing equations The dilemma of diagnostics being either meaningless or so model-dependent that they can be neither standardized nor inter-compared This rule aims at going out of the flux vs. tendencies dilemma by communicating on the basis of three statusses for each variable: reference, initial and final In order to have a maximum of flexibility one could introduce the concept of processes and schemes which requires a set-up system that attributes names of routines contributing to a mandatory tendency into a table indexed on this tendency

3. Rule A: existing time-step organisations Dynamics Physics Physics Dynamics

4. Rule A: other options Current models are too straight forward in their time-step organisation (having only one in mind). An interface hosting different models should therefore be flexible to allow all logic organisations: - Physics applied before or after the dynamics - Sequential or parallel physics - Physics at the departure or arrival point of the semi-lagr. trajectory - Using stored information from the previous time-step The control of the time-step should be at high-level in such a way that the low-level routines are unaware of the followed time-step strategy. The big advantage of this way of working is that the current models can be implemented immediately and also allow small-step-by-small-step changes towards theoretical ideal solutions (of which there is currently hardly any literature available).

5. Rule B: the governing equations dm = 0 dm = 1 qi qi qi qv qv qv qa qa qa ql ql ql qs qs qs qr qr qr -Pi -Pl Pi Pi Pi Pl Pl Pl Rule B states that all low-level routines should obey a single set of governing diabatic equations and that any inconsistency with this set of equations has to be corrected at the lower level. This set of equations includes: - a mass-weighted (barycentric) point of view - the option to conserve or vary atmospheric mass (δm = 0,1)

6. Rule B: phase changes all phase changes go through the vapor phase qi qs Pi'' (autoconv) Pi''' Pi' Pi (snow) (subli) (freez) qv (evap) (cond) Pl''' Pl' ql qr Pl'' (autoconv) (rain) Pl

7. Rule B: equations The conservation of the different mass-species (dry air, water vapour, cloud water, rain water, cloud ice and snow) (no hail or graupel) The thermodynamic equation in a flux conservative form

8. Rule B: equations (2) The more physical option (in case of compressible air) where any heat source is projected on both temperature and pressure changes. This means replacing the following set of equations by with

9. Rules C: diagnostics dilemma Some routines will need to add so-called ‘diagnostic-equivalent’ variables to their local input/output stream. For instance, to solve the set of equations presented earlier, we need 6 time integrals which will give us together with the 6 tendencies a set of 12 equations with 12 unknowns (which is fortunately easy to solve). To minimize all these modifications at the level of the DDH-machinery, low-level routines or intermediate ‘communication’ routines should of course prepare the necessary output. (Only recently a new proposal was made which will change the type of required fluxes. This is however still under study but will surely have implications on this rule and rule D) Instead of calculating the diagnostic-equivalent fluxes in the low-level routines it is proposed to calculate a number of the necessary fluxes in a high-level routine such that the total fluxes are equal to the tendencies.

10. (Rule D: tendencies vs fluxes) ... Fini Ffinal Time sequence Fini Fini = Fref The sum of the fluxes (the total tendency) is not equivalent with the sum of the intermediate tendencies. Proposal: communicate with three 'statusses': a reference input, an initial input and a final output. but Fref also available Total or intermediate tendencies This value is updated (if sequential) but the reference value stays.

11. Rules E: Processes and schemes The following components connected to a flexible way of running and experimenting on physics are proposed: Processes: in order to avoid unrealistic experiments it seems very natural to define the different processes which the model may describe. For a given model run, a number of parameterizations are chosen such that they do not overlap in the sense that the same process would be described twice. For example, some convection schemes also include precipitation release so that they cannot be combined with another microphysics scheme. One may imagine an ASCII file defining the different processes, e.g. dynamics, turbulence, shallow convection, deep convection, grid scale condensation, cloud cover parameterization, precipitation release including phase changes, solar radiation, thermal radiation, surface processes…

12. Rules E: Processes and schemes (2) Schemes: as used today may often consist of several subroutines that describe one or more processes. In a revised system the idea is that a scheme is characterized by a unique assigned number (integer) followed by information e.g. integers associated with the processes that are described by the parameterization. For example, a convection scheme that can be run with or without precipitation release defines possibly two separate schemes. The one without precipitation release can be combined with a more advanced precipitation release, the other not. In order for the user to actually choose a meaningful combination of schemes an offline checking program could be made verifying that some combinations are sensible. In order to make the suggested system work it will require some rewriting of communication parts and namelists.

13. Conclusions - Outlook 5 rules have been presented of which the ALADIN-2 community thinks that they are necessary to construct a flexible physics-dynamics interface that can host various models (HIRLAM among others). Recently there was a new proposal (more about it Wednesday) with the aim to rely on the existing and have a progressive implementation strategy. This is of course also favourable for a possible HIRLAM cooperation. However, we cannot force these rules on the HIRLAM community without any input from them. Therefore we propose that the working session on Wednesday morning should be the opportunity for people to ask questions, make suggestions, formulate technical/political remarks and so on… A small document which can serve as a guideline will be distributed.