HIRLAM Workshop on convection and clouds - Tartu
Towards a consistent
formulation of
interfaces between
dynamical and physical
processes
Bart Catry
(ALADIN-2)
24-26 January 2005, Tartu
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HIRLAM Workshop on convection and clouds - Tartu
Interface Coding Rules
A) 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
B) The link between individual tendencies and the projection of a summed
forcing on the dynamical terms obey a single set of governing equations
C) The dilemma of diagnostics being either meaningless or so modeldependent that they can be neither standardized nor inter-compared
D) 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
E) 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
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HIRLAM Workshop on convection and clouds - Tartu
Rule A: existing time-step organisations
ARPEGE/ALADIN
ECMWF/HIRLAM
Inv. FFT, inv. Legendre transform.
Call physics (APLPAR)
Inv. FFT, inv. Legendre transform.
Physics
Update tendencies
Compute departure point D
Linear dynamics, non-linear residual R(t) and R(t-Δt)
Dynamics
Compute departure point (D)
Interpolate to D
Interpolate to D
Adiabatic explicit tendencies at arrival point (A)
Interpolation diabatic tendencies of radiation, convection
Physics
and clouds at t to D
Tendencies of parameterized processes
Explicit part of dynamics
Dynamics
FFT, Legendre transformation
Helmholtz solver, Horizontal diffusion
Add tendencies of adiabatic and diabatic processes
FFT, Legendre transformation
Helmholtz solver, Horizontal diffusion
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HIRLAM Workshop on convection and clouds - Tartu
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-smallstep changes towards theoretical ideal solutions (of which there is
currently hardly any literature available).
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HIRLAM Workshop on convection and clouds - Tartu
Rule B: the governing equations
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)
dm = 0
dm = 1
qi
qi
qv
qs
Pi
qr
Pl
qv
qa
ql
qs
Pi
qv
qa
ql
-Pl -Pi
qi
qs
qr
Pl
qa
ql
Pi
qr
Pl
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HIRLAM Workshop on convection and clouds - Tartu
Rule B: phase changes
qi
qs
Pi'' (autoconv)
Pi'''
Pi'
Pi (snow)
(subli)
(freez)
all phase changes
go through the
vapor phase
qv
ql
(cond)
(evap)
Pl'
Pl'''
Pl'' (autoconv)
qr
Pl (rain)
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HIRLAM Workshop on convection and clouds - Tartu
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
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HIRLAM Workshop on convection and clouds - Tartu
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
with
by
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HIRLAM Workshop on convection and clouds - Tartu
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.
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HIRLAM Workshop on convection and clouds - Tartu
(Rule D: tendencies vs fluxes)




c pd  qv (c pv  c pd )  ...T   g F 
t
p
 F   q / T
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.
...
Fini
Time
sequence
Ffinal
but Fref also
available
Fini
Fini
= Fref
This value is updated (if sequential)
but the reference value stays.
Total or
intermediate
tendencies
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HIRLAM Workshop on convection and clouds - Tartu
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…
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HIRLAM Workshop on convection and clouds - Tartu
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.
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HIRLAM Workshop on convection and clouds - Tartu
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.
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