Summary of Microscale Modelling Workshop held on 27^th March 2003, 10:30 - 16:00, at the UK Met Office .

Presentations

1. Introduction: Stephen Mobbs. (link to presentation). Background to the project and an analysis of the rationale, project objectives, issues to be addressed. The project timetable was presented. Copies of the original project specification approved by UWERN were distributed, but it was appreciated that there was now a need to incorporate recent scientific progress.

2. Workshop structure: Alan Gadian. (link to presentation). Workshop outline, programme for the day, general issues and what are we trying to achieve.

3. Equation Sets: Andy White. (introductory slide). Specific analysis of the continuity equation with reference to fully compressible set, anelastic set (many variants), pressure set and hydrostatic pressure set. A tabular summary, with specific reference to acoustic modes is provided. (link to presentation).

4. Validity of anelastic and other equation sets as inferred from normal-mode analysis: Nigel Wood. (link to presentation). This work will appear in the QJRMS, but a preprint version is temporarily available (link to preprint). Discussion of the importance of use of complete compressible equation sets for linearised solutions was presented for different scales. For meso-scale applications, pseudo in-compressible and compressible set were satisfactory; for NWP full equation set is necessary.

5. Blasius and orography issues: Andy Brown (link to presentation). Discussion of the Blasius grid structure, UM grid structure and scale dependent decay.

6. High order non-oscillatory schemes for advection-reaction equations: Tito Toro (link to presentation) Examination of the reaction advection equation and the ADER approach. An extension to a multi-dimension approach, with comparison with numerical solutions was given.

7. Data Assimilation: some thoughts: Ian Roulstone (link to presentation). Some thoughts on Data Assimilation, high resolution modelling. General Data Assimilation issues and those specifically related to the Microscale Model were discussed.

8. Microscale Modelling from an Engineering perspective: Rex Britter & Bill Dawes (link to presentation). N.B. slides not in the original order . This talk presented the need and relevance for a building scale model. Aspects of Dispersion modelling from the Roy. Met Soc Policy Statement, comments relating to COST 615, EU model evaluation group, the Saturn project and current confidence predictions from the QNET-CFD newsletter. Willing to assist with a Building scale model development.

9. Issues about a fine scale model Mike Cullen (given by AG): (link to presentation). What is the model for? Data assimilation requirements? Developments of the UM and what can and can't be covered by the UM.

10. Experience of modelling atmospheric flows with a commercial CFD package: Christiane Montavon: (link to presentation). Examples of the CFX models and what was achieved over a relatively short time scale was presented. Examples of applications from ventilation of a railway station to foehn effects and flow over isolated hills was given.

Discussion

Questions were raised during each talk, and for a prolonged session at the end of the afternoon. The main comments are collated below.

Specific discussion areas in the workshop

What is the model for?
This issue was raised throughout the workshop in the presentations and in the discussion. It was agreed that there was a need to create a model to look at local scale flows over steep terrain and also a model for use in urban type conditions. the need for an urban type model was expressed by both the engineering and the Met Office communities. The need for a model that could be applied for flow over steep orography was agreed. How "steep" was not resolved. These two objectives could not be achieved by the same model. There was an opinion presented that there should be two possible models, if resources allowed.

Equation set?
Significant input from Piotr Smolarkiewicz and Martin Miller, provided useful input from non-UK sources. The presentations provided an important foundation for the discussion. The advantages of the compressible equation set were proposed by several of the speakers. The use of an anelastic set several was also feasible. Some suggested that perhaps the Lipps - Hemler anelastic set could be equally used. However, many suggested that comparability with the UM equation set would be of advantage. There are issues of having to cope with sound waves, e.g. by solving an elliptic equation as in the UM, or using some time splitting scheme as in RAMS etc. The advantages of the Laprise hydrostatic pressure co-ordinate system were presented. However, there was a consensus feeling that the UM equation set would the current preferred option.

UM development and use for a microscale model.
A need for the microscale model was variable horizontal (or nesting) and vertical grids. It was stated that this was in line with UM developments and that there would be development of the UM in this direction. The ability of the pressure solvers to cope with steep terrain following co-ordinates was discussed. It was he view of of several people that the pressure solvers could cope with steep slopes , but not overhangs. It was suggested that investigations were made to test the applicability of existing pressure solvers.

Data assimilation.
Discussion of the needs for data assimilation was discussed. The role of DA as opposed to "coupling". For a steep terrain model, DA would be of great advantage, and would usefully link in to Met Office future developments. If the UM / new dynamics equation set were used this would be of advantage.

Grid systems.
Limitations were seen in current grid systems. For an urban code, unstructured grids, adaptive grids, are options and further discussion is required. For a steep terrain model, development of variable horizontal grids are envisaged. (see notes below)

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Overall summary and actions following the workshop

1. There would be a web collection of the presentations and conclusions of the meeting produced (this document).

2. An "urban scale" model and "steep terrain" model were not compatible. If both objectives are to be attained, then separate models would need to be constructed. The feasibility of this will be discussed at the next workshop.

3. Email sub-groups, managed by Alan Gadian, would be convened to look at specific aspects of the project. These groups would examine the following topics. The groups are to be formed from those wishing to participate. Suggested topics are listed below, but are not meant to be exclusive. Participation, and structure of email groups is to be arranged and are only suggestions drawn from the workshop discussion.

Urban Code

Numerical Techniques. This includes all aspects of model numerics, eg pressure solvers and advection. schemes. The workshop considered these issues would become more important in later stages of the project, and were not critical at this stage. Is this agreed and what criteria / methods should be used. If using the UM as a basis code, would it be useful to test elements; e.g. pressure solvers
Email Group?
NW, JT, SV, TT, PM , AG ?????

4. Test Cases. The generation of set test cases should be considered by the working groups, and ideas discussed at future workshops. Specific ideas have been generated in the pre-workshop background, the pre-workshop agenda and in the "additional submissions" section below.

5. The project was designed to include aspects of UWERN activity, and current and future holders of grants would be approached to develop specific areas. This would be discussed by university groups and for the future UWERN / JIF proposal.

6. The next workshop would be held in the autumn to address conclusions of the sub-groups, and progress made.

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Additional submissions

A. Tito Toro.
CARTESIAN CUT CELLS
The Cartesian Cut Cell (CCC) approach offers some advantages over the mature Unstructured Gridding (UG) approach. Note that CCC is also unstructured. CCC can deal with arbitrarily complex geometries. In 2D the "boundary fidelity" is optimal; in 3D some compromises are necessary. The degree of automation of CCC Bl with arbitrarily complex geometries. In 2D the "boundary fidelity" is optimal; in 3D some compromises are necessary. The degree of automation of CCC is claimed to be higher than that of UG; the cost of grid generation is lower for CCC. Also, I believe CCC is more suitable for Adaptive Mesh Refinement (AMR) applied in both space and time. Some attempts have been made to include adaptation in time in the UG approach, but I am not aware of these being successful.
I have some experience in CCC as applied to three-phase (3 velocity vectors) problems including combustion and moving boundaries in 2D axi-symmetric very complicated configurations. I'm very satisfied with the performance of the whole approach.
General remarks:
A research tool (in the form of a mathematical model, numerical method, grid generation system, initial, boundary conditions, closure relations, etc.) is different from an application tool. This is specially true in weather forecasting, where efficiency is fundamental. This is not so for research different from an application tool. This is specially true in weather forecasting, where efficiency is fundamental. This is not so for research purposes, where "accuracy" should be an overriding priority.

B. John Thuburn
General Comments:
It seems unlikely that we'll be able to come up with a single model that will do everything everybody wants, but maybe two models between them will do most things most people want. Certainly I think the New Dynamics should be one of the models considered.
With conversations with Terry Davies, the New Dynamics could be adapted to have vertical no-flow boundaries, i.e. to cope with cliffs, buildings etc. He thought it shouldn't be too much work. If that can be done then the modified New Dynamics on its own might satisfy most needs. This would have the advantage of short-circuiting a lot of model development. One limitation of the New Dynamics is that its schemes are not inherently conservative. That might not matter for the overall accuracy for the kinds of problems to be looked at, but it would affect the ability to diagnose closed budgets, e.g. of moisture. (We recently had quite a job closing the moisture budget in the current LEM, which is supposedly conservative.)
My main comments are on testing models, which I've been thinking about on and off over the years. While everyone else has been devising their favourite test cases, I've been trying to advocate that first we need to decide what is the purpose of any test case and what is it trying to test. Below is my latest version of my evolving thoughts on the subject. Not all are completely relevant to the microscale model, but many are.
Issues that proposed test cases might address
(1) Detect coding errors
(2) Domain and gridding: Poles and other grid inhomogeneities / anisotropies;
Bottom boundary (may be non-trivial e.g. for theta-coords with mass-less layers);
Top boundary? It's probably impossible to treat the top boundary "correctly" in a model with a finite number of layers, so test cases should be designed so that the top boundary isn't an issue, or test the effectiveness of any proposed "sponge" layer.
(3) Advection:
Horizontal;
Vertical.
[Includes things like "age of air", spurious numerical mixing, behaviour of tracer-tracer correlations,...]
(4) Conservation:
Mass of air; mass of tracer; angular momentum; energy. Arguably, whether or not we demand conservation of potential enstrophy (and possibly even energy?) should depend on whether or not there is a cascade to small scales; Material conservation of potential temperature and potential vorticity, tracer mixing ratio, (equivalent potential temperature in the moist case); non-trivial if these are not advected or used as coordinates). [E.g. examine correlations of these quantities with passive tracers initialised to look the same; compare with trajectory calculations]
(5) Rossby wave propagation:
Horizontal and vertical; Zero and non-zero phase speeds.
(6) Gravity wave propagation:
Horizontal and vertical;
Zero and non-zero phase speeds;
Sensitivity to time step, especially with semi-implicit schemes.
(7) Acoustic wave propagation (for non-hydrostatic models):
Horizontal and vertical;
Zero and non-zero phase speeds;
Sensitivity to time step, especially with semi-implicit schemes.
(8) Balance:
Maintenance of an initially balanced flow (straightforward e.g. for steady zonally symmetric flow, but less obvious for more complicated flows where balance is a fuzzy concept) [e.g. monitor mean square divergence tendency?];
Include case in which nonlinear terms are important part of balance;
Adjustment to balance, bearing in mind application to climate models where convection and gravity wave drag schemes, for example, force at the grid scale.
For non-hydrostatic models consider quasi-hydrostatic balance as part of the overall balance.
(9) Orography:
Pressure gradient term;
Orographic resonance.
(10) Cascades to unresolved scales:
Layerwise 2D flow;
Frontogenesis;
(Fully 3D turbulence? smallest scale eddies are more energetic in this case, so a subgrid parameterization might be essential, whereas in 2D the smallest scales are essentially passive and we just need sufficient dissipation to soak up the cascade.) [E.g. monitor PV field, energy and enstrophy spectra, look for grid-scale noise.]
(11) Non-hydrostatic effects:
e.g. gravity wave reflection.
(12) Buoyancy-driven flow: E.g. Buoyant bubble (may be tricky if it generates 3D turbulence); Other buoyancy-driven flow relevant to climate, e.g. cold surge type flow?
(13) Elliptic solvers:
e.g. Poisson equation; Helmholz equation.
(14) Computational modes:
Could be associated with horizontal or vertical grid or time stepping. I'm not sure how one could systematically test for them (perhaps by forcing on the grid scale for one time step? careful analysis of the discrete equations?)
(15) Convergence with improving time and space resolution.
(16) Cost / efficiency.
(17) For adaptive models one could add a list of issues related to the adaptivity.

C. Other comments(Mike Cullen)
Since people had reservations about the cost of maintaining any model, I would have thought that using 2 models was even more a problem.

Urban code: A key issue is to be able to maintain a state of rest in hydrostatic balance.

Steep terrain code: I suspect the new dynamics can do this job, with extra terms included in the coordinate transformation. If not, then a different sort of grid structure has to be used and we get the urban code.

Generalised coordinate systems: The real question is whether terrain following coordinates can be used in the steep terrain model. Need to establish when this approach ceases to be viable. This is best addressed by test cases, but then need reliable reference solution to check against.

Data assimilation: I would expect standard techniques to work, with some multivariate coupling to maintain closeness to hydrostatic balance when appropriate.

Numerical techniques: Either anelastic or compressible models need a pressure solver. The new dynamics uses an efficient solver for non-symmetric problems, so can cover the requirement if the grid is structured. Solvers for unstructured grids should be available from enginerring type codes. I doubt whether advection schemes are an issue, given that quasi-monotone upwind schemes are now in general use. These are certainly appropriate for fine scales. Conservation is only an over-riding issue for high speed flows, not our case.

D. Other Comments (2)
Test cases are a good point for collaboration. i
Variable horizontal grid for the UM is a key element of the research and development plans for the MO and so there could be no problems on that front (other than possibly time scales).

Attendees:

Alan Burns , Rex Britter , Andy Brown , Xiaoming Cai , Peter Clark , Terry Davies , Bill Dawes , Alan Gadian , Sue Gray , Brian Golding , Roy Kershaw , Paul Mason , Christiane Montavon , Stephen Mobbs , Martin Miller , Ian Roulstone , Chris Smith , Piotr Smolarkiewicz , Andrew Staniforth , John Thuburn , Tito Toro , Simon Vosper , Neils Wedi , Andy White , Nigel Wood.

Unable to attend:
Stephen Belcher , Ken Carslaw, Ian Castro, Mike Cullen , Martin Jukes, Doug Parker , Ian Renfrew, Glenn Shutts

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Further details email Alan Gadian