Working group on evolution of physics from synoptic to mesoscale

participants: Javier, Toon,  Ernesto, Stefan, Eric, Gerard, Mariken, Timo, Andres,  Laura
chairman: Gerard, notes: Laura

In the group, some important problems were discussed, but not all questions mentioned in the agenda, which was based on the introductory remarks. Main points are summarized here.

Externalized surface code

In the beginning, externalized soil and surface code was shortly discussed. Such code allows combination of surface analysis with forecast of soil processes with all needed details, interactions and definition of the (unnecessary to the atmospheric model) parameters. The soil model may be directly driven by relevant observations, mainly precipitation, instead of relying on forecasted parameters also in the beginning of integrations. Such a code may be able to utilize more fully the fine-resolution surface description, where a lot of presently unused information for the model is available. Technically, externalization allows a flexible coupling of atmosphere and surface via surface fluxes and other coupling variables. Still more technically, this code exists in AROME and it may be necessary to use this framework in order to implement possible HIRLAM elements of surface parametrizations.

Evolution or revolution?

In moving from the present synoptic-scale physics some parametrizations may evolve smoothly towards mesoscale, some should be replaced by new approaches. Development of three-dimensional moist turbulence and shallow convection parametrizations is considered necessary in the horizontal resolutions smaller than 2-3 km. These developments may be based on the existing turbulence code and mass flux convection parametrizations (evolution). Parametrization of deep convection will not be needed anymore but completely new microphysics with the solid and liquid cloud condensate and precipitation as prognostic variables is needed (revolution, but done in AROME). Surface scheme may be developed based on the present code (evolution), but improvement of the vertical and horizontal resolution require special solutions because the atmosphere is not in equilibrium with its own each surface element. Especially stable shallow boundary/surface layers require new approaches when the vertical resolution is increasing.  A three-dimensional radiation  parametrization capable of interacting with the new microphysics may be developed from the existing code (evolution). Some totally new for HIRLAM processes need to be included into the model, like aerosol or carbon dioxide. These would bring new interactions into the model (aerosol - radiation - CCN - cloud microphysics etc) (closer to revolution).

Towards portable physics

Our aim is to develop portable physics, which adapts itself to the different model resolutions and can be flexibly used and combined with other model components. There are examples of code capable to gradually switch itself off or on when the model resolution  changes (like subgrid-scale orography parametrizations, Straco, the planned sloping-surface radiation in HIRLAM). It is possible that in the present HIRLAM physics code there are hidden scale-dependencies, which should be revealed and analysed systematically. Automatic adaptation to different resolutions is not produced automatically, but requires that this is taken as a starting point when developing the parametrizations. Different solutions may exist, there may also be code where this approach does not work (no examples were mentioned, though).

Problem solving methods

Present physical parametrizations evolve basically in two ways: (1) based on the experience of the developers of a specific scheme who find possibilities to improve them individually (2) based on the need to solve specific model problems where many processes are involved in interaction. Different combinations of model modifications may lead to a solution of the problem so that there may be more than one "correct answer".Working with the problems needs team work, understanding the interactions, use of the expertise of different people, different from the present way of planning and reporting.  Four problems important to both the present HIRLAM and AROME were identified:

Nordic Temperature Problem

Active team work is ongoing to attack the different aspects - snow surface, stable shallow boundary layer, condensation, LW radiation - of the problem with the aid of Sodankylä observations. HIRLAM people and Eric participating, so this is a common effort. Also, a lot of work has already been done during several years especially in the surface part which now gives a possibility to find a solution within a short time. With this task we have learned a lot of coordination, communication and share of work between specialists.

The problem of fog and low clouds

Has been indicated by forecasters in many countries to be one of the most important forecasting problems. Not much systematic work done in HIRLAM, some ongoing in AROME, maybe something related to UM?. The problem of this problem is that it is difficult and immediate solutions may not exist. The problems seems not to be solved by fine resolution or advanced microphysics. A possibility to attack together: to define the problem and model components involved, find suitable observations, diagnostic tools.

The problem of weak and strong winds

There are indications in the verification scores and in comments from forecasters and wave modellers that overpredicting weak and underpredicting of the strong winds is a problem in HIRLAM. Some aspects of this problem are attacked (Charnock constant over sea, orographic momentum fluxes etc), but not much is done systematically. This problem is evidently very much scale-dependent. Is there a need for a coordinated study?

Prediction of heavy precipitation location and intensity

A lot of development work is going on, mainly outside HIRLAM. A possibility to benefit of the experience of others.

Contributions to AROME physics

Three or four components of the present HIRLAM physics were identified as candidates to be further developed for AROME:

Recoding of physics

The aim is to recode the HIRLAM physics in the IFS-framework according to the rules defined in cooperation with the ALADIN-AROME developers. The first exampe of a recoded routine exists, because Bent has written a version of Straco to fit this framework. The recoding work should be done in a coordinated and well planned way, starting from the analysis of the present code and study of the target IFS structures and rules. People responsible for the individual schemes should participate, together with some system people. A possible way to start the work is to prepare questions for the analysis of the present code. Important aspects here are the variables used and transmitted, interfaces, hidden scale dependencies, increase of readability etc. A working week in a good place should be arranged with participation of needed people, in cooperation with the HIRLAM mesogroup and somebody from AROME and ALADIN (interfacing and equations experts).