LINK TO NEW ALADIN WEB
Lampe ALADIN
 ALADIN Consortium
 ALADIN Documents
 ALADIN Model

The scientific contents of

Modelling aspects of meteorology will rely more and more on tools developed in an operational environment, which will require more and more « industrial » methods for their production and maintenance. In that respect, the ALADIN environment anticipates the working conditions that atmospheric science will witness in the 10 coming years. Nevertheless, that environment is still there to foster scientific and technical progress.


The scientific medium-term and annual plans, the scientifical reports


Common points between ARPEGE and ALADIN :

  • Both models use the spectral technique for the horizontal representation of fields. This means that special provisions have to be taken for the use of a bi-Fourier LAM representation. The solution chosen here is that of Machenhauer and Haugen that requires the so-called bi-periodicization of fields through a fictitious « extension zone ».
As an example, on the figure (colour or bw) that features the « extended » orography of the ALADIN-LACE application, one notices the smoothness and isotropic character of the transition form one side to the other through the additional zone. It is sometimes claimed that spectral methods are unsuitable for LAM applications and/or that they cannot represent sharp features for lack of « locality ». The other figure (colour or bw ) representing 18 hour forecasts of 10 m winds (the colour of the arrows depends on 2m temperature), is a good counter example : no apparent boundary problems exist but many realistic features over land as well as over sea at the two grid-length scale do (plotting) ;
  • The vertical discretization is hydrid (going progressively from « p » to « sigma » ) ;
  • The time stepping now uses the semi-implicit semi-Lagrangian scheme with two-time-levels solution ;
  • The physics is for the time being identical with ARPEGE (which also benefited from ALADIN work) ; (see Geleyn et al., 1994).
  • The DFI technique is used, the latest, most efficient version (Lynch, Giard and Ivanovici, 1997) having been suggested in the framework of ALADIN ;
  • The CANARI and Full-Pos applications are mirrors of the ARPEGE ones, with specific steps linked to the LAM geometry.

The points specific to ALADIN :

  • The bi-periodicization is accomplished only on the files interpolated from the coupling model (i.e. as seldom as possible) and can thus be performed with a quite sophisticated iterative combination of splines and filters ;
  • The form of the coupling function (i.e. the relative weight taken at each time step by the larger-scale solution near the boundaries of the LAM) that has been optimized as much as possible in the spectral context ;
  • The implementation of the semi-Lagrangian scheme in the case of trajectories originating in the « extension zone » which required an original treatment of the link between semi-implicit time stepping and coupling (Radnoti, 1995 ) as well as a specific adaptation of Rochas's idea about Coriolis terms in the two-time-level algorithm ;
  • The existence of a non-hydrostatic option, based on Laprise's « hydrostatic pressure » type of vertical coordinate, which also requires a redefinition of the « Simmons-Burridge » vertical discretization operators ( Bubnova et al., 1995).

Finally around the project itself :

  • A special study (Caian and Geleyn, 1997) was performed to evaluate the respective merits of the stretched solution in ARPEGE and of the coupled solution represented by ALADIN ; the conclusion was that the combination of moderate stretching for the global part and of local adaptation via the LAM solution is the best combination, given the current computing constraints ;
  • ALADIN was used as a tool for adjoint sensitivity studies on frontogenetic problems (Horanyi and Joly, 1996 ) ;
  • A simplified physics package and its adjoint version for future mesoscale 4D variational data assimilation are currently being developed ( Janiskova, Thépaut and Geleyn, 1996) ;
  • Other important (past and present) topics of scientific activity are : the behaviour of the model's physics at the limit of validity of the hydrostatic assumption (and beyond) ; the conditions for a successful dynamical adaptation process ; and the intrinsic properties of the semi-Lagrangian time-stepping scheme.






 Home