Progress on Development of a Nonhydrostatic
Version of HIRLAM at Tartu Observatory
A nonhydrostatic (NH) version of the HIRLAM model is under development at
Tartu Observatory, Estonia, by the Group of Dynamic Meteorology (GDM).
Tartu Observatory is an scientific research institution under administration of the Estonian
Ministry of Education. It is associated with Tartu University and the
Estonian Academy of Sciences. Its main area of activity is
fundamental research into atmospheric science and astronomy.
The NH version of the HIRLAM model is based on the complete, non-hydrostatic,
pressure-coordinate model of atmospheric dynamics (Rõõm, 1989)
and its anelastic approximations (Miller 1974, White 1989, Salmon and Smith 1994).
The anelastic pressure-coordinate model is
presented both in the continuous and discrete versions in the spherical
geometry, hybrid-coordinate presentation by Männik, Rõõm, Gustafsson and
Ivarsson (1998). It is close to the Laprise model, but different in that it
employs the full nonhydrostatic pressure as the vertical coordinate.
The main advantages of the model are:
- High precision for a broad range of dynamic scales.
The model is applicable from 100 m to planetary scale processes, and the
grid resolution can be varied in limits 100m to 100km (or larger).
The hydrostatic regime is reached automatically for large-scale processes
and for low grid resolutions.
- The NH version can be obtained by extension of the existing HS HIRLAM model,
by inclusion of NH effects. There is no need for parallel maintenance of
different HS and NH models. After extension, the same model is capable
of describing both HS and NH regimes and events. Which of these two regimes
will be dominant depends automatically on the horizontal grid resolution
and dominant dynamic scales of the process.
An attempt to install the HIRLAM at the Tartu Observatory was made in
September, 1997, but there was a failure due to the old operating system of
our SUN workstation.
Real work began on January 15, 1998, after we obtained the new operating
system Solaris 2.6. The mini-version of the HIRLAM, prepared by the Swedish
colleagues (550 X 550 km, 24 layers, horizontal grid-step 11 km) was installed
on the updated SUN at the end of January. Soon after that the same
model was installed under LINUX at the GDM's PENTIUM-II Workstation.
It is worth noting that the Pentium HIRLAM is three times faster than
the SUN version. Also, we succeeded in installing the Metgraf in two versions,
the SMHI version and the Linux-version from DMI.
During February-April, 1998 we have developed the detailed theory for
a discrete, NH, hybrid coordinate, spherical geometry, three time-level,
explicit (leapfrog) time-stepping scheme for the HIRLAM (Männik, Rõõm,
Gustafsson, Ivarsson 1998) and a similar theory for the semi-implicit
time-stepping scheme (not yet published elsewhere). In parallel, an
eight hour lecture-course "Numerical schemes of the HIRLAM model"
was presented by Rõõm to the students of Tartu University.
Beginning in May, the numerical realization of the explicit scheme is in
The first (beta-)version of the explicit scheme should be
finished in June, 1998. Its testing is planned in June-July. In parallel,
the creation of a semi-implicit scheme should be started in July.
As the theory shows, the implicit time stepping has an advantage in
comparison with the explicit (enabling larger time-step without loss of
accuracy) if the horizontal grid-step is larger than 4 or 5 km.
As the grid steps 5 to 10 km are of largest practical
importance, the semi-implicit time-scheme should be worth development.
[Below the critical resolution of approximately 4 km, the nonhydrostatic
model has an internal stabilizing mechanism which
is more effective than the one provided by the implicit scheme.]
For testing of both models in real conditions, the area of integration
should be 5 or 10 times larger than in the present realization (550 km).
For that a more powerful computing facility is required (approx. 100 times
faster than our fastest Pentium-II work-station).
Männik A., R. Rõõm, N. Gustafsson, K.-I. Ivarsson, 1998:
Nonhydrostatic experimental version of the HIRLAM. EGS XXIII General Assembly,
Nice, 20-24 April 1998, Abstracts.
Miller, M. J., 1974: On the use of pressure as vertical co-ordinate in modeling
convection. QJRMS, v. 100, 155-162.
Rõõm R., 1989:
General form of dynamical equations of the atmosphere in the isobaric
coordinate space. Proc. Estonian Acad. Sci., v. 38, p. 371.
Salmon R., L. M. Smith, 1994: Hamiltonian derivation of the nonhydrostatic
pressure-coordinate model. QJRMS, v. 120, 1409-1413.
White A. A. ,1989: An extended version of nonhydrostatic, pressure coordinate
model. QJRMS v. 115, 1243-1251.