Geofizika, Vol. 15 No. 1, 1998.
Izvorni znanstveni članak
Refinement of the vertical diffusion scheme in the ARPÈGE/ALADIN model
Maja Telišman Prtenjak
Antun Marki
Pierre Benard
Sažetak
In atmospheric numerical weather prediction (NWP) models, the use of
long time-steps as allowed by efficient numerical/dynamical schemes can
lead to spurious oscillations due to the parameterized physical part. Typical
examples of this are the oscillations associated with simplified parameterization
schemes for vertical diffusion or shallow-convection, such as usually
used for NWP.
The oscillations generated by K-type vertical-diffusion schemes are well
documented, and being called fibrillations; they are characterized by high
temporal and vertical frequencies. Since they are linked to high vertical
resolution, these spurious oscillations are generally found in the low-levels
of model’s domain.
In ARPÈGE, the MÉTÉO-FRANCE NWP global model, and in ALADIN,
its limited-area model (LAM) version developed in cooperation with
Eastern European countries, and also used for operational NWP purpose,
some oscillations still remained in the evolution of the forecast fields, despite
the fact that a first anti-fibrillation scheme (AFS) had been included.
This study was made to examine the possible sources of these oscillations,
through the 1-D (vertical) version of these models.
First, the parameterization of shallow-convection (which is in fact part
of the vertical diffusion scheme) was found to be an important source of oscillations,
and some solutions for eliminating this problem are proposed.
Second, the original AFS is shown not to completely prevent the generation
of fibrillations, and a more efficient formulation is derived.
All AFSs basically consist in a temporal first-order decentering of the
diffusion equation, keeping an explicit form for the exchange coefficient itself.
The AFS correction thus always improves the stability at the expense of
the accuracy in some way. In the new AFS proposed here, the number of
grid-points which need a correction is lessen from almost 90% to some 5%,
resulting in a more accurate scheme. Unlike AFSs proposed in the literature,
the correction has now to be applied not only for grid points of atmospheric
stable conditions (i.e. Richardson number, Ri > 0) but also for atmospheric
slightly unstable conditions (Ri < 0).
Ključne riječi
NWP model; ARPÈGE/ALADIN model; shallow-convection; Richardson number; fibrillation; anti-fibrillation scheme
Hrčak ID:
17703
URI
Datum izdavanja:
1.12.1998.
Posjeta: 1.007 *