| Advent of global cloud-resolving model (CRM) achieved by Japanese Earth Simulator often
leads to an extremely misleading argument that all the parameterization problems will be
replaced by explicit high-resolution modelling. However, this type of arguments totally forget
the fact that CRM itself is built up on various subgrid-scale parameterizations. Thus we need
to move beyond this type of "brutal" high resolution modelling approaches by seeking
methodologies (not necessarily parameterization) for correctly and more efficiently
representing complex atmospheric processes of smaller and smaller scales.
In order to advance towards this goal, we propose the approach of NAM-SCA:
Nonhydrostatic Anelastic Model under Segmentally-Constant Approximation. The idea for
this model is inspired from various different sources. First of all, a branch of mathematics
called the multiresolution analysis provides a philosophical basis for pursuing this
possibility: in the same sense as wavelet can extensively compress an image, the
multiresolution analysis provides extensive possibilities for compressing numerical models.
Application of this principle into practice leads to a very flexible time-dependent
mesh refinement or nesting, far more extensively than conventional approaches can
provide. A "deconstruction" analysis of the mass flux convective analysis, on the other
hand, reveals that the mass flux decomposition itself can be used for this purpose:
NAM is simply decomposed into an ensemble of mass flux modes, purely as a
geometrical representation, under a spirit of multiresolution analysis, but without any
further approximations. We call this representation as SCA due to its geometrical
constraint.
NAM-SCA can run much efficiently than conventional CRM by adopting high resolutions
only where they are required, and potentially it can achieve a much higher resolution than the
current CRM can achieve. A two-dimensional version is already operational available.
Possibility of three-dimensional version will also be discussed, as well as implications for the
downscaling problem.
References:
Yano, J.-I., P. Benard, F. Couvreux, and A. Lahellec, 2010: NAM-SCA: Nonhydrostatic
Anelastic Model under Segmentally-Constant Approximation. Mon. Wea. Rev., 138,
1957-1974.
J.-I. Yano, 2010: Downscaling, Parameterization, Decomposition, Compression: A
Perspective from the Multiresolutional Analysis. Adv. Geophy., 23, 65-71. |