Name:			Aime' Fournier
Citizenship:		USA
Organization:		NCAR & U MD CCMPS Meteorology
Mailing Address:	NCAR
			POB 3000
			Boulder, CO 80307-3000
Tel. No.:		303 497 1614
Fax No.:		303 497 1700
E-Mail:			fournier@ucar.edu

Title and abstract of presentation:

Persistent quasi-stationary ridges in the
Spectral Element Atmospheric Model

A. Fournier, M. A. Taylor and J. Tribbia

The occurrence, frequency, strength and duration of
atmospheric blocking events appear to be important aspects
of intraseasonal weather/climate prediction which are not
adequately (re)produced by state-of-the-art GCMs.  The
present study focuses on the effect of model horizontal
resolution on blocking-like phenomena in a
primitive-equation, dynamical-core-only framework, the
Spectral Element Atmospheric Model (presented at
PDEs-On-The-Sphere '98).  We extend the results of several
GCM-based studies on this topic in the literature, to the
higher resolutions easily attainable by SEAM, but only
including Held-and-Suarez forcing and topography, not
physical parameterizations.  As Tibaldi proposed for GCMs,
we propose and demonstrate a set of quasi-stationary-ridge
dynamics statistics as a useful test case of dynamical-core
performance, combining purely statistical measures such as
that of Held and Suarez and purely dynamic measures such as
proposed by Fournier et al. in an accompanying presentation.

We review and compare with the studies of D'Andrea et al.
and Doblas-Reyes et al., who found blocking-frequency
modeling to somewhat improve with increasing horizontal
resolution, up to T106.  Tibaldi et al. obtained similar
results, and specifically that improved physical
parameterizations do not improve blocking modeling unless
horizontal resolution is also increased (a motive for the
present study), but that this increase alone appears to be
sufficient to improve Euro-Atlantic sector model blocking. 

In the present study geopotential-height (Z) fields 
from 4 runs of 3600 model days each are used, with 
and without T42 topography, and at 2 horizontal resolutions 
up to an equivalent of T181.  Owing to SEAM's 
idealizations, quasi-objective diagnostics are
employed throughout, rather than traditional ones.  First
the sigma=0.25 level, from around 35 to 65 deg latitude, is
determined to be the zone of greatest Z variance.  Averaging
over this zone yields a 2d longitude-time distribution of Z,
whose maxima translate with a distribution of "ridge speeds."
This distribution's full width at half-max. is estimated as
a criterion for extracting "quasi-stationary ridge" events,
the analog of blocking in this system.  The frequency
distribution of events vs duration decreases exponentially,
as found by Dole and Gordon and other authors, but with a
2-to-8 times smaller e-folding time of only 18h. 
Stratifying the events by longitude shows some increase in
frequency just upstream or in the vicinity of mountain ranges,
while stratifying by amplitude suggests that longer-lasting
events tend to be strong, but the strongest events need not
last the longest.  Increasing model resolution increases
frequency, as found by the cited studies.

Special requirements or comments:
To be presented by AF.  Two transparency projectors would be 
preferable, if possible.