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41.
The
ability of a regional climate model (RCM) to successfully reproduce the
fine-scale features of a regional climate during summer is evaluated using an
approach nick-named the “Big-Brother Experiment” (BBE). The BBE establishes a
reference virtual-reality climate with a RCM applied on a large and
high-resolution domain: this simulation is called the Big-Brother (BB)
simulation. This reference simulation is then downgraded by filtering
small-scale features that are unresolved in today’s global objective analyses.
The resulting fields are then used as nesting data to drive the same RCM, which
is integrated, at the same high resolution as the BB, only over a sub-area of
the larger BB domain, hence, producing the Little-Brother simulation (LB). With
the BBE approach, differences between the two simulated climates (BB and LB)
can be unambiguously attributed to errors associated with the dynamical
downscaling technique, and not to model errors or observational limitations.
The current study focuses on the summer over the West Coast of North America.
Results of the stationary and transient parts of the fields, decomposed by
horizontal scales, are presented for the month of July, for 5 consecutive years
(1990–1994). Three degrees of spatial filtering (roughly equivalent to the
global spectral resolution of T30, T60 and T360) as well as two update
intervals (3 and 6 h) of the lateral boundary conditions (LBC) have been
employed. This study establishes that the maximum acceptable resolution of
driving data for summer is T30, with improved results employing the T60
resolution of LBC. There is little improvement by reducing the time interval
from 6 h to 3 h. These results are generally in agreement with previous studies
carried out for winter. The good correlation between LB and BB simulations is
more difficult to achieve during the summer season, mostly due to weaker
control exerted by LBC. Poor correlations are more pronounced for the transient
parts than they are for the stationary parts of the fields. This is especially
true for the precipitation field, where differences can be attributed to higher
temporal variability during the summer due to the presence of
convection. 相似文献