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The scaled-decomposed atmospheric water budget over North America is investigated through the analysis of 25 years of simulation
by the Canadian Regional Climate Model (CRCM) driven by the NCEP–NCAR reanalyses for the period 1975–1999. The time average
and time variability of the atmospheric water budget for the winter and summer seasons are decomposed into their large-scale
and small-scale components to identify the added value of the regional model. For the winter season, the intra-seasonal transient-eddy
variance is the main temporal variability. The large- and small-scale terms are of the same order of magnitude, and are large
over both coasts and weak over the continent. For the summer season, the time–mean atmospheric water budget is rather different
to that of winter, with maximum values over the south-eastern part of the continent. The summer intra-seasonal variance is
about twice stronger than in winter and also dominates the variability, but the inter-monthly variance is non-negligible and
can be in part associated to North American Monsoon System. Over the continent, the intra-seasonal climatological variance
is dominated by the variability of the small scales. The small scales, that is those scales that are only resolved in the
regional model but not in the reanalyses, contribute to the added value in a regional climate simulation. In the winter season,
the added value of the CRCM is large and dominated by oceanic forcing, while in summer, it is dominant (larger than the large
scales) and controlled mainly by convective processes. 相似文献
2.
Soline Hallier Emmanuel Chaljub Michel Bouchon Haruko Sekiguchi 《Pure and Applied Geophysics》2008,165(9-10):1751-1760
The basin edge effect, i.e., the interference of the direct S wave with the surface wave diffracted off the basin edge has been invoked by many authors to explain the damage distribution during the January 17, 1995 Hyogo-Ken Nanbu (Kobe) earthquake. Here we present the results of numerical experiments obtained with the spectral element method in 2-D geometry. Our results confirm that the amplification of horizontal motion close to the basin edge can be twice as large as the one measured in the center of the basin. This additional amplification is shown to depend strongly on the edge geometry and on frequency, due to physical dispersion of diffracted surface waves. In particular we obtain maximal amplification around 3 Hz, at frequencies critical for buildings. 相似文献
3.
General circulation models still show deficiencies in simulating the basic features of the West African Monsoon at intraseasonal,
seasonal and interannual timescales. It is however, difficult to disentangle the remote versus regional factors that contribute
to such deficiencies, and to diagnose their possible consequences for the simulation of the global atmospheric variability.
The aim of the present study is to address these questions using the so-called grid point nudging technique, where prognostic
atmospheric fields are relaxed either inside or outside the West African Monsoon region toward the ERA40 reanalysis. This
regional or quasi-global nudging is tested in ensembles of boreal summer simulations. The impact is evaluated first on the
model climatology, then on intraseasonal timescales with an emphasis on North Atlantic/Europe weather regimes, and finally
on interannual timescales. Results show that systematic biases in the model climatology over West Africa are mostly of regional
origin and have a limited impact outside the domain. A clear impact is found however on the eddy component of the extratropical
circulation, in particular over the North Atlantic/European sector. At intraseasonal timescale, the main regional biases also
resist to the quasi-global nudging though their magnitude is reduced. Conversely, nudging the model over West Africa exerts
a strong impact on the frequency of the two North Atlantic weather regimes that favor the occurrence of heat waves over Europe.
Significant impacts are also found at interannual timescale. Not surprisingly, the quasi-global nudging allows the model to
capture the variability of large-scale dynamical monsoon indices, but exerts a weaker control on rainfall variability suggesting
the additional contribution of regional processes. Conversely, nudging the model toward West Africa suppresses the spurious
ENSO teleconnection that is simulated over Europe in the control experiment, thereby emphasizing the relevance of a realistic
West African monsoon simulation for seasonal prediction in the extratropics. Further experiments will be devoted to case studies
aiming at a better understanding of regional processes governing the monsoon variability and of the possible monsoon teleconnections,
especially over Europe. 相似文献
4.
NCEP/GFS analysis is used to investigate the scale dependence and the interplay between the terms of the atmospheric water
budget over West Africa using a dedicated decomposition methodology. The focus is on a 2-month period within the active monsoon
period of 2006. Results show that the dominant scales of seasonal mean precipitation and moisture flux divergence over West
Africa during the monsoon period are large scales (greater than 1,400 km) except over topography, where mean values of small
scales (smaller than 900 km) are strong. Correlations between moisture flux divergences in monsoon and African Easterly Jet
layers and precipitation indicate that precipitation is strongly correlated to moisture flux divergence via both large-scale
and small-scale processes, but the correlation signal is quite different depending on the region and vertical layer considered.
The analysis of the scales associated with the rainfall and the local evaporation over 3 different regions shows that positive
correlation exists over the ocean between precipitation and evaporation especially at large scale. Over the continent south
of the Sahel, the correlation is negative and driven by large scale. Over the northern part of Sahel, positive correlation
is found, only at small scales during the active monsoon period. Lag correlation reveals that the maximum evaporation over
the Sahel occurs 1–3 days after the maximum precipitation with maximum contribution from small-scale processes during the
first day. This study shows that NCEP/GFS reproduces well the known atmospheric water budget features. It also reveals a new
scale dependence of the relative role of each term of the atmospheric water budget. This indicates that such scale decomposition
approach is helpful to clarify the functioning of the water cycle embedded in the monsoon system. 相似文献
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