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Current-climate downscaling of winds and precipitation for the
Eastern Mediterranean
Scott Swerdlin 1 , Thomas Warner 1 , Andrea Hahmann 2 , Dorita Rostkier-Edelstein 3 and Yubao
Liu 1
1 National Center for Atmospheric Research, Boulder, Colorado,USA; swerdlin@ucar.edu
2 Risø National Laboratory for Sustainable Energy, Technical University of Denmark
3 Israel Institute of Biological Research, Ness-Ziona, Israel
1. Introduction
The use of a four-dimensional data-assimilation
(FDDA) system for generating mesoscale
climatographies is demonstrated. This dynamical
downscaling method utilizes the Penn State
University – National Center for Atmospheric
Research Mesoscale Model Version 5 (MM5),
wherein Newtonian-relaxation terms in the
prognostic equations continually nudge the model
solution toward surface and upper-air observations.
When applied for mesoscale climatography
development, the system is called Climate-FDDA
(CFDDA). In this example application of CFDDA,
it is used to downscale weather in the Eastern
Mediterranean region for January and July. This
region was chosen because of its interesting weather
and climate (e.g., geographic contrasts, inter-annual
and inter-seasonal variability).
We verify the performance of the downscaling
method by using independent gridded observations
of monthly rainfall, QuikSCAT ocean-surface
winds, TRMM and gauge rainfall, and hourly
boundary-layer winds from near-coastal tall-tower
sites. The verification focuses on the ability of the
mesoscale model to represent the frequency
distributions of atmospheric states, rather than the
simple zero-order statistics. Earlier results for this
study are published in Hahmann et al. (2007).
2. The model configuration
The model uses 36 computational levels, with
approximately 12 levels within the lowest 1 km and
with the model top at 50 HPa. Figure 1 displays the
geographic location of the model grids, and the
surface elevation on each domain. The model has a
horizontal grid increment of 45 km on the outer grid
(D1), which covers most of the Mediterranean Sea
and extends eastwards to cover the Black and
Caspian Seas. The first nested grid (D2), which has
a grid increment of 15 km, covers the Eastern
Mediterranean and Middle East regions. A third
nested domain (D3), with a grid increment of 5 km,
is located along the easternmost portion of the
Mediterranean coast. The three domains contain 81
× 111, 124 × 178, and 102 × 102 grid points,
respectively.
Figure 1. The model grids used for the downscaling
simulations. See the text for details.
3. Example results
The downscaling method represents very well the
statistics of precipitation over this region. Not only
are monthly averages and their geographic
distribution well simulated, but also the frequency
distributions of daily rainfall are accurately
diagnosed for various regions of the Levant. The
inter-annual variability of the January rainfall is also
well represented for the regions of heavy
precipitation in the Levant and southern Turkey.
The verification with QuikSCAT-estimated wind
speeds over the ocean shows some systematic
errors. The CFDDA-simulated winds tend to be
slower than those defined by QuikSCAT, especially
in winter. These errors are easily corrected by
simple regression equations. In contrast, over land,
the C-FDDA-derived wind climatography
overestimates the boundary-layer wind speeds,
especially in winter. However, most of the errors
can be explained by poorly resolved orography and
coastal configuration in the mesoscale model.
Figure 2 shows the QuickSCAT-observed and
model downscaled winds during July for the period
2000-2007.