Liang, X.-Z., K.E. Kunkel, and A.N. Samel,
2001: Development of a regional climate model for U.S. Midwest applications.
Part 1: Sensitivity to buffer zone treatment. J. Climate., 14,
4363-4378.
ABSTRACT. A regional climate model (RCM) is being developed for U.S.
Midwest applications on the basis of the newly released Pennsylvania State
University-NCAR Fifth-Generation Mesoscale Model (MM5), version 3.3. This study
determines the optimal RCM domain and effective data assimilation technique to
accurately integrate lateral boundary conditions (LBCs) across the buffer
zones. The LBCs are constructed from both the NCEP- NCAR and ECMWF reanalyses
to depict forcing uncertainties. The RCM domain was chosen to correctly
represent the governing physical processes while minimizing LBC errors.
Sensitivity experiments are conducted for the Midwest
1993 summer flood to investigate buffer zone treatment impacts on RCM
performance.
The results
demonstrate the superiority of the buffer zone treatment that consists of the
physically based domain choice and revised assimilation technique. Given this
treatment, the RCM realistically simulates both temporal variations and spatial
distributions in the major flood area (MFA). This success is identified with
the accurate representation of both the midlatitude
upper-level jet stream and Great Plains
low-level jet (LLJ). The RCM reproduces different climate regimes, where
observed rainfall was identified with the periodic (5 day) passage of midlatitude cyclones in June and persistent synoptic
circulations in July. The model also correctly simulates the MFA rainfall
diurnal cycle (with the peak amount at 0900 UTC), which follows the LLJ cycle
by approximately 3 h. On the other hand, RCM performance is substantially
reduced when the southern buffer zone extends to the Tropics, where large
forcing errors exist. In particular, the RCM generates a weaker LLJ and, as a
consequence, a decreased amount and delayed diurnal cycle of the MFA rainfall.
In addition, the MM5 default LBC data assimilation technique produces
considerable model biases, whereas the revised technique improves overall RCM
performance and reduces sensitivity to domain size.
Annual Cycle
Liang, X.-Z., L. Li, K.E. Kunkel, M. Ting,
and J.X.L. Wang, 2004: Regional climate model simulation of U.S.
precipitation during 1982-2002. Part 1: Annual cycle. J. Climate (in press).
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ABSTRACT. The
MM5-based regional climate model (CMM5) capability in simulating the United States
precipitation annual cycle is evaluated with a 1982-2002 continuous baseline
integration driven by the NCEP-DOE AMIP II reanalysis. The causes for major
model biases (differences from observations) are studied through supplementary
seasonal sensitivity experiments with various driving lateral boundary
conditions (LBCs) and physics representations. It is demonstrated that the CMM5
has a pronounced rainfall downscaling skill, producing more realistic regional
details and overall smaller biases than the driving global reanalysis. The
precipitation simulation is most skillful in the Northwest, where orographic
forcing dominates throughout the year; in the Midwest, where mesoscale
convective complexes prevail in summer; and in the central Great
Plains, where nocturnal low-level jet and rainfall peaks occur in
summer. The actual model skill, however, is masked by existing large LBC
uncertainties over data-poor areas, especially over oceans. For example, winter
dry biases in the Gulf States
likely result from LBC errors in the south and east buffer zones. On the other hand, several important
regional biases are identified with model physics deficiencies. In particular, summer
dry biases in the North American monsoon region and along the east coast of the
United States
can be largely rectified by replacing the Grell with
the Kain-Fritsch cumulus scheme. The latter scheme,
however, yields excessive rainfall in the Atlantic Ocean but large deficits
over the Midwest. The fall dry biases over the
lower Mississippi
River basin, common to
all existing global and regional models, remain unexplained and the search for
their responsible physical mechanisms will be challenging. In addition, the
representation of cloud-radiation interaction is essential in determining the
precipitation distribution and regional water recycling, for which the new
scheme implemented in the CMM5 yields significant improvement.
Diurnal Cycle
Liang,
X.-Z., L. Li, A. Dai, and K.E. Kunkel, 2004: Regional climate model simulation
of summer precipitation diurnal cycle over the United States. Geophys. Res. Lett. (submitted).
ABSTRACT. MM5-based
regional climate model (CMM5) simulations of the diurnal cycle of U.S. summer
precipitation are found to be sensitive to the choice of cumulus
parameterization schemes, whose skills are highly regime selective. The Grell scheme realistically simulates the nocturnal precipitation maxima and
their associated eastward propagation of convective systems over the Great
Plains where the diurnal timing of convection is controlled by the
large-scale tropospheric forcing; whereas the Kain-Fritsch scheme is more accurate for the late afternoon
peaks in the southeast U.S.
where moist convection is governed by the near-surface forcing. In radar rainfall data and the
simulation with the Grell scheme, another weaker eastward- propagating diurnal
signal is evident from the Appalachians to the
east coast. The result demonstrates the importance of cumulus schemes
and provides the first realistic simulation of the central U.S. nocturnal precipitation
maxima.
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