Low (web) Quality - BALTEX

125
4. Preliminary results for downscaling one CFS
ensemble member with WRF
Thus far we have executed two tests dynamically
downscaling one CFS ensemble member for the 1993 warm
season. For the first test, only standard lateral boundary
nudging is applied to the outer five grid points of the model
domain, equivalent to what is traditionally done in WRF
numerical weather forecast applications and oftentimes in
regional climate modeling applications. For the second test,
both standard lateral boundary nudging and internal nudging
(at all wavelengths) is applied according to the standard
WRF FDDA options.
Precipitation results for June 1993 are shown for the
original CFS ensemble member (Figure 1), and the WRF
test downscaling experiments described above (Figure 2).
The precipitation for the original CFS ensemble member
demonstrates that the CFS model is able to capture warm
season teleconnections that lead to increased rainfall in the
central U.S., consistent with the precipitation observations at
this time. The WRF experiment with only lateral boundary
nudging, however, produces a diminished amount of
precipitation in the central U.S. as compared to the original
CFS ensemble member. Thus, this experiment appears to
actually take away value from CFS model. An analysis
showed that the large-scale circulation patterns in this WRF
experiment are significantly different than in the driving
CFS model ensemble member data, in a manner consistent
with Castro et al. (2005). By an improved representation of
the large-scale circulation, the WRF experiment with lateral
boundary nudging and internal nudging dramatically
improves the forecast June precipitation. The positioning of
the precipitation maximum in the central U.S. shifts slightly
southward and the amount of precipitation there is increased
compared to the original CFS ensemble member, more
closely matching the observed precipitation. Later months
in the summer (not shown) also show a much improved
representation of the North American Monsoon in the
Southwest U.S.
System (RAMS), J. Geophys. Res., 110,
D05108, doi:10.1029/2004JD004721, 2005.
Castro, C.L., R.A. Pielke, Sr., J.O. Adegoke, S.D
Schubert, and P.J. Pegion. Investigation of the
Summer Climate of the Contiguous U.S. and
Mexico Using the Regional Atmospheric
Modeling System (RAMS). Part II: Model
Climate Variability. J. Climate, 20, 3888-3901,
2007.
Miguez-Macho, G., G.L. Stenchikov, and A. Robock.
Regional Climate Simulations over North
America: Interactions of Local Processes with
Improved Large-Scale Flow. J. Climate, 18,
1227-1246, 2005.
Figure 1. June 1993 CFS precipitation (mm day -1 ) from
sample ensemble member.
5. Conclusions and ongoing work
The preliminary results of dynamically downscaling a
CFS ensemble member with WRF for the warm season in
North America are quite promising. Provided that the
regional model is able to retain the variability in the large
scale circulation fields, WRF used as a RCM can potentially
add value to representation of the warm season climate.
This is primarily realized by an improved representation of
warm season convective precipitation. These results appears
to validate the hypothesis posed by Castro et al. (2007) that
RCMs can add value to the representation of warm season
climate provide the driving global model produces
reasonably accurate teleconnection patterns and that these
are retained in the RCM. We anticipate the results here will
improve further with the incorporation of spectral nudging
in the CFS-WRF simulations.
6. Acknowledgements
This work is supported by National Science Foundation
under grant number ATM-0813656. We thank Dr. Gonzalo
Miguez-Macho of the University of Santiago de Compostela
for providing the spectral nudging code for WRF.
Selected References
Castro, C.L., R.A. Pielke, Sr., and G. Leoncini, Dynamical
Downscaling: Assessment of value restored and
added using the Regional Atmospheric Modeling
Figure 2. June 1993 CFS-WRF downscaled precipitation
(mm day -1 ) using lateral boundary nudging only (top) and
interior nudging (bottom).