Introduction
Precipitation is difficult to realistically simulate over North America using
present day climate models. Three main factors contribute to this challenge: model
resolution, model physics, and season. Climate models using coarse spatial
resolution are challenged to represent both spatial patterns of time-averaged
precipitation and daily precipitation events (Iorio et al. 2004). Small scale
precipitation events including extreme events are not captured on larger model
spatial scales. In regions such as the western United States where topography is
important for precipitation processes, increased resolution does improve model
results. Climate models also more accurately simulate precipitation produced by
large scale mechanisms as opposed to convective schemes which are related to both
the model resolution and model physics. Iorio et. al. (2004) argues that
improvements in model physics rather than, or in addition to, further resolution
refinement will be required to achieve significant reductions in precipitation errors.
Lastly, there is also a seasonal component in accurately modeling precipitation.
Seasonal mean precipitation in the Southeast U.S. is highly dependent on spatial
resolution in DJF and SON when precipitation is primarily produced from large scale
mechanisms. In MAM and JJA precipitation is produced mainly from convective
schemes and increasing resolution shows little improvement (Iorio et al. 2004).
This study analyzes all three simulation challenges in the context of the
Bermuda High (BH), which is an atmospheric dynamical feature important for
simulating and predicting summertime precipitation in the eastern United States.
The BH is a semi permanent subtropical high in the North Atlantic Ocean and is strongest
during northern hemisphere summertime months. This high pressure system extends
westward into the Gulf of Mexico and contributes to two jets that bring significant
moisture into the continent. The East Mexican easterly Low Level Jet (EMLLJ)
represents the westward penetration of the BH into Mexico. Flow of the western
edge of the BH from the south contributes to the Great Pains Low Level Jet which
transports one third of the moisture that enters the United States annually (Helfand
and Schubert 1995). The GPLLJ is confined to the eastern side of the Rocky
Mountains travelling northward to the Great Plains and in more intense months the
jet continues northeasterly. Due to the location of the jet, it is primarily a moisture
source for precipitation in the eastern half of the United States. Precipitation forms
at converging regions along the jet and precipitation in the interior of the eastern
U.S. is strongly tied to disturbances off the Front Range of the Continental Divide
(Schubert et al. 1998).
The GPLLJ is also difficult to simulate in climate models. Cook et. al. (2008)
studied 18 coupled atmosphere-ocean GCMs to determine which models best
represent the GPLLJ in the twentieth century to be used for predicting changes in
the GPLLJ due to increased greenhouse gases in the twenty-first century. She found
that all of the models produced a GPLLJ and most simulated its longitudinal position
fairly well but with varying degrees of accuracy. The Community Climate System
Model (CCSM3), which is the combination of many component models including the
Community Atmosphere Model (CAM) and one of the models our study analyzes,
outperformed many of the other models and was selected for their twenty-first
century simulation (Cook et al. 2008).
CAM was selected in this study to be compared against reanalysis and
observational data in order to better define the model’s strengths and weaknesses
in representing the role of the BH in bringing moisture into North America. The
main motivating question is how does the variability of the BH in the observations
compare to that of the BH in the model and does this variability increase, decrease,
or stay the same with increasing model resolution? The degree of influence
surrounding the accuracy of variability in the model will be studied in the
precipitation patterns for the eastern half of the United States as compared with
observations.
Cook, K. H., E. K. Vizy, Z. S. Launer & C. M. Patricola (2008) Springtime Intensification
of the Great Plains Low-Level Jet and Midwest Precipitation in GCM
Simulations of the Twenty-First Century. Journal of Climate, 21, 6321-6340.
Helfand, H. M. & S. D. Schubert (1995) CLIMATOLOGY OF THE SIMULATED GREATPLAINS LOW-LEVEL JET AND ITS CONTRIBUTION TO THE CONTINENTAL
MOISTURE BUDGET OF THE UNITED-STATES. Journal of Climate, 8, 784-806.
Iorio, J. P., P. B. Duffy, B. Govindasamy, S. L. Thompson, M. Khairoutdinov & D.
Randall (2004) Effects of model resolution and subgrid-scale physics on the
simulation of precipitation in the continental United States. Climate
Dynamics, 23, 243-258.
Schubert, S. D., H. M. Helfand, C. Y. Wu & W. Min (1998) Subseasonal variations in
warm-season moisture transport and precipitation over the central and
eastern United States. Journal of Climate, 11, 2530-2555.