Low (web) Quality - BALTEX

203
Atmospheric river induced heavy precipitation and flooding in the
NARCCAP simulations
L. Ruby Leung and Yun Qian
Pacific Northwest National Laboratory, Richland, WA, USA; Ruby.Leung@pnl.gov
1. Introduction
The western U.S. receives precipitation predominantly
during the cold season when storms approach from the
Pacific Ocean. Several studies in recent years have clarified
the role of atmospheric rivers (AR) in producing heavy
precipitation and floods in the mountainous regions of the
West. Atmospheric rivers are narrow bands of enhanced
water vapor associated with the warm sector of extratropical
cyclones over the Pacific and Atlantic oceans (Zhu and
Newell 1998; Ralph et al. 2004]. Because of the strong
winds and neutral stability, atmospheric rivers often lead to
heavy precipitation due to large orographic enhancement
during landfall on the U.S. west coast. This study aims to
investigate how global warming may affect the frequency
and water vapor fluxes of atmospheric rivers and the
potential impacts on heavy precipitation and flooding in the
western U.S.
2. Numerical Simulations
As part of the North American Regional Climate Change
Assessment Program (NARCCAP), the Weather Research
and Forecasting (WRF) model (Skamarock et al. 2005) has
been used to simulate the regional climate of North
America. In this study, we first analyzed the simulation
driven by the NCEP/DOE global reanalysis for 1980-1999.
We used the standard NARCCAP domain that covers North
America and the adjacent oceans at 50 km grid resolution. A
simple nudging scheme was used to blend the lateral
boundary conditions from the global reanalysis with the
WRF simulation in a 10-grid point wide buffer zone, with
nudging coefficients following a linear-exponential function.
Several AR events in this simulation have been compared to
elucidate atmospheric and hydrologic ingredients that lead to
heavy precipitation and flooding in the western U.S.
A second set of simulations, in which WRF was driven by
the current (1970-2000) and future (2040-2070) climate
simulated by the Community Climate System Model
(CCSM), are being analyzed to investigate the potential
impacts of climate change on AR characteristics and the
resulting heavy precipitation and flooding in the
mountainous regions.
3. Comparison of Two AR Events
Atmospheric rivers have large impacts on heavy
precipitation and flooding along the west coasts where the
complex terrains effectively extract the low-level moisture
from the ARs. To examine the hydrologic impacts of AR,
we selected two specific AR events, the 1986 President Day
(PD) event and the 1997 New Year Day (ND) event, to
contrast the precipitation and flooding conditions. Our
analysis highlights the role of both atmospheric and land
surface conditions in flooding associated with AR.
Atmospheric stability plays a role in determining the spatial
distribution of precipitation of the 1986 PD and 1997 ND
events, as atmospheric stability can modify the orographic
precipitation signature through changes in low level flow
over terrain. This study suggests that the Froude number,
defined by atmospheric stability and low level wind speed,
can be a useful parameter for predicting or diagnosing
orographic precipitation pattern. This may also be true for
heavy precipitation such as those associated with AR, as
even small deviations from moist neutral stability can lead
to important differences in both precipitation amounts and
spatial distributions.
For precipitation to generate floods, our results underscore
the important role of antecedent soil moisture, as well as
precipitation phase (or snow level), which depends on
temperature, and possibly melting of existing snowpack
due to rain-on-snow. The 1997 ND event was found to
produce much larger flooding than the 1986 PD event
because the former was characterized by high antecedent
soil moisture, warmer temperature, which produces a
higher ratio of rainfall to snowfall, and larger existing
snowpack, which may increase runoff through rain on
snow. For a climate simulation to realistically characterize
extreme precipitation and flood in the western U.S., the
model must be able to simulate AR and its atmospheric
structures, as well as the land surface conditions. The
latter requires realistic simulation of both temporal and
spatial variability of precipitation.
4. Potential Changes of AR in the Future
Climate
Analysis is being performed to compare the AR
characteristics simulated by CCSM for the current and
future conditions. Preliminary results suggest a small
increase in AR frequency, particularly AR that makes
landfall in the Pacific Northwest, in the future (2040-
2070) compared to the current (1970-2000) conditions. In
addition, the ARs in the future are associated with more
water vapor content, which is consistent with the warmer
temperature that can hold more moisture. An increase in
AR frequency and water vapor flux suggests that heavy
precipitation and flooding may increase in the future
climate. Analysis is being performed using the WRF
downscaled climate change scenarios to examine how AR,
the associated precipitation, and land surface conditions
may be affected by climate change to alter extreme
precipitation and flooding in the western U.S.
References
Ralph, F.M., P.J. Neiman, and G.A. Wick (2004), Satellite
and CALJET aircraft observations of atmospheric
rivers over the eastern North-Pacific Ocean during
the winter of 1997/98, Mon. Wea. Rev., 132, 1721-
1745.
Skamarock, W.C., J.B. Klemp, J. Dudhia, D.O. Gill, D.M.
Barker, W. Wang, and J.G. Powers (2005), A
Description of the Advanced Research WRF Version
2. NCAR Technical Note NCAR/TN-468+STR,
88pp.
Zhu, Y., and R.E. Newell (1998), A proposed algorithm
for moisture fluxes from atmospheric rivers, Mon.
Wea. Rev., 126, 725-735.