Determining the Local Implications of
Global Warming
For Urban Precipitation and Flooding
Clifford Mass and Eric Salathe,
Richard Steed
University of Washington
Although relatively rare, serious
urban flooding does occur in the
Puget Sound Region
• July 13, 1993: Nearly stationary
thunderstorms dumped heavy rain over
King and southern Snohomish Counties.
• Streets flooded with 1-2 feet of water and
several ramps to I-405 closed.
• Storm totals: 1-3.5 inches.
• A forest service employee reported 3.2
inches in 30 minutes.
December 13, 2006: The
Madison Valley Storm
Nearly 1 inch in an hour
Do We Expect Such
Events to Increase
in Frequency or
Intensity Under
Global Warming?
From general principles one could argue
BOTH for increasing and decreasing
threats.
Increasing or Decreasing Threat?
• On one hand, global warming will increase the
amount of moisture in the atmosphere, thus
potentially causing heavier or more frequent
showers. GREATER THREAT
• On the other hand, global climate model
simulations and recent satellite data suggest that
the jet stream…and its associated storms…may
move northward with global warming. LESSER
THREAT
Regional Climate Modeling
• As will be explained in this talk, there is
one approach that offers the potential to find
the best answer to this important question:
– Using General Circulation Models
(GCM’s)…global climate models… to drive
high resolution regional models capable of
simulated local meteorology.
Regional Climate Prediction
• To understand the impact of global warming, one
starts with general circulation models (GCMs) that
provide a view of the global evolution of the
atmosphere.
• GCMs are essentially the same as global weather
prediction models but are run with much coarser
resolution and allow the composition of the
atmosphere to vary in time (e.g., more CO2)
• Even leading GCMs only describe features of
roughly 500 km or larger in scale.
•Northwest weather is
dominated by terrain
and land-water
contrasts of much
smaller scale.
•In order to understand
the implications of
global changes on our
weather, downscaling
of the GCM predictions
considering our local
terrain and land use is
required.
Model Topography and Resolution
MM5 Topo (15 km)
GCM: ECHAM5 Topo (150km)
Downscaling
• There is only one way to do this right…
running full weather forecasting models at
high resolution over extended periods, with
the large scale conditions being provided by
the GCMs….this is called dynamical
downscaling.
• Such weather prediction models have very
complete physics and high resolution, so
they are capable of handling any “surprises”
Downscaling
• Computer power and modeling
approaches are now powerful enough
to make dynamical downscaling
realistic.
• Takes advantage of the decade-long
work at the UW to optimize weather
prediction for our region.
UW Regional Climate Simulations
• Makes use of the same weather prediction
model that we have optimized for local
weather prediction: the MM5.
• 10-year MM5 model runs nested in the
German GCM (ECHAM).
• MM5 nests at 135 km, 45 km, and 15 km
model grid spacing.
MM5 Model Nesting
• 135, 45, 15 km MM5 domains
• Need 15 km grid spacing to model local weather features.
Regional Modeling
• Ran this configuration over
several ten-year periods:
• 1990-2000-to see how well the
system is working
• 2020-2030, 2045-2055, 20902100
Details on Current Study: GCM
• European ECHAM model with resolution roughly
equivalent to having grid points spaced ~ 150 km apart.
Can resolve features of roughly 600 km size or more.
• IPCC climate change scenario A2 -- aggressive CO2
increase (doubling by 2050)
IPCC Report, 2001
IPCC Report, 2001
Change in
Water
Of
Snowpack
(%)
Precipitation over Western
Washington
• Bottom Line: No Large Regional Trends in
Annual Precipitation
• Some seasons gain precipitation (summer/fall),
others dry out.
• Modest increase in the number of days with
heavy rain.
More Heavy Rain in Seattle?
JJA Percent
Change in
Precipitation
SON Percent Change
in Precipitation
Summary
• The viability of the approach…using high resolution
numerical prediction models forced by large-scale
general circulation climate models (GCMs)… has
been demonstrated.
• Careful evaluation of the GCM output is
required…there are deficiencies, with GCM output
and our regional models are not perfect by any
means.
• Although there is general warming over the region
for all seasons, the terrain and land water contrasts
of the region enhance or weaken the warming in
certain areas.
Summary
• Precipitation changes are more modest then
temperature changes.
• There will be a substantial loss of snow pack,
during the next century.
• At this point, there does not appear to be a
radically increased threat of short-term heavy
precipitation leading to urban flooding.
The End
The surprise heavy rains in November flooded this intersection in Seattle.
(Photo by Casondra Brewster, USGS Seattle District)