Implications of global climate
change over the mountain areas of
western North America
Professor Clifford Mass, Eric Salathe, Richard Steed
University of Washington
Questions
How do the western mountain barriers and
the proximity of the Pacific combine to
modulate the impact of global warming over
western North America?
Are there some potential surprises
due to terrain effects?
•Rockies
•Coast
Mountains/Cascades/Sie
rras
•Coastal Range
Downscaling of GCMs
Downscaling of GCMs: Statistical or
Dynamical Approaches?
• The starting point for this work: GCM runs
available through the IPCC and other sources
• Statistical downscaling is computationally
efficient, but are not effective if the
relationship between large scale flow and
mesoscale structures change under a
different climatic regime.
• Could be deceiving or wrong.
Dynamical Downscaling
• The alternative, dynamical downscaling
using high-resolution mesoscale models
nested in GCM fields, integrated over
decades, is now quite viable, and handle
non-linearities and “surprises” in the
downscaling.
• Dynamical downscaling used here (MM5
and WRF).
To Handle Western Terrain Need
Roughly 12-15 km Grid Spacing
36-km
12-km
4-km
UW Regional Climate Simulations:
First Attempt: MM5
• 10-year MM5 model runs nested in the
PCM and ECHAM GCMs.
• MM5 nests at 135km, 45km, and 15 km
model grid spacing.
• First tested with contemporary periods to
evaluate the fidelity of the system.
Terrain Lesson 1: Crazy Cold Waves
and Getting Your Domains Right
• Some lowerresolution GCM’s
had crazy cold
waves over coastal
N. America
• The reason:
Rockies too low to
stop cold air from
continental interior
Plumber’s Delight
• Surface temp, 850
mb heights
Solution
• Higher resolution GCMs
• Smart nesting structure that gives enough
resolution to sufficiently define the blocking
effects of the Rockies in the MM5 or WRF
domains.
MM5 Model Nesting
• 135, 45, 15 km MM5 domains
15
45
135
Regional Modeling
• Ran this configuration forced
by ECHAM GCM 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
• IPCC climate change scenario A2 -- aggressive CO2
increase (doubling by 2050)
Some Results, with emphasis on
the influence on terrain
Why Such Strong Warming on
Mountain Slopes.
Particularly in Spring?
The Answer:
Snow melt on lower/mid-slopes
resulting in more solar heating.
Change in
Water
Of
Snowpack
(%)
Snow and Ice Reflect Much of
The Incoming Solar Radiation
Solar Radiation
Now
Global Warming Causes Snow level to Rise
Resulting In Absorption of Solar Energy on
Melted Slopes
Solar Radiation
Future
=WARMING
Cooling and cloudier west of the
Cascades?
• Low clouds due to more onshore flow from
off the cool, cloud Pacific as interior warms.
• More low clouds over Pacific due to a
stronger anti-cyclone
• The Montereyization of the western
lowlands! And enhancement on windward
slopes of terrain.
Precipitation
• Bottom Line: No Large Regional Trends
• Less evidence of topographic feedbacks
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.
• 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
• Warming is enhanced on the upper windward slopes
due to snow melt.
• Springtime warming is lessened west of the Cascade
crest due to more low clouds.
• Precipitation changes are more modest then
temperature changes.
• There will be a substantial loss of snowpack,
reaching catastrophic decreases by 2090.
The END