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/Sierras•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 lower-resolution 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

45

135

15

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, 2090-2100

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 inWaterOfSnowpack(%)

Solar Radiation

Snow and Ice Reflect Much of The Incoming Solar Radiation

Now

Solar Radiation

Global Warming Causes Snow level to Rise Resulting In Absorption of Solar Energy on Melted Slopes

=WARMINGFuture

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