1. Mesoscale “Surprises” in Complex Terrain Revealed by Regional Climate Simulations
Cliff Mass, Atmospheric Sciences
University of Washington

2. Most climate impacts evaluation is based on GCM’s with grid spacing of 100-150 km

3. Too coarse to simulate the effects of critical terrain/coastal interfaces or mesoscale features
Climate Model Terrain

4. Might unresolved mesoscale circulations alter the global warming story locally?
Terrain-forced mesoscale circulation can have a complex, non-linear relationships with changing synoptic flow.
Also interactions with land use and land/water interfaces
Other complexities with the relationship of convection and changing large-scale flows.

5. Two Main Approaches to Downscaling GCMs to Secure Mesoscale Impacts
Statistical: using contemporary relationships between large scale fields (e.g., temperature or precipitation) and their high-resolution distribution.
Dynamical: using high resolution mesoscale models (a.k.a, regional climate models) with boundary conditions driven by GCMs

6. Dynamical Downscaling Not Statistical Downscaling
Only fully dynamical downscaling can simulate the non-linearities and complexities of the mesoscale response to global warming.
Examples:
Location and distribution of precipitation as stability changes.
Onshore flow and coastal marine clouds enhanced by greater onshore pressure gradients
Albedo feedbacks as mountain snow melts.

7. Some surprises based on currently available GCMs and a limited number of high-resolution dynamically downscaled runs

8. 12-km RCM downscaling of ECHAM5 GCM
Change in Winter Surface Air Temperatures (F) 8

9. 9

10. 10

11. Why local hot spots? Regions of melting snow on terrain

12. A major surprise: synoptic deamplification and its effects on downslope flow off terrain, offshore flow, and heat waves

13. There is a strong correlation between easterly offshore flow and heat waves in western WA Easterly flow brings continental influence and subsidence warming on western slopes

14. Onshore/offshore flow is mainly controlled by synoptic variability
Heat waves in Seattle and Portland are preceded by inland high pressure, lower pressure offshore, and strong easterly flow descending the Cascades.

15. CMIP5 simulations indicate a robust deamplification of summer synoptic amplitude

16. Impact: LESS extremes in onshore/offshore flow (850 hPa shown)
Zonal Wind Histogram
Western Oregon/WA
Blue=
Light brown=
Dark brown=overlap

18. Global warming deamplification of summer synoptic amplitude works AGAINST downslope/offshore flow forced heat extremes
Weaker maxima for offshore flow works against high temperature extremes over western WA/OR
Very significant on the coast where offshore flow is most critical for heat waves.

19. Let’s Examine the Impacts Using a High-Resolution Regional Climate Model (WRF, 12-km)

20. Mesoscale Location Makes All the Difference for How Extreme Temperatures Will Change 1970–99 and 2040–69 (CCSM3 Model)

22. Very Warm Tails Don’t Change Much

23. The extremes do increase substantially inland from GW

24. Surprise! More low clouds during spring/early summer west of Cascades

25. March-April-May Changes

27. Why more clouds in spring?
The interior of the continent warms up more/faster than the eastern Pacific, resulting preferential hydrostatic pressure declines over the interior.
GCMs enhance eastern Pacific anticyclone
Enhanced onshore pressure difference pushes marine air inland.

28. More extreme precipitation and flooding from atmospheric rivers?

29. Global warming will intensify atmospheric rivers
Examined a large collection of climate model simulations for changes between to based on “business as usual”, RCP 8.5 greenhouse gas emissions.
Precipitation during extreme atmospheric river days increases by 15-39% .

30. Need high resolution RCMs to explore the impacts, which will be localized on coastal terrain

31. More Mesoscale Complexities
With warming there will be less snow, so buffering or water storage in the snowpack.
Thus, heavier rainfall could lead to even greater flooding on major rivers.

32. And even more…
Heavier precipitation shifts EARLIER in the fall (more in October)

33. Many other potential mesoscale surprises under global warming
Changes in sea breeze circulations
Weakening of onshore marine pushes
Changes in convection
Changes in the Southwest Monsoon circulation and convection
…and many more

34. Now producing dozens of high-resolution dynamically downscaled runs to explore mesoscale impacts of global warming over the western U.S.

35. Opportunity
Big opportunity for mesoscale meteorologists to work with our climate-oriented colleagues to gain insights into the regional and local impacts of global warming.
Climate change research is rapidly going downscale to the mesoscale.

36. The END