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Determining the Local Implications of Global Warming For Urban Precipitation and Flooding

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Determining the Local Implications of Global Warming For Urban Precipitation and Flooding Clifford Mass and Eric Salathe, Richard Steed University of Washington – PowerPoint PPT presentation

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Title: Determining the Local Implications of Global Warming For Urban Precipitation and Flooding


1
Determining the Local Implications of Global
Warming For Urban Precipitation and Flooding
Clifford Mass and Eric Salathe, Richard
Steed University of Washington
2
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.

3
December 13, 2006 The Madison Valley Storm
Nearly 1 inch in an hour
4
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.

5
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 streamand its associated stormsmay
    move northward with global warming. LESSER THREAT

6
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 (GCMs)global
    climate models to drive high resolution regional
    models capable of simulated local meteorology.

7
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.

8
  • 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.

9
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10
Model Topography and Resolution
MM5 Topo (15 km)
GCM ECHAM5 Topo (150km)
11
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

12
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.

13
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.

14
MM5 Model Nesting
  • 135, 45, 15 km MM5 domains
  • Need 15 km grid spacing to model local weather
    features.

15
Regional Modeling
  • Ran this configuration over several ten-year
    periods
  • 1990-2000-to see how well the system is working
  • 2020-2030, 2045-2055, 2090-2100

16
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
17
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20
Change in Water Of Snowpack ()
21
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22
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.

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25
More Heavy Rain in Seattle?
26
JJA Percent Change in Precipitation
27
SON Percent Change in Precipitation
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31
Summary
  • The viability of the approachusing high
    resolution numerical prediction models forced by
    large-scale general circulation climate models
    (GCMs) has been demonstrated.
  • Careful evaluation of the GCM output is
    requiredthere 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.

32
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.

33
The End
The surprise heavy rains in November flooded this
intersection in Seattle. (Photo by Casondra
Brewster, USGS Seattle District)
34
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