Downscaling for Climate Change

1

• Downscaling for Climate Change Water Resources
• Mike Dettinger, Hugo Hidalgo Dan Cayan
• USGS _at_ SIO, La Jolla, CA
• Quick review of western trends projections
• USGS downscaling needs
• New downscaled scenarios

2

• Review of recent trends projections
• Mostly results from

USGS Hydroclimatology (Global Change
Hydrology) Program-funded studies Funded
by Office of Surface Water Thanks to Harry
Lins Bill Kirby Julie Kiang San Francisco Bay
PES efforts
3
2. USGS downscaling needs Mostly lessons from
Efforts contributing to the 2006 California
Climate Change Assessment activities Planning
for 2008 California Climate Change Assessment
http//meteora.ucsd.edu/cap/pdffiles/CA_climate_Sc
enarios.pdf
4
SF Bay PES CASCaDE projects
3. New downscaled scenarios A product of
Computational Assessments of Scenarios of Change
in the Delta Ecosystem
5
1. Review of recent trends projections
The western states have been warming in recent
decades.
6
Geographically, here is the pattern of warming.
(Cayan et al., 2001)
Annual-mean warming, 1979-2005 (IPCC 4th WG1 Ch.
4)
7
Warming already has driven hydroclimatic trends.
Observed Less spring snowpack
TRENDS (1950-97) in April 1 snow-water content
at western snow courses
Observed Less snow/more rain
Knowles et al., 2006
Mote, 2003
-2.2 std devs LESS as snowfall
1 std dev MORE as snowfall
Observed Earlier greenup
Observed Earlier snowmelt runoff
Cayan et al., 2001
Stewart et al., 2005
8
Under projected greenhouse forcings, all climate
models yield warmer futures for California
and the West.
Dettinger, 2005
9
Under even the coolest of these scenarios
(2.5ºC), observed hydrologic trends in most
Western streams are projected to continue
in 21st Century.
Stewart et al., 2004
Center-of-Timing of Streamflow

• With coming addition of
• increased flood risks in Pacific coast states
• low baseflows
• warm streams
• drier summer soils fuels
• potentially large changes in mountain
• recharge

Knowles Cayan, 2004
Knowles and Cayan 2002
10

• Three Stages of Climate-Change Response
• Is there a problem?
• What should I do?
• What are my risks and options?

11
2. USGS downscaling needs
The downscaling problem
Global Climate Model (actually, general
circulation model)
12
USGS PES SF Bay (CASCADE) project needs
Downscaled series needed for cascade of
hydrologic, hydrodynamic, geomorphologic,
geochemical ecological modelers Daily max
min temperatures Daily precipitation
?
San Francisco
Monterey
13

• Downscaling options
• Broad-brush ?T and P changes
• Scenario options
• Synthetic statistical
• Describe the probability distributions of
  downscaled
• variables conditional on statistical relations
  between GCM and downscaled variables, and
  resample
• accordingly
• Deterministic statistical
• Describe spatio-physical relations between GCM
  and
• downscaled variables, and interpolate directly
• Dynamical
• Simulate same physical processes as in GCM but
  at higher resolution and (today) over smaller
  spatial
• domains

14

• Our downscaling wish list
• -- Resolution Daily time resolution, 10-km
  spatial resolution, multiple climate variables
• -- Accuracy Reproduction of high-resolution
  historical records
• -- Feasibility Not too computationally
  burdensome (?)
• -- Synchronicity Downscaled weather synchronized
  with climate model weather (just a Delta SLR
  thing?)
• -- Theoretical Doesn't constrain future
  'weather' to be same as historical
• -- Aesthetic Climate ( trends) arise from
  entire weather field rather than being imposed
  GCM-grid cell by grid cell
• -- Practical Ability to downscale to grids
  stations at same time, maintaining internal
  consistency

15

• The Delta
• The Delta encompasses 2,177 km2 of agricultural
  land Central Valley, including the Delta, yields
  31B/yr (45 of the US produce)
• 6 km3/yr of water is pumped from the Delta by
  State Feds for agriculture SoCal urban uses
  (22M people)

Lower salinity inflows
Seawater intrusion
Higher salinity freshwater inflows
Export to south
16

• LEVEE STABILIZATION
• 1800 km of levees provide the dry land for 57
  manmade islands
• Aging levees, sea-level rise, storm surges
  flows, oxidizing island peats, failing ecosystems
  earthquakes threaten uncontrolled breaches
• Levee breaches continue, despite massive
  structural controls in the 20th Century

Florsheim Dettinger, 2005
17
Sources of extreme water levels from San
Francisco to Sacramento
NORTH
Sacramento
San Joaquin R
Sacramento R
Delta
Bay
San Francisco
Monterey
18
From 2006 California Climate Change Assessment
Projected extreme high sea-level stands
(black) Extreme high sea-level stands coinciding
with low-pressure systems (red) A distant proxy
for measuring the coincidences of high seas
floods
Cayan et al, in press, Clim. Chg
PROBLEM BCSD-based downscaling yields enhanced
floods, but downscaled floods not synchronized
with GCM storm surges
19
Downscaled Projected Trends in December
Precipitation by Two Approaches (GFDL CM2.1, A2
emissions, 21st Century)
Ours
BCSD
Bias correction spatial downscaling, from Ed
Maurer, SCU
20

• Our approach towards downscaling climate change
• Downscale weather day by day
• Downscale enough (daily) weather and you get
  downscaled climate
• Downscale enough climate and you get downscaled
  climate change
• Don't impose climate or climate change after the
  fact !

21
The constructed-analogs method
Hidalgo, H.G., Dettinger, M.D., and Cayan, D.R.,
in review, Downscaling using constructed
analogues daily US precipitation and
temperatures J. Climate, 24 p.
22
Hidalgo, H.G., Dettinger, M.D., and Cayan, D.R.,
in review, Downscaling using constructed
analogues daily US precipitation and
temperatures J. Climate, 24 p.
23
Hidalgo, H.G., Dettinger, M.D., and Cayan, D.R.,
in review, Downscaling using constructed
analogues daily US precipitation and
temperatures J. Climate, 24 p.
24
(No Transcript)
25
Skill of downscaling as indicated by application
of method to historical OBSERVATIONS
Skill at monthly average scale
26
Distributions of daily precip at selected sites
Application of method to historical OBSERVATIONS
shows that even extremes are captured accurately
Hidalgo, H.G., Dettinger, M.D., and Cayan, D.R.,
in review, Downscaling using constructed
analogues daily US precipitation and
temperatures J. Climate, 24 p.
27

• Our constructed analogs offer
• -- Resolution
• -- done
• -- Accuracy
• -- just illustrated
• -- Computationally feasible
• -- yes
• -- Synchronized with GCM
• -- yes
• -- Doesn't constrain future 'weather' to be same
  as historical
• -- done
• -- Source of trends
• -- everything fitted to the whole field at daily
  level with no trends imposed externally
• -- Downscaling fields stations
• -- direct extension

28
3. A downscaling offer

• Available by end of (calendar) year 2007
• -- Daily downscaled Tmin, Tmax Precip fields
  for 1950-2100 across CONUS on 12-km grid will be
  online and available from PES/CASCADE
• From GFDL CM2.1 climate model (one of the
  warmest models) under heavy emissions (A2) and
  moderate emissions (B1) scenarios
• From NCAR PCM climate model (one of the
  coolest models) under heavy emissions (A2) and
  moderate emissions (B1) scenarios
• Planned by beginning of summer 2008
• -- Same for between 4 and 6 more models

29
Caveats with this approach -- If you don't like
the PRISM temperature precipitation
interpolations in your area, you won't like the
downscaled fields -- If the GCM's weather
(historical or otherwise) is flakey, this
downscaling won't fix it -- CA needs daily GCM
fields and an archive of observed weather data
(analogs) -- U of WA USBR are hurrying to
distribute another set of downscaled fields
(BCSD), which may become the de facto default
30
What do our constructed-analogs downscaled
climate changes look like?
A2
Dettinger, 2005
31
One day in the 21st Century
(GFDL A2)?
Downscaled
Original GCM values
32
Trends, December temperatures
Trends, June-August temperatures
33
Trends, December precipitation
Trends, June-August precipitation
34
A2
B1
35
A2 Tmin Trends
B1 Tmin Trends
36
A2 Precip Trends
B1 Precip Trends
37

• TO SUMMARIZE re clim.chg.
• Warming by 2 to 6ºC,
• likely more so at higher altitudes
• Uncertain precipitation changes,
• maybe less over much of western mountains, mostly
  small declines
• Significant changes in
• rain-vs-snow storms
• snowpack amounts
• snowmelt timing
• flood risk
• streamflow timing
• low flows
• growing seasons
• recharge?
• total flow (drought?)

Already detected
PES/CASCADE is producing a new generation of
downscaled scenarios for CONUS
38
(No Transcript)
39

• REFERENCES
• Bales, R., Molotch, N., Painter, T., Dettinger,
  M., Rice, R., and Dozier, J., 2006, Mountain
  hydrology of the western United States Water
  Resources Research, 42, W08432,
  doi10.1029/2005WR004387, 13 p.
• Cayan, D.R., Kammerdiener, S., Dettinger, M.D.,
  Caprio, J.M., and Peterson, D.H., 2001, Changes
  in the onset of spring in the western United
  States Bulletin, American Meteorological
  Society, 82, 399-415.
• Dettinger, M.D., 2005, From climate-change
  spaghetti to climate-change distributions for
  21st Century California San Francisco Estuary
  and Watershed Science, 3(1), http//repositories.c
  dlib.org/jmie/sfews/ vol3/iss1/art4.
• Dettinger, M.D., and Cayan, D.R., 1995,
  Large-scale atmospheric forcing of recent trends
  toward early snowmelt in California Journal of
  Climate 8(3), 606-623.
• Dettinger, M.D., Cayan, D.R., Meyer, M.K., and
  Jeton, A.E., 2004, Simulated hydrologic responses
  to climate variations and change in the Merced,
  Carson, and American River basins, Sierra Nevada,
  California, 1900-2099 Climatic Change, 62,
  283-317.
• Hidalgo, H.G., Cayan, D.R., and Dettinger, M.D.,
  in preparation, Variability of spring-summer
  drought in regions of high and low evaporative
  efficiency for submission to J. Hydrometeorology
• Intergovernmental Panel on Climate Change, 2007,
  Climate change 2007, The physical science
  basisSummary for Policymakers IPCC Secretariat,
  http//www.ipcc.ch, 21 p.
• Knowles, N., and D. Cayan, 2002, Potential
  effects of global warming on the Sacramento/San
  Joaquin watershed and the San Francisco estuary
  Geophys. Res. Lett., 29 18-21.
• Knowles, N., and D. Cayan, 2004, Elevational
  Dependence of Projected Hydrologic Changes in the
  San Francisco Estuary and Watershed. Climatic
  Change, 62, 319-336.
• Knowles, N., Dettinger, M., and Cayan, D., 2006,
  Trends in snowfall versus rainfall for the
  Western United States Journal of Climate,
  19(18), 4545-4559.
• Mote, P.W., 2003, Trends in snow water equivalent
  in the Pacific Northwest and their climatic
  causes. Geophysical Research Letters, 30, DOI
  10.1029/2003GL0172588.
• Stewart, I., Cayan, D., and Dettinger, M., 2005,
  Changes towards earlier streamflow timing across
  western North America Journal of Climate,18,
  1136-1155.