Downscaling for Climate Change & Water Resources

Mike Dettinger, Hugo Hidalgo & Dan CayanUSGS @ SIO, La Jolla, CA

Quick review of western trends & projections USGS downscaling needs New downscaled scenarios

USGS Hydroclimatology (Global Change Hydrology)Program-funded studies

Funded byOffice of Surface Water:Thanks to Harry LinsBill KirbyJulie Kiang

& San Francisco Bay PES effortsReview of recent trends & projections:Mostly results from:

Efforts contributing to the2006 California Climate Change Assessment activities

Planning for 2008 California Climate Change Assessment2. USGS downscaling needs

Mostly lessons from:

http://meteora.ucsd.edu/cap/pdffiles/CA_climate_Scenarios.pdf

SF Bay PES & CASCaDE projects:ComputationalAssessments ofScenarios ofChange in theDeltaEcosystem3. New downscaled scenarios: A product of

The western states have been warming in recent decades.1. Review of recent trends & projections:

Geographically, here is the pattern of warming. (Cayan et al., 2001)Annual-mean warming, 1979-2005(IPCC 4th WG1 Ch. 4)

Warming already has driven hydroclimatic trends.Knowles et al.,2006-2.2 std devsLESS as snowfall+1 std devMORE as snowfallObserved: Less snow/more rainMote, 2003TRENDS (1950-97) in April 1 snow-water content at western snow coursesObserved: Less spring snowpackObserved: Earlier snowmelt runoffStewart et al., 2005Observed: Earlier greenupCayan et al., 2001

Under projected greenhouse forcings,all climate models yield warmer futures for California andthe West.Dettinger, 2005

Stewart et al., 2004Center-of-Timing of StreamflowUnder even the coolest of these scenarios (+2.5C), observed hydrologic trends in most Western streams are projected to continue in 21st Century.Knowles & Cayan, 2004Knowles and Cayan 2002With coming addition of increased flood risks in Pacific coast stateslow baseflowswarm streamsdrier summer soils & fuelspotentially large changes in mountain recharge

Three Stages of Climate-Change Response

Is there a problem?

What should I do?

What are my risks and options?

The downscaling problem2. USGS downscaling needsGlobal Climate Model(actually, general circulation model)

Downscaled series needed for cascade of hydrologic, hydrodynamic, geomorphologic, geochemical & ecological modelers:

Daily max & min temperaturesDaily precipitationUSGS PES SF Bay (CASCADE) project needs:?San FranciscoMonterey

Downscaling options:

Broad-brush T and %P changes

Scenario options:

Synthetic statisticalDescribe 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 DynamicalSimulate same physical processes as in GCM but at higher resolution and (today) over smaller spatial domains

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

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)Higher salinity freshwater inflowsLower salinity inflowsSeawater intrusionExport to south

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

Sources of extreme water levels from San Francisco to SacramentoSacramento RSan Joaquin RBayNORTHSan FranciscoMontereySacramentoDelta

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 + floodsCayan et al, in press, Clim. ChgFrom 2006 California Climate Change Assessment:PROBLEM: BCSD-based downscaling yields enhanced floods, but downscaled floods not synchronized with GCM storm surges

Downscaled Projected Trends in December Precipitation by Two Approaches(GFDL CM2.1, A2 emissions, 21st Century)OursBCSDBias correction & spatial downscaling, from Ed Maurer, SCU

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 !

The constructed-analogs methodHidalgo, H.G., Dettinger, M.D., and Cayan, D.R., in review, Downscaling using constructed analogues daily US precipitation and temperatures: J. Climate, 24 p.

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.

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.

Skill of downscaling as indicated by application of method to historical OBSERVATIONSSkill at monthly average scale

Application of method to historical OBSERVATIONS shows that even extremes are captured accuratelyHidalgo, H.G., Dettinger, M.D., and Cayan, D.R., in review, Downscaling using constructed analogues daily US precipitation and temperatures: J. Climate, 24 p.Distributions of daily precip at selected sites

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

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 3. A downscaling offer:

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

A2What do our constructed-analogs downscaled climate changes look like?Dettinger, 2005

(GFDL A2)One day in the 21st CenturyDownscaledOriginal GCM values

Trends, December temperaturesTrends, June-August temperatures

Trends, December precipitationTrends, June-August precipitation

A2B1

A2 Tmin TrendsB1 Tmin Trends

A2 Precip TrendsB1 Precip Trends

TO SUMMARIZE re: clim.chg.

Warming by +2 to +6C,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?)

just in response to temperatures* Already detected& PES/CASCADE is producing a new generation of downscaled scenarios for CONUS

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, doi:10.1029/2005WR004387, 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.cdlib.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. HydrometeorologyIntergovernmental 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.