Climate Change, Atmospheric Rivers, and Future California...
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Climate Change, Atmospheric Rivers, and Future California Floods Michael D. Dettinger , Ph.D. Research Hydrologist U.S. Geological Survey Scripps Institution of Oceanography La Jolla, CA 92093-0224 Tel: (858) 822-1507 Fax: (858) 822-2028 Email: [email protected] Web: http://tenaya.ucsd.edu/~dettinge BIOGRAPHICAL SKETCH Dr. Michael Dettinger is a research hydrologist for the U.S. Geological Survey, Branch of Western Regional Research, and a research associate of the Climate, Atmospheric Sciences and Physical Oceanography Division at Scripps Institution of Oceanography. He has degrees from the University of California, San Diego (Physics), Massachusetts Institute of Technology (Civil Engineering), and a Ph.D. from the University of California, Los Angeles (Atmospheric Sciences). Dettinger has monitored, evaluated, and researched the water resources of the West for over 25 years, focusing on regional surface water and groundwater resources, watershed modeling, causes of hydroclimatic variability, and climatic-change influences on western water resources. Among other activities, he was Nevada State Ground-Water Specialist for the USGS in the 1980s, received a Vice President’s National Performance Review Award for physical-sciences leadership in Mojave Desert Ecosystems science and data management planning efforts in 1996, was an Associate Editor of the journal Water Resources Research from 1998 to 2000, was the program chair and fundraiser for the annual Pacific Climate (PACLIM) Workshops, 1998-2004, is a founding member of the multi-institution CIRMONT Western Mountain Climate Sciences Consortium, and served on the CALFED Water Management Science Board. ABSTRACT Historically, the most dangerous storms in California have been warm wet storms that strike in winter, producing intense rains over large areas and unleashing many of the State’s largest floods. The most commonly recognized of these storms have been described as “pineapple express” storms because of the way that they appear (in weather satellite imagery) to draw warm, moist air from the tropics near Hawaii northeastward into California. Recent studies, though, have shown that pineapple express storms are just one version of a common feature of midlatitude weather, called “atmospheric rivers” (ARs). We now know that, globally, about 90% of all the water vapor transported towards the poles across the midlatitudes is transported within the narrow, intense filamentary bands of moist air that form these ARs. Because AR storms are increasingly understood to have been the source of most of the largest floods in California, an evaluation of the future of floods under climate change must attempt to project the future frequencies and intensities of ARs. Using a locally-based strategy for detecting AR-type storms along the California coast, developed at the NOAA Earth System Research Lab, climate simulations from seven global-climate models (GCMs) were analyzed to compare frequencies and magnitudes of AR storms arriving in California under simulated historical and climate-changed conditions. First, numbers of AR episodes in the climate models and in the observational record were compared to find that, although on average most of the models generate more ARs than observed, the general distribution of AR days per winter were not so different as to preclude evaluations of the projected changes. Next, in comparing historical to future climate simulations, changes in AR storms in the models were found to occur mostly at the extremes: Years with many AR storms become more frequent in most of the climate-change projections, but the average number of such storms per year are not projected to change much. Similarly, although the average intensity of the storms is not projected to increase much in most models, occasional much-stronger-than-historical-range storm intensities are projected to occur under the warming scenarios. The simulated AR storms also warm along with the winter- mean temperatures in the seven models. Together these findings suggest that California flood risks from the warm-wet, atmospheric-river storms may increase beyond those that we have known historically, mostly in the form of occasional more-extreme-than-historical storm seasons.
Transcript of Climate Change, Atmospheric Rivers, and Future California...
Climate Change, Atmospheric Rivers, and Future California Floods
Michael D. Dettinger , Ph.D. Research Hydrologist U.S. Geological Survey Scripps Institution of Oceanography La Jolla, CA 92093-0224
Tel: (858) 822-1507 Fax: (858) 822-2028 Email: [email protected] Web: http://tenaya.ucsd.edu/~dettinge
BIOGRAPHICAL SKETCH
Dr. Michael Dettinger is a research hydrologist for the U.S. Geological Survey, Branch of Western Regional Research, and a research associate of the Climate, Atmospheric Sciences and Physical Oceanography Division at Scripps Institution of Oceanography. He has degrees from the University of California, San Diego (Physics), Massachusetts Institute of Technology (Civil Engineering), and a Ph.D. from the University of California, Los Angeles (Atmospheric Sciences). Dettinger has monitored, evaluated, and researched the water resources of the West for over 25 years, focusing on regional surface water and groundwater resources, watershed modeling, causes of hydroclimatic variability, and climatic-change influences on western water resources. Among other activities, he was Nevada State Ground-Water Specialist for the USGS in the 1980s, received a Vice President’s National Performance Review Award for physical-sciences leadership in Mojave Desert Ecosystems science and data management planning efforts in 1996, was an Associate Editor of the journal Water Resources Research from 1998 to 2000, was the program chair and fundraiser for the annual Pacific Climate (PACLIM) Workshops, 1998-2004, is a founding member of the multi-institution CIRMONT Western Mountain Climate Sciences Consortium, and served on the CALFED Water Management Science Board.
ABSTRACT
Historically, the most dangerous storms in California have been warm wet storms that strike in winter, producing intense rains over large areas and unleashing many of the State’s largest floods. The most commonly recognized of these storms have been described as “pineapple express” storms because of the way that they appear (in weather satellite imagery) to draw warm, moist air from the tropics near Hawaii northeastward into California. Recent studies, though, have shown that pineapple express storms are just one version of a common feature of midlatitude weather, called “atmospheric rivers” (ARs). We now know that, globally, about 90% of all the water vapor transported towards the poles across the midlatitudes is transported within the narrow, intense filamentary bands of moist air that form these ARs. Because AR storms are increasingly understood to have been the source of most of the largest floods in California, an evaluation of the future of floods under climate change must attempt to project the future frequencies and intensities of ARs. Using a locally-based strategy for detecting AR-type storms along the California coast, developed at the NOAA Earth System Research Lab, climate simulations from seven global-climate models (GCMs) were analyzed to compare frequencies and magnitudes of AR storms arriving in California under simulated historical and climate-changed conditions. First, numbers of AR episodes in the climate models and in the observational record were compared to find that, although on average most of the models generate more ARs than observed, the general distribution of AR days per winter were not so different as to preclude evaluations of the projected changes. Next, in comparing historical to future climate simulations, changes in AR storms in the models were found to occur mostly at the extremes: Years with many AR storms become more frequent in most of the climate-change projections, but the average number of such storms per year are not projected to change much. Similarly, although the average intensity of the storms is not projected to increase much in most models, occasional much-stronger-than-historical-range storm intensities are projected to occur under the warming scenarios. The simulated AR storms also warm along with the winter-mean temperatures in the seven models. Together these findings suggest that California flood risks from the warm-wet, atmospheric-river storms may increase beyond those that we have known historically, mostly in the form of occasional more-extreme-than-historical storm seasons.
CLIMATE CHANGE, ATMOSPHERIC RIVERS &FUTURE CALIFORNIA FLOODS
Michael Dettinger (USGS)with contributions from Marty Ralph (NOAA) & Dale Cox (USGS)
Extreme Precipitation SymposiumSacramento, CA
24 June 2009
Outline
• Perspectives on Atmospheric Rivers (ARs)
• Detecting Atmospheric Rivers
• ARs in climate-change projections
• Conclusions
OF MORE THAN 40 CM
Atmospheric Rivers as Hazards
Atmosphericriver
Relationship of ARs and Flooding
From Ralph, et al., 2006
Dettinger, Extr PrecipSympos, 2004
Atmosphericriver
From Ralph, et al., 2006
Atmospheric Rivers as Water Supply
Atmospheric Rivers as Water Supply
Atmosphericriver
From Ralph, et al., 2006
California AR Landfalls
Atmospheric Rivers as Water Supply
Atmosphericriver
From Ralph, et al., 2006
Recognizing Atmospheric RiversAtmospheric
river
Vertically IntegratedWater Vapor Transports
SSMI Vertically Integrated
Water Vapor Content(> 2000 km long; < 1000 km wide;
IWV > 2 cm)
IVT > 500 kg/m/s
Recognizing Atmospheric Rivers Atmosphericriver
From Ralph, et al., 2006
Rain >10 mm/h:>12.5 m/s; >2 cm
Observed Rainfall Intensity (colors) vsForcings (upslope wind) and Fuel (water vapor) at Cazadero
Neiman et al 2009
Recognizing Atmospheric Rivers
Simulated Forcings (upslope wind) and Fuel (water vapor) on Central CA Coast
Atmosphericriver
Future Atmospheric Rivers Atmosphericriver
Cayan et al, 2009, CEC
Future Atmospheric RiversSimulated Forcings (upslope wind) and Fuel (water vapor) on Central CA Coast
Atmosphericriver
From Ralph, et al., 2006
Future Atmospheric RiversNumbers of AR storms on Central California coast in
7 climate-change projections & historical record
Future Atmospheric Rivers
Numbers of AR storms on Central California coast in 7 climate-change projections & historical record
Average # per yr
# yrs < 5 ARs
# yrs > 15 ARs
# yrs > 20 ARs
Reanalysis 1961-2000
5.8 days/yr 42 % of yrs 3 % of yrs 0 % of yrs
Projections1961-1980 8.5 25 16 51981-2000 9.0 27 16 82046-2065 11.6 12 28 102081-2100 11.7 16 32 12
30%increase
53% decrease
100% increase
85% increase
Future Atmospheric Rivers
Projected trends in numbers of AR storms on Central California coast in 7 climate-change projections
CCC (Canada)
CNRM (France)
ECHAM (Germany)
GFDL (USA)
GISS (USA)
MIROC
(Japan)
MRI (Japan
)
Change in number of ARs/yr in 21st
Century
+ 7 days
+2 days
+4.5 days
+0.4 days
+0.3 days
+2 days
+4 days
Future Atmospheric Rivers
Expect more moisture content in many storms
Future Atmospheric Rivers
Expect weaker upslope winds in many storms
Future Atmospheric Rivers
Let Product of Vapor * Winds = Intensity
Projected: Not so much change in mean intensities, but more extreme outliers
Future Atmospheric Rivers
Broad warming of AR storms, with some warmer than any historicals
From Ralph, et al., 2006
Future Atmospheric Rivers
Projected: AR warming along with general winter temperatures, but not as fast
Future Atmospheric Rivers
Lengthening of AR seasons?
Observed(Dettinger 2004)
Summary
• The future of landfalling atmospheric rivers is important for the future of flood hazards AND water resources in California
Projections of 21st Century climates suggest: • More years with lots of ARs, fewer with few• Moister ARs with weaker upslope winds• Overall average intensities don’t change much but
occasional much stronger than historical ARs• ARs warmer by about +2C on ensemble avg• AR season extends
Implication
• Thus, preparations for extreme storms (like the one being dramatized by the ARkStorm effort) is all the more prudent in view of current climate-change projections
For broader discussion of projected flood regimes, see:
Dettinger, M.D., Hidalgo, H., Das, T., Cayan, D., and Knowles, N., 2009, Projections of potential flood regime changes in California: California Energy Commission Report CEC-500-2009-050-D, 68 p.
