precipitation extremes in western u.s. urban areas: how reliable are regional climate model...
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Precipitation Extremes in Western Precipitation Extremes in Western U.S. Urban Areas: How Reliable U.S. Urban Areas: How Reliable
are Regional Climate Model are Regional Climate Model ProjectionsProjections
Vimal MishraVimal Mishra11, Francina Dominguez, Francina Dominguez22, and Dennis P. , and Dennis P. LettenmaierLettenmaier11
1Civil and Environmental Engineering, University of Washington, Seattle, WA
2University of Arizona, Tucson, AZ
PNW Climate Science Conference,2011 [email protected] 1
Introduction
“there is now mounting evidence to suggest that a warmer climate will be one in which the hydrological cycle will in general be more intense, leading to more heavy rain events” (IPCC, 1996)
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Intensified Hydrologic Cycle
More Precipitation Extremes
Warmer Climate
Precipitation Extremes
4Groisman et al., 2005, J. of Climate
Trends in heavy and very heavy rain events
Human contribution to more-intense precipitation extremes
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S-K Min et al. Nature 470, 378-381 (2011) doi:10.1038/nature09763
Given that atmospheric water-holding capacity is expected to increase roughly exponentially with temperature—and that atmospheric water content is increasing in accord with this theoretical expectation—it has been suggested that human-influenced global warming may be partly responsible for increases in heavy precipitation.
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Need for the regional climate models for precipitation extremes
Source: NCAR
Spatial resolution of GCMs is too coarse to resolve the processes that control precipitation extremes
RCMs partially resolved finer scale variability related to topography and land cover
However, RCMs have been evaluated for precipitation extremes in urban areas
Precipitation Extremes and Urban Areas
• About 80% of U.S. population lives in urban areas
• Urban areas have large infrastructure and wealth and precipitation extremes may cause more damage than expected
• Increasing precipitation extremes may lead to substantial changes in drainage water infrastructure
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August 8, 2007, 8:49 AMFlooding Cripples Subway System (New York Times)
HEAVY RAIN CAUSES MORE FLOODING IN CHICAGO SUBURBSFriday, August 24, 2007
Precipitation Extremes in Urban Areas
Introduction
• So far relatively little work has been done evaluating the ability of RCMs to simulate precipitation extremes in major urban areas in U.S.
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Science QuestionHow effective are RCM-downscaled climate simulations in reconstructing observed sub-daily to daily scale precipitation extremes over the major urban areas in the western United States?
Study Region
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20 major urban areas across the Western United States Hourly precipitation data from the nearby stations
Data
Observed hourly precipitation data from NCDC RCMs simulated 3-hourly data from North
American Regional Climate Change Program (NARCCAP)
GCM boundary conditions [1968-1999] Reanalysis boundary conditions [1979-2003] Spatial Resolution– 50 km Models: RCM3, CRCM, HRM3, MM5I, ECPC
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Analysis
• Data quality check- years with more than 10% missing data were removed
• Extracted observed and RCMs simulated precipitation maxima at 3, 6, 12 and 24 hour durations
• Used areal reduction factor to convert point precipitation measurement to their areal equivalent
• Estimated annual precipitation maxima for 2 to 100 years return period using the Generalized Extreme Value (GEV) distribution
• Estimated ensemble mean bias in characteristics of precipitation maxima from the RCMs 12
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Ensemble mean percentage bias in annual precipitation maxima
Reanalysis GCM
Overestimation in the Interior west
Underestimation in the southwest
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Ensemble mean percentage bias in variability in annual precipitation maxima
Overestimation and underestimation of variability in annual precipitation maxima in majority of urban areas
Reanalysis GCM
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Ensemble mean percentage bias annual precipitation maxima at 5 and 100 years return period
Overestimation in the Interior west
Underestimation in the southwest
Reanalysis GCM
Conclusions
• Regardless of the boundary conditions (reanalysis/GCM), most of the NARCCAP RCMs are unable to reproduce the seasonality of precipitation extremes
• RCMs underestimate annual precipitation maxima in urban areas located in the coastal and southwest regions, while overpredict in the interior
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Conclusions
• RCMs overestimated (underestimated) precipitation maxima at 5 and 100 years return intervals for most of the urban areas in the interior (southwest)
• The deficiencies in RCMs may be attributable to the coarse resolution and parameterization related to the processes of precipitation extremes
• Further improvements are needed in RCMs before they can be considered reliable source of information for engineering design purpose.
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