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 vmishra@hydro.washington.edu 1

Introduction

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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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Can RCMs reproduce the timing of precipitation maxima ?

Winter Summer

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Can RCMs reproduce the timing of precipitation maxima ?

Winter Summer

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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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