Precipitation Extremes in Western U.S. Urban Areas: How Reliable are Regional Climate Model Projections Vimal Mishra 1, Francina Dominguez 2, and Dennis P. Lettenmaier 1 1 Civil and Environmental Engineering, University of Washington, Seattle, WA 2 University of Arizona, Tucson, AZ PNW Climate Science Conference,2011 1

Introduction 2

“ 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) 3 Intensified Hydrologic Cycle More Precipitation Extremes Warmer Climate

Precipitation Extremes 4 Groisman et al., 2005, J. of Climate Trends in heavy and very heavy rain events

Human contribution to more-intense precipitation extremes 5 S-K Min et al. Nature 470, (2011) doi: /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.

6 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 7

8 August 8, 2007, 8:49 AM Flooding Cripples Subway System (New York Times) HEAVY RAIN CAUSES MORE FLOODING IN CHICAGO SUBURBS Friday, 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. 9 Science Question How 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 10  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 [ ]  Reanalysis boundary conditions [ ]  Spatial Resolution– 50 km  Models: RCM3, CRCM, HRM3, MM5I, ECPC 11

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

13 Can RCMs reproduce the timing of precipitation maxima ? Winter Summer

14 Can RCMs reproduce the timing of precipitation maxima ? Winter Summer

15 Ensemble mean percentage bias in annual precipitation maxima Reanalysis GCM  Overestimation in the Interior west  Underestimation in the southwest

16 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

17 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 18

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. 19