High Rainfall Events and Their Changes in the Hawaiian Islands

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1 High Rainfall Events and Their Changes in the Hawaiian Islands Pao-Shin Chu, Xin Zhao, Melodie Grubbs, Cheri Loughran, and Peng Wu Hawaii State Climate Office Department of Meteorology School of Ocean and Earth Science and Technology University of Hawaii-Manoa Presented at the PRIDE Workshop, August 9-10, 2005, East-West Center

2 Project Objective To determine the frequency of occurrence of extreme rainfall events, their return periods, their long-term trends, and locations where extreme events are most likely to occur. To investigate spatial patterns and temporal trends of extremely high rainfall events in the Hawaiian Islands.

3 Problems in heavy rainfall and flooding The New Year s Eve flood on Oahu, December 31, 1987 January 1, 1988 ($35 million damage) Torrential rain with 24-h totals as high as 37 caused flooding in the southeastern part of the Island of Hawaii in November 2000 ($88 million damage) The October 2004 Halloween flood in Manoa ($75 million damage for the UH alone) Of the ten costliest natural disasters in Hawaii history, five are associated with heavy rainfall and floods (Honolulu Advertiser, 2004).

4 Potential Benefits State and county planners/managers will know better which areas are vulnerable to heavy rainfall and flooding. Products also will be useful for engineering design purposes, for real estate development, and for flood insurance guidance.

5 Relevance of the Project To establish a showcase so that the methodologies and analysis procedures used in Hawaii could be applied to the American flag territories because they have similar problems in heavy rainfall and floods as Hawaii To complement the overall goal of the NOAA Climate & Global Change Program which is to understand climate variability and change to enhance society s ability to plan and respond. This study also supports the NOAA s Pacific Islands Regional Integrated Science and Assessment (Pacific RISA) under the leadership of Eileen Shea which emphasizes on reducing Pacific island vulnerability to climate-related extreme events such as drought, floods, and tropical cyclones. To respond to the initial PRIDE call. The third thematic category of PRIDE (i.e., Risk Management) is support the provision of new data sets illuminating past, current, and future trends in patterns of climate and weather-related extreme events such as tropical cyclones, flooding, drought, Such applications are not only immediately applicable to the Pacific Islands region around Hawaii, but techniques and resulting integrated data products can be extensible to other coastal areas.

6 Methodologies to Identify Extreme Rainfall Events (Karl et al., 1996; Zwiers and Kharin, 1998; Groisman et al., 1999; Groisman et al., 2001) Heavy Rainfall Events Daily precipitation with amounts > 2 inches (based on a fixed precipitation value) 90th percentile of precipitation days (based on a specific threshold) Return period of 1-yr event associated with the annual maximum 24-h precipitation Generalized Extreme Value (GEV) Distribution with the Method of L- moments

7 There are 3 parameters for GEV: Location: Scale: Shape: ξ α k CDF: F( x) = exp{ exp( y)} where, y = 1 k ln{1 k(x ξ)/α} k 0 (x ξ)/α k = 0

8 Very Heavy Rainfall Events Daily precipitation with amounts > 4 inches 99th percentile of precipitation days Return period of a 20-yr event associated with the annual maximum 24-hr precipitation (GEV Distribution with the L-moments)

9 NCDC TD3200 Data Set Initially 294 stations in the Hawaiian Islands with daily rainfall records for 20 yrs Finally selected 158 stations (for winters (Nov to Apr) with no more than 45 days missing)

10 8 Lyon Arbor in Manoa, Oahu Year Frequency of "very high" rainfall events

15 On-going Research Winter, Summer 6-h rainfall statistics Temporal changes in extreme rainfall events Future Opportunities Streamflow gages in the Hawaiian Islands U.S. Affiliated Pacific Islands (e.g., Guam, Kwajalein)

Precipitation Extremes in the Hawaiian Islands and Taiwan under a changing climate

Precipitation Extremes in the Hawaiian Islands and Taiwan under a changing climate Pao-Shin Chu Department of Atmospheric Sciences University of Hawaii-Manoa Y. Ruan, X. Zhao, D.J. Chen, and P.L. Lin December