Operational Flash Flood Warning Systems with Global Applicability

Transcription

1 International Environmental Modelling and Software Society (iemss) 7th International Congress on Environmental Modelling and Software San Diego, California, USA, D.P. Ames, N. Quinn (Eds.) Operational Flash Flood Warning Systems with Global Applicability Theresa M. Modrick, Rochelle Graham, Eylon Shamir, Robert Jubach, Cristopher R. Spencer, Jason A. Sperfslage, and Konstantine P. Georgakakos Hydrologic Research Center, San Diego, CA Abstract: The availability of global meteorological information (observations and forecasts) and widespread digital data on the land surface now affords the use of hydrometeorological modelling in an operational environment to provide numerical guidance for local forecasts and warnings of flash flooding by meteorological and disaster management agencies. Flash floods are among the deadliest natural disasters and can have significant impact on the natural environment and infrastructure. Working with national meteorological and hydrologic services, the Hydrologic Research Center has developed and implemented regional software systems to convey remotely sensed data and guidance products for rapid decisions regarding flash flood warnings. The systems integrate satellite-based and in-situ observations of precipitation with hydrologic models of the land surface to provide real-time assessment of hydrologic conditions at small spatial scales characteristic of flash flood occurrence. System design includes regional and local components to accommodate (a) the need for computational power at the regional level to process spatially distributed real-time data, and (b) the capability for local forecaster adjustments in real-time a quick warning dissemination through appropriate local channels. Regional or country operational systems have been implemented (or are under development) in Central America, Southern Africa, Southeast Asia, the Black Sea-Middle East region, Southeastern Europe, South Asia, Romania, Mexico, and the Republic of South Africa. This paper highlights the system design, capabilities, and challenges faced, including the characterization and conveyance of local forecast uncertainty in decision making by NMHSs, effective forecaster training, and effective dissemination to disaster response organizations. Keywords: flash flood forecasting; operations; remotely-sensed precipitation; distributed hydrologic modelling. 1 INTRODUCTION Flash floods are among the world s deadliest natural disasters with more than 5000 lives lost annually and result in significant social, economic and environmental impacts (Jonkman, 2005). Accounting for approximately 85% of the flooding cases, flash floods also have the highest mortality rate (defined as the number of deaths per number of people affected) among different classes of flooding (e.g., riverine, coastal). Flash floods have a different character than river floods, notably short time scales and occurring in small spatial scales, which make forecasting of flash floods quite a different challenge than traditional flood forecasting approaches. As an example, the United States National Weather Service (U.S. NWS) defines a flash flood as a rapid or extreme flow of high water into a normally dry area or a rapid rise of water level in a stream or creek above a

2 predeterminedflood level, beginning within 6 hours of the causative event (NWS, 2014). In forecasting flash floods, we are focused on two causative events, 1) intense rainfall and 2) rainfall on saturated soils. Flash floods occur throughout the world, and the time thresholds vary across regions from minutes to several hours depending on land surface, geomorphological, and hydroclimatological characteristics of the region. However, for the majority of these areas there exists no formal process for flash flood warnings and a lack of capacity to develop effective warnings for these quick response events. Advances in global datasets of land surface characteristics and in the availability of remotely sensed meteorological information, notably satellite estimation of precipitation, makes possible the use of hydrometeorological modelling in an operational environment for the evaluation and development of flash flood warnings by local forecasters. In 2001, the Hydrologic Research Center (HRC), in collaboration with the U.S. NWS, began development of a regional system to support flash flood warnings for seven countries of Central America. This effort was in response to the devastating impacts of Hurricane Mitch (1998), which caused widespread flooding, flash flooding, landslides, thousands of deaths, and billions of dollars in damages. The regional system would incorporate remotelysensed precipitation with the local but relatively sparse surface observation network, and distributed hydrologic modelling with high spatial resolution to produce guidance products pertaining to the threat of small scale flash flooding. The Central America Flash Flood Guidance (CAFFG) system became the first operational, regional flash flood guidance system with deployment in This paper discusses the design and advancement of regional flash flood guidance systems with the ability to be applied within any region throughout the globe. Several example output products are highlighted. The systems are designed for use by trained operational forecasters from National Meteorological and Hydrologic Services (NMHSs). This paper then discusses general challenges faced during implementation of several regional systems. 2 DESIGN OF FLASH FLOOD GUIDANCE SYSTEMS WITH GLOBAL APPLICABILITY With recognition of a lack of capacity and an expressed need for flash flood warnings worldwide, a Memorandum of Understanding (MOU) was signed among the World Meteorological Organization (WMO), the U.S. Agency for International Development/Office of U.S. Foreign Disaster Assistance (USAID/OFDA), the U.S. National Oceanic and Atmospheric Administration (NOAA), and HRC in February 2009, towards a cooperative initiative to implement flash flood guidance systems. The foundational approach in these systems is the concept of flash flood guidance (FFG), which involves the real-time comparison of observed (or forecast) rainfall with a characteristic volume of rainfall of a given duration for a given watershed which is sufficient to produce minor flooding at the watershed outlet (Georgakakos, 2006). Rainfall in excess of this characteristic amount is likely to produce flooding. This characteristic rainfall volume is termed flash flood guidance and is determined through hydrologic principles and accounting for the current state of the watershed in terms of capacity to absorb rainfall. The key elements of the design of the FFG system with applicability in any region of the globe are: Regional computational capacity that includes technical components of o the use of remotely-sensed precipitation (satellite and possibly radar); o bias-correction of remotely-sensed precipitation using available realtime surface observations; and,

3 o land-surface hydrologic modelling with relatively high spatial resolution at scales relevant for flash flood occurrence. Dissemination to local (national) forecasters to support local flash flood warning generation including o rapid evaluation of the FFG system products; o capability for the local forecaster to modify products based on up-tothe-minute local observations and other information, such as observer reports; and, o dissemination protocols for disaster management agencies. Figure 1 presents the flow of information from the key technical elements. This includes components for ingest of remotely sensed rainfall data, primarily satellite precipitation estimates for global applicability but with option for regional weather radars, and local raingauge data. Satellite precipitation estimates have the advantages of near global coverage and of providing estimates for regions that are not observed by traditional raingauge networks. The primary source for satellite precipitation estimates used is the Hydro-Estimator product (Scofield and Kuligowski, 2003) produced by the NOAA National Environmental Satellite, Data, and Information Service (NESDIS), due to the operational mandate and short latency time. This product is available within 30 minutes of observation. The rainfall data are quality controlled, merged, corrected for regional bias using the local real-time observations, and used to compute mean areal precipitation (MAP) for small watersheds defined for the region. In addition to considering observed precipitation for flash flood warnings (nowcasting), many systems also include quantitative precipitation forecasts (QPF) based on mesoscale numerical weather prediction models for short term flash flood forecast warnings or advisories, as indicated in the lower portion of Figure 1. The hydrologic technical components are indicated on the center and right hand side of the figure. These include modelling components for (a) estimating the characteristic runoff volume for each watershed associated with bankfull conditions at the watershed outlet (termed threshold runoff), (b) accounting for current soil moisture conditions, and (c) modelling of snowpack, if applicable. These hydrologic modelling components require a variety of land surface and meteorologic input in addition to precipitation as noted. The additional inputs are developed from global datasets with adjustment by regional information if provided by the NMHSs. These components feed the flash flood guidance model, which defines for each watershed and each time step, the current FFG. Again, flash flood guidance is defined as the volume of rainfall of a given duration and falling over a given watershed which is needed to produce minor flooding (defined in this formulation by bankfull conditions) at the watershed outlet. Watersheds are defined through GIS spatial analysis of digital elevation data with a resolution generally of km 2 on average. The determination of this scale is influenced by the source of precipitation forcing (e.g., for radar-based systems with higher resolution of precipitation forcing, the average scale maybe reduced to 25-50km 2.) The second key element of the systems indicated in the lower left of the figure is the local forecaster. These systems are designed to be a tool and provide guidance to the forecaster; the system is NOT designed to automatically produce flash flood warnings. The products are designed to be evaluated and potentially modified by knowledgeable forecasting personnel. Forecasters may have last minute information, which may include local observations not available to the regional system or reports from local observers. They also have knowledge of local meteorological and hydrologic response in their region that can influence their decision on whether to issue a flash flood warning or advisory to the public or disaster management agencies. Forecasters are trained on how to make adjustments to MAP products and FFG products.

4 Real-time Precipitation Inputs Satellite Rainfall Radar (as available) Gauge (as available) Rainfall Data Processing Quality Control Merging Bias Adjustment Basin Precipitation Air Temperature Snow Model Spatial GIS Data Analyses Flash Flood Guidance Model Soil Moisture Model Basin Delineation Parameter Estimation (Terrain, LULC, soils, streams) Flash Flood Guidance Threshold Runoff Model Potential Evapotranspiration (Climatological) Forecaster Input Flash Flood Threat Rainfall Forecasts (Mesoscale Model) (Forecaster Produced) Figure 1. Schematic of the FFG Systems technical components. The FFG approach and system design addresses requirements of operational flash flood prediction, which is fundamentally different than usual hydrologic modelling and meteorological forecasting approaches. First, prediction of flash floods differs from that of large river floods notably in the short time scales afforded in flash flooding. Prediction of the potential flash flood occurrence is most important, whereas in large river flooding concern is often with the magnitude of the flood (peak discharge). The catchment response times and flood wave routing allow for longer forecast lead time (several hours to days) in large river flooding, which in terms allows time for coordination of response and mitigation actions. Coordination of flash flood forecasting and response is challenging in real-time and requires careful coordination planning and protocols. Large river flood forecasting can be done with hydrologic models able to produce entire hydrographs with low uncertainty given good quality data. In flash flood prediction, local information is highly valuable, and in contrast to traditional meteorological forecasting approaches, requires knowledge and understanding of the hydrologic conditions including the sensitivity of uncertain input on the output of distributed hydrologic models (Carpenter and Georgakakos, 2006; Ntelekos et al, 2006). Flash floods are truly hydrometeorological events. As such, a critical part of the system design includes the collaboration with and training of personnel from the National Meteorological Hydrologic Services (NMHSs). Collaboration is necessary during the development period to understand and characterize the application region for the modelling components. Training of both meteorological and hydrology personnel aids in their understanding of the unique aspects of flash flood occurrence. Through time, training has evolved to include short-term workshops, longer term hands on training with system developers, and e-learning training modules. Sustainability of the systems requires commitment within the region, where trained forecasters become the trainers for future system users and end-users (such as disaster management). 3 REGIONAL SYSTEMS Table 1 indicates the regional FFG systems implemented, or under development, under the 2009 MOU. These include a total of 49 countries, serving a total population of more than 1 billion people. Additional local or country-specific

5 systems have been developed by HRC and are also noted in the table. Due to the nature of flash floods, the computation component of the systems are implemented within a regional center, typically designated as one of the National Meteorological Services which have 24/7 operation and capability for good communications and computational environment. Access to FFG system products are made through a secure web site, from which forecasters may view or download product images and system data. Figure 2 presents a few images of example system products. These images were extracted from the SARFFG system for the date of The top row of the figure includes gridded satellite precipitation from HydroEstimator and biascorrected MAP. The systems usually present precipitation products for 1-, 3-, 6-,and 24-hour accumulations, which are updated every hour. These products are intended to give the forecasters real-time and up-to-date best estimates of observed precipitation falling over their small watersheds. If forecast precipitation is available to the system, gridded and MAP forecast precipitation for these time durations are also presented. The second row of the figure present hydrologic modelling output including an estimate of the average soil water content in the surface soil and the current FFG values (for a 6-hour duration) for each small Table 1. Regional FFG Systems Deployed or Under Development by HRC. System Name Location Countries Served Precipitation Input CAFFG Central America Belize, Costa Rica, Satellite El Salvador, Guatemala, Honduras, Nicaragua, Panama MRCFFG Lower Mekong Cambodia, Laos, Satellite River Vietnam, Thailand MMFFG Mexico State of Chiapas, MX Single radar HDRFFG Haiti/Dominican Haiti, Dominican Satellite Republic Republic BSMEFFG Black Sea- Middle East Armenia, Azerbaijan, Bulgaria, Georgia, Iraq, Satellite, multiple radars Lebanon, Syria, Turkey SARFFG Southern Africa Botswana, Malawi, Mozambique, Namibia, South Africa, Zambia, Zimbabwe Satellite PAKFFG Pakistan Pakistan Satellite SEEFFG + Southeastern Satellite Europe Albania, Bosnia- Herzegovina, Croatia, Macedonia, Moldova, Montenegro, Romania, Serbia, Slovenia SAsiaFFG + South Asia Afghanistan, India, Bangladesh, Nepal, Pakistan, Sri Lanka CAsiaFFG + Central Asia Kazakhstan, Kyrgyzstan, Tajikistan Turkmenistan, Uzbekistan Satellite Satellite + (Systems under deployment) OTHER SYSTEMS NOT COVERED BY THE MOU ROFFG Romania Romania Multiple radar SAFFG South Africa Select regions of Republic of South Africa Multiple radar and satellite

6 watershed. The average soil water is a fractional value and indicates the level of saturation in the surface soil (which is most relevant for flash flood generation). The current FFG values are produced for 1-, 3-, and 6-hour rainfall durations, and, as defined above, indicate the amount of rainfall (of appropriate duration) necessary to produce minor flooding at the watershed outlet. The soil water content and current FFG values are typically updated every 6 hours in the systems. The lower row shows an initial indication of regions where the (observed or forecast) precipitation exceeded the corresponding FFG value. To highlight detail, these images are shown for the countries of Mozambique and the Republic of South Africa. These images show flash flood threat products and give indication of specific watersheds that may be at risk, which forecasters can then evaluate in further detail. It is noted that communications pertaining to the FFG System products are via secure internet channels. 4 CHALLENGES FACED The primary purpose of the FFG systems is to provide real-time informational guidance products pertaining to the threat of potential small-scale flash flooding to trained forecasters at NMHSs. To this end forecasters must be educated and trained to understand the unique challenges associated with flash floods. The objective of advanced operations training is to provide meteorologists and hydrologists with the science background and the interpretation/use of the products available in the flash flood guidance system for operations towards flash flood warning or advisory generation. For forecasters who are new to flash flood forecasting, the hydrological components of soil moisture, threshold runoff, watershed delineation, stream flow, and the concept of bankfull flow are the biggest challenge. As stated, this requires a fundamentally different approach than traditional operational forecasting. Our implementation experience has shown that as the knowledge and experience of trained forecasters increases, they are able to identify their individual strengths and weaknesses in relation to their abilities in flash flood forecasting and identify areas where their local knowledge and FFG system provide applicable and realistic results, as well as gaining a sense of the meteorological and hydrologic conditions likely to lead to flash flooding for their country. An additional and equally important as the identification of the causative factors that lead to flash flooding is the verification of flash flood events. Sources of verification of an individual event can take the form of reports from newspapers, local news reports, observers, and disaster response agencies. By verifying the impact of a forecast flash flood event (one in which a warning or advisory was issued), along with the severity of the impact, forecasters build experience and confidence in both their own skill and the FFG system. Evaluations for the Central America and Black Sea-Middle East systems over several months of flash flood warnings have been produced by operational forecasters using the guidance products. The evaluations were made in terms of traditional forecast performance measures of hit ratio and false alarm rate. The results indicate hit ratio of 65-70% and a false alarm rate of 0-32%. The work for the CAFFG System indicated that forecaster experience and adjustment to FFG system products yielded significant improvement in the performance measures. Shamir et al (2013) also show the utility of post-event analysis, concluding that reliable flash flood risk forecasts can be useful in managing disaster preparedness during a large scale event. The final challenge highlighted here is for forecasters to develop effective concept of operations and appropriate dissemination of the warnings to disaster response agencies and other interested stakeholders. The challenge is in having other agencies understand that flash floods are predictable on only very short time scales (< 6hrs) and the anticipation of the event requires preparedness and response management by organizations and people (Faulkner and Ball, 2007).

7 Figure 2. Example system products for the SARFFG System for For flash flooding, preparedness means developing response structures well in advance of events by establishing mechanisms for rapid and orderly action which limits impacts. Furthermore, public awareness and education efforts also limit impacts as the general public learns appropriate response to but official and unofficial warnings (including self-warning signs such as rising water levels). These are all important for flash flood management and developing preparedness strategies. 5 CONCLUSIONS

8 Flash floods are among the worst natural disaster worldwide, yet vast flash flood prone areas lack capacity for flash flood warning generation and adequate surveillance resulting in an unmitigated threat for millions of people. Developing and least developed countries often face the greatest challenges and suffer the greatest loss of life due to the lack of technical and financial resources. This work highlights the development of systems designed to address the need to provide early warnings for flash floods under a regional approach that has global applicability. Over the past five years, several regional and country-based operational systems have been deployed with others currently under development. The regional approach is efficient and targeted to aid developing countries. Initial evaluations of the systems have concluded that the systems do provide useful information for timely flash flood forecasts and warnings. Further systematic verification of forecast warnings is desired by or in collaboration with operational forecasters. The main challenges are (a) training of forecasters and disaster managers in the hydrometeorological aspects and response to effectively mitigate flash flood hazards, and (b) the development of programs for evaluation of realtime forecasts to provide additional guidance to forecasters for making real-time adjustments. ACKNOWLEDGMENTS Funding for the FFG Systems noted in the paper has come from various sources. We acknowledge ongoing support from and collaboration with USAID/OFDA, NOAA, and WMO. We gratefully acknowledge the continued collaboration with many individuals of the NMHSs of the countries served. In particular, for this paper, we acknowledge the forecast verification analysis performed by Ms. Rosario Alfaro, formerly with the National Meteorological Institute of Costa Rica (IMN), Ms. Lorena Soriano, of the El Salvador Ministero de Medio Ambiente y Recursos Naturales (MARN), and Mr. Ayhan Sayin of the Turkish State Meteorological Service (TSMS). REFERENCES Carpenter, T.M., Georgakakos, K.P., Discretization scale dependencies of the ensemble flow range versus catchment area relationship in distributed hydrologic modelling. J. Hydrology, 328, Faulkner, H., Ball, D., Environmental hazards and risk communications: prologue. Environ. Hazards, 7, Jonkman, S.N., Global perspectives on loss of human life caused by floods. Natural Hazards, 34, Georgakakos, K.P., Analytical results for operational flash flood guidance. J. Hydrology, 317, Ntelekos, A.A., Georgakakos, K.P., Krajewski, W.F., On the uncertainties of flash flood guidance: toward probabilistic forecasting of flash floods. J. Hydromet., 7, NWS, National Weather Service Glossary, U.S. National Weather Service. (last accessed ). Scofield R.A., Kuligowski, R.J., Status and outlook of operational satellite precipitation algorithms for extreme-precipitation events. Mon. Wea. Rev., 18, Shamir, E., Georgakakos, K.P., Spencer, C., Modrick, T.M., Murphy, M.J. Jr., Jubach, R., Evaluation of real-time flash flood forecasts for Haiti during the passage of Hurricane Tomas, November 4-6, Natural Hazards. DOI /s