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North Carolina Forecast System

Design and Implementation of a Real-Time, Event-Triggered Storm Surge and Flood Forecasting Capability for the State of North Carolina

Craig Mattocks¹, Cristina Forbes² and Limei Ran^1
1Institute for the Environment, University of North Carolina, Chapel Hill, North Carolina
²Institute of Marine Sciences, University of North Carolina, Morehead City, North Carolina

A new real-time, event-triggered storm surge prediction system* has been developed for the State of North Carolina to assist emergency managers, policy-makers and other government officials with evacuation planning, decision-making and resource deployment during tropical storm landfall and flood inundation events. Scientists at the Institute for the Environment and the Institute of Marine Sciences designed and built the North Carolina Forecast System (NCFS) from high-resolution versions of the ADCIRC (Advanced Circulation) coastal ocean model and the Weather Research & Forecasting (WRF) numerical weather prediction model. A rapid response assessment of hurricane threat is accomplished by driving the storm surge model with winds from a synthetic asymmetric gradient wind vortex generated from the National Hurricane Center (NHC) forecast advisories the moment they are inserted into the real-time weather data stream, maximizing the number of hours of forecast utility.

This approach has several important operational advantages. The use of a synthetic wind field allows a forecast simulation to be launched as soon as an advisory is issued by NHC, which maximizes the number of hours of forecast utility. The asymmetric vortex winds are generated on the fly from the gradient wind formula, so they can be directly coupled to the ocean model at every time step in the model simulation. Since these winds are available at "perfect" analytical resolution and "true" intensity, they can be calculated at the exact locations of the ADCIRC storm surge model's computational nodal points in the finite element mesh. This prevents interpolation errors that can cause distortions in the shape of the isotachs and an artificially weak representation of the tropical storm.

The eight primary lunar and solar tidal harmonic constituents (M2, S2, K2, N2, K1, O1, P1, Q1) were prescribed at the open water boundaries and applied as tidal potential constituents in the interior of the ADCIRC ocean model domain. A directional surface roughness parameterization that modulates the wind speed at a given location based on the types of landuse encountered upwind and a spatially varying distribution of Manning's roughness coefficient used for computing the bottom/channel bed friction are included in the storm surge predictions.

Comparisons of the model results against actual sea surface elevations measured by NOAA tide gauges along the NC coast indicate that this new system produces remarkably realistic predictions of storm surge. Improvements to the grid, the wind forcing and the ocean model and a more extensive verification/calibration of the system's performance are currently underway. These will enable the NCFS to be used in the future as a mission-critical tactical decision aid (TDA) by state and county emergency management agencies.

References

Mattocks, C, C. Forbes, L. Ran, 2006: "Design and Implementation of a Real-Time Storm Surge and Flood Forecasting Capability for the State of North Carolina". UNC-CEP, Technical Report #2-52130.

Mattocks C. and C. Forbes, 2008:"A real-time, event-triggered storm surge forecasting system for the State of North Carolina", Ocean Modelling, doi:10.1016/j.ocemod.2008.06.008 (in press).

Acknowledgments

The authors wish to thank Dr. Rick Luettich, Director of UNC's Institute of Marine Sciences in Morehead City, NC, for his insights on ADCIRC modeling issues. Ms. Crystal Fulcher, UNC-IMS, developed and refined the high-resolution NC finite-element grid for ADCIRC. Hurricane Specialists James Franklin and Lixion Avila and IT Specialist Brian Maher at the National Hurricane Center were extremely helpful in providing detailed information about NHC's forecast/guidance products, their generation, timing and accuracy. Ms. Limei Ran processed the 30-m NLCD land cover data and computed the directional surface roughness lengths and Manning's-n friction coefficients for use as ADCIRC nodal attributes.

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^*Experimental testbed for research purposes only.