Modeling Tropical Cyclone Gridded Wind Field Forecasts 26th Conference on Hurricanes and Tropical Meteorology 3—7 May 2004 Miami, FL Ken Waters Joel Cline National Weather Service Honolulu, Hawaii

Modeling Tropical Cyclone Gridded Wind Field Forecasts Background Background Process/Algorithm Process/Algorithm Issues Issues Results Results Future Plans Future Plans

Background: Purpose The goal: The goal: –An hourly forecast estimate of surface wind speeds and direction for 31 island points in the West Pacific Needed for: Needed for: –Terminal Aviation Forecasts (TAFs) for 24-hour period –Hourly wind forecast needed to help determine start/stop times for Tropical Storm and Typhoon Watches and Warnings –Wind grid information for possible initialization of digital wind grids (National Digital Forecast Database)

Background: Warning Points

Official Tropical Cyclone Warnings are produced by the DoD Joint Typhoon Warning Center in Pearl Harbor Official Tropical Cyclone Warnings are produced by the DoD Joint Typhoon Warning Center in Pearl Harbor Forecast of position and maximum sustained wind speed for Hours 00, 12, 24, 36, 48, 72, 96, and 120 Forecast of position and maximum sustained wind speed for Hours 00, 12, 24, 36, 48, 72, 96, and 120 Estimated wind radii by quadrant for Hours Estimated wind radii by quadrant for Hours Background: Source Warnings

Background: JTWC forecast Max Wind 12 Hour Forecast Lat/Long Max Wind Initial Position Lat/Long 24 Hour Forecast

Background: JTWC forecast – Example Graphic Showing Wind Radii Information

Process/Algorithm: Rankine Vortex Anthes (1982): Anthes (1982): V(r) = Velocity at distance “r” from center of storm V(R 0 ) = Maximum Wind Speed at Radius of Maximum Wind, R 0 x = exponential “form factor”, typically between 0.5 and 0.7 r 0 = Radius of storm influence

Process/Algorithm: Rankine Vortex For winds inside the Radius of Maximum Wind (RMW), use: For winds inside the Radius of Maximum Wind (RMW), use:

Process/Algorithm: Interpolate storm forecast to obtain hourly positions and maximum wind speed Interpolate storm forecast to obtain hourly positions and maximum wind speed Apply the wind formula for a regular grid centered upon the storm at each hourly position as well as for selected point locations (e.g., islands) at each hour based on distance from storm to the island Apply the wind formula for a regular grid centered upon the storm at each hourly position as well as for selected point locations (e.g., islands) at each hour based on distance from storm to the island Compute simple wind direction orthogonal to line between the storm and the point Compute simple wind direction orthogonal to line between the storm and the point

Process/Algorithm: National Digital Forecast Database requires offices to produce surface wind grids of at least 5 km resolution every 3 hours National Digital Forecast Database requires offices to produce surface wind grids of at least 5 km resolution every 3 hours Following example shows a method of populating the wind field based solely on the wind radii (quadrant) information inside the warning --- not using Rankine vortex Following example shows a method of populating the wind field based solely on the wind radii (quadrant) information inside the warning --- not using Rankine vortex

Issues: Determination of Radius of Maximum Wind (RMW) - -- not part of forecast but input to the formula Determination of Radius of Maximum Wind (RMW) - -- not part of forecast but input to the formula Determination of the exponential “shape” factor Determination of the exponential “shape” factor How rapidly does wind speed fall off as one moves away from center? How rapidly does wind speed fall off as one moves away from center? Variances include spectrum from huge storm down to “midget” typhoons Variances include spectrum from huge storm down to “midget” typhoons Inherent dangers with using a highly deterministic approach to describe difficult-to-predict tropical cyclone movement and intensity Inherent dangers with using a highly deterministic approach to describe difficult-to-predict tropical cyclone movement and intensity

Results: Typhoon Sudal Apr 9, 2004 Warning #20 SYMMETRIC MODEL PREDICTED WINDS FOR: Koror Ngulu Yap HOURLY POSITION/CLOSEST POINT: # Lat Lon Wind Mvmt Dstnc/Dirctn from (naut mi) SSE Yap SSE Yap SSE Yap S Yap S Yap SSW Yap SSW Yap SW Yap SW Yap N Ngulu N Ngulu

Future Plans Asymmetric wind structure (moving storm) Asymmetric wind structure (moving storm) Environmental blending (e.g., SW monsoon flow) Environmental blending (e.g., SW monsoon flow) Verification (using…? QuikSCAT? Recon?) Verification (using…? QuikSCAT? Recon?) Create netCDF grids for possible use in initializing National Digital Forecast Database wind grids Create netCDF grids for possible use in initializing National Digital Forecast Database wind grids Explore methods to estimate exponential factor and Radius of Maximum Wind Explore methods to estimate exponential factor and Radius of Maximum Wind

Questions? Text data files can be found at: Text data files can be found at: – –Graphics planned for this spring/summer Ken Waters Regional Scientist National Weather Service, Pacific Region 737 Bishop St., Ste Honolulu, HI , cell: