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NHC Track and Intensity Models Mark DeMaria NESDIS/CIRA RAMM Team COMET Presentation May 1999. Outline. TPC/NCEP Guidance Models Track, Intensity Forecast Procedure/ Products Forecast Examples Discussion. Factors Affecting TC Motion. Zero Order - Cons. of relative vorticity
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NHC Track and Intensity Models Mark DeMaria NESDIS/CIRA RAMM Team COMET Presentation May 1999
Outline • TPC/NCEP Guidance Models • Track, Intensity • Forecast Procedure/ Products • Forecast Examples • Discussion
Factors Affecting TC Motion • Zero Order - Cons. of relative vorticity • Vortex Moves with “Steering Flow” • First Order - Cons. of absolute vorticity • Vortex induces beta-gyres and affects motion • General Model • Vertical structure is important • Interaction with orography, friction, convection
Track Guidance Models • Zero Order - CLIPER, NHC90/91 • First Order - BAM, LBAR, VICBAR • General Models - GFDL, AVN, UKMET, NOGAPS, ETA
CLIPER (CLIMatology and PERsistence) Statistical track model developed in 1972 Required Input: Current/12 h old speed/direction of motion Current latitude/longitude Julian Day, Storm maximum wind Avg. 24, 48, 72 h errors: 210, 450, 650 km Used as benchmark for other models
NHC90 (Atlantic), NHC91 (East Pacific) • Statistical-Dynamical track model • Required Input • CLIPER forecast tracks • Deep layer mean heights (1000-100 mb) from NCEP global model forecast • New version for Atlantic with vortex removal scheme (NHC98)
BAM (Beta and Advection Model) • Follows trajectory in aviation run of MRF • Includes correction for “beta”-effect • Horizontal smoother (T25) applied to MRF wind fields • Three versions (BAMS, BAMM, BAMD) • Shallow: 850-700 mb • Medium: 850-400 mb • Deep: 850-200 mb
LBAR (Limited-area BARotropic) • Shallow water equations on Mercator projection solved using sine transforms • Initialized with 850-200 mb average winds/heights from NCEP global model • Sum of idealized vortex and current motion vector added to large-scale analysis • Boundary conditions from global model
GFDL Model • Sigma vertical coordinate (18 levels) • Finite difference model on lat/lon grid • 3 nested meshes (111, 37, 18 km) • Boundary layer, cumulus and radiation parameterizations • Large-scale analysis, boundary conditions from aviation run of MRF model • Sophisticated vortex initialization
Total Wind Environment Vortex
Vortex from global Analysis Vortex from spin-up procedure
AVN 1000 mb Initial Wind/Vorticity 9/21/98 00 UTC (X indicates observed center of Hurricane Georges) X X
1998 Gulfstream IV Missions • 1 in July: Alex (1) • 7 in August Bonnie (4), Danielle (3) • 9 in September Earl (1), Georges (7), Hermine (1) • 2 in October Mitch (2) • 19 Total Missions
Tropical Cyclone Intensity Change • Internal Control - evolution of the inner core • Thermodynamic Control - maximum intensity depends on SST and trop. temp. • External Control - inner core is modulated by environmental interactions
Intensity Forecast Models • SHIFOR - Statistical, analogous to CLIPER • SHIPS - Statistical-Dynamical • GFDL - Primitive Equation • Inland Wind Decay - Parametric model
SHIPS - Statistical Hurricane Intensity Prediction Scheme (Updated 1997) • Thermodynamic Predictors • SST potential (MPI-Vmax), 200 mb temp. • Synoptic Predictors • 850-200 mb shear, 200 mb zonal wind, 200 mb eddy momentum flux, 850 mb environmental vorticity, 200 mb divergence • Climatology and Persistence Predictors • Julian Day, Previous 12 h intensity change • SST evaluated along storm track, synoptic predictors from model forecast to 48 h
Inland Wind Decay Model • Simple exponential decay function • Decay parameters fit to NHC best track intensities for U.S. landfalling storms • Developed from 1967-1993 storms • Separate algorithm for New England region (1938-1991 sample) • Forecast depends on initial intensity, speed of motion
Improvements in Intensity Forecast Skill (Land Cases Excluded)
Advisory Preparation Schedule (4 per day) • 00:00 Data collection, evaluate synoptic situation • 00:30 Satellite classification, recon fixes • 00:45 Prepare input, initiate guidance models • 01:15 Forecast preparation, NWS coordination • 02:00 NCEP/NWS/Navy coordination • 02:15 NAWAS, Caribbean coordination • 02:45 Complete and transmit advisory
Overview of the Dvorak Technique • Visible and Infrared Technique • Uses patterns and measurements as seen on satellite imagery to assign a number (T number) representative of the cyclone’s strength. • The T number scale runs from 1 to 8 in increments of 0.5. • Additional rules determine current intensity
Typical Examples of Patterns in Visible Dvorak Method Curved Band Ivan 1998 Shear Bertha 1996 Eye Georges 1998 Central Dense Overcast Georges 1998