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An Overview of Tropical Cyclone Intensity Guidance Models Used by NHC

An Overview of Tropical Cyclone Intensity Guidance Models Used by NHC. Jamie Rhome and Chris Landsea, NCEP/NHC Mark DeMaria, NOAA/NESDIS/StAR Andrea Schumacher, CIRA/CSU Bernard Meisner, NOAA/NWS/Central Region. An Overview of NHC Intensity Guidance Models. GENERAL OBJECTIVES

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An Overview of Tropical Cyclone Intensity Guidance Models Used by NHC

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  1. An Overview of Tropical Cyclone Intensity Guidance Models Used by NHC Jamie Rhome and Chris Landsea, NCEP/NHC Mark DeMaria, NOAA/NESDIS/StAR Andrea Schumacher, CIRA/CSU Bernard Meisner, NOAA/NWS/Central Region

  2. An Overview of NHC Intensity Guidance Models GENERAL OBJECTIVES By the end of this module, you should be able to… • Describe the spectrum of NHC TC intensity models • Describe the general strengths and weaknesses of each model • Give an example of each

  3. Types of TC Intensity Forecast Models Statistical Models: SHIFOR (Statistical Hurricane Intensity FORecast). Based solely on historical information - climatology and persistence. (Analog to CLIPER.) Statistical/Dynamical Models: SHIPS, (Statistical Hurricane Intensity Prediction Scheme): Based on climatology, persistence, and statistical relationships to current and forecast environmental conditions. LGEM (Logistic Growth Equation Model, variation on SHIPS, relaxes intensity to MPI) Dynamical Models: HWRF,GFDL, GFDN, GFS, UKMET, NOGAPS, ECMWF. Solve the governing equations for the atmosphere (and ocean). Ensemble, Consensus Models

  4. Statistical Intensity ModelsSHIFOR (Statistical Hurricane Intensity FORecast) • Based solely on climatology and persistence • Originally developed in 1979 • Updated in 1988, revised and extended to 120 hr in 2003 • Base Period 1967 – 1999 • Named storms (35 kt or greater) at least 30 n mi. from land • Predictor Variables • Julian day • Initial intensity, latitude and longitude • Intensity change (last 12 hrs) • Zonal and meridional storm motion (last 12 hrs) • Decay SHIFOR • Climatological decay rate over land using CLIPER track • SHIFOR, D-SHIFOR used as skill baseline

  5. Statistical/Dynamical Intensity ModelsSHIPS (Statistical Hurricane Intensity Prediction Scheme) • Multiple regression model • Predictors from climatology, persistence, atmosphere and ocean • Atmospheric predictors from GFS forecast fields • SST from Reynolds weekly fields along forecast track • Predictors from satellite data • Oceanic heat content from altimetry (Atlantic only) • GOES IR window channel brightness temperatures • Decay SHIPS • Climatological wind decay rate over land

  6. The SHIPS Model Predictors* (+) SST POTENTIAL (VMAX-V): Difference between the maximum potential intensity (depends on SST) and the current intensity. (-) VERTICAL (850-200 MB) WIND SHEAR: Current and forecast. (+) PERSISTENCE: If it has been strengthening, it will probably continue to strengthen, and vice versa. (-) UPPER LEVEL (200 MB) TEMPERATURE: Warm upper-level temperatures inhibit convection (+) THETA-E EXCESS: Related to buoyancy (CAPE); more buoyancy is conducive to strengthening (+) 500-300 MB LAYER AVERAGE RELATIVE HUMIDITY: Dry air at mid-levels inhibits strengthening *Red text indicates most important predictors

  7. The SHIPS Model (Cont…) • (+) 850 MB ENVIRONMENTAL RELATIVE VORTICITY: Vorticity averaged over large area (r <1000 km). Intensification is favored when the storm is in an environment of cyclonic low-level vorticity. • (+) GFS VORTEX TENDENCY: 850 hPa tangential wind (0-500 km radial average). Intensification favored when GFS spins up storm. • (-) ZONAL STORM MOTION: Intensification is favored when TCs are moving west • (-) STEERING LAYER PRESSURE: intensification is favored for storms that are moving more with the upper level flow. This predictor usually only comes into play when storms get sheared off and move with the flow at very low levels (in which case they are likely to weaken). • (+) 200 MB DIVERGENCE: Divergence aloft enhances outflow and promotes strengthening • (-) CLIMATOLOGY: Number of days from the climatological peak of the hurricane season

  8. Satellite Predictors added to SHIPS in 2003 • GOES cold IR pixel count 3. Oceanic heat content from • GOES IR Tb standard deviation satellite altimetry • (TPC/UM algorithm) • Cold IR, symmetric IR, high OHC favor intensification

  9. The Logistic Growth Equation Model (LGEM) • Applies a simple differential equation to constrain the max winds between zero and the maximum potential intensity • Intensity growth rate predicted using SHIPS model input • More responsive than SHIPS to time changes of predictors such as vertical shear • More sensitive track errors • More difficult to include persistence

  10. LGEM Improvement over SHIPS2006-2007 real-time runs

  11. SHIPS text output file SHIPS and LGEM forecasts Includes predictor information Also includes Rapid Intensity Index Note: SHIPS and RII output available on-line in real time ftp://ftp.tpc.ncep.noaa.gov/atcf/stext

  12. Dynamical Intensity Models GFS: U.S. NWS Global Forecast System < relocates first-guess TC vortex UKMET: United Kingdom Met. Office global model < bogus (syn. data) NOGAPS: U.S. Navy Operational Global Atmospheric Prediction System global model < bogus (synthetic data) ECMWF: European Center for Medium-range Weather Forecasting global model (no bogus) GFDL: U.S. NWS Geophysical Fluid Dynamics Laboratory regional model <bogus (spin-up vortex) GFDN: Navy version of GFDL model < bogus (spin-up vortex) HWRF:NCEP Hurricane Weather Research and Forecast regional model (vortex relocation and adjustment)

  13. Dynamical Intensity Models Limitations Sparse observations, especially in inner core Inadequate resolution, especially global models Data assimilation on storm scale Representation of physical processes (PBL, microphysics) Ocean interactions Predictability

  14. A Note on Bogussing • Since the globally analyzed vortex does not typically represent the structure of a true TC, “Bogussing” is often employed. • Involves an analysis of synthetic data to describe the TC vortex. • Can significantly affect the surrounding environment • Vertical shear • Creating and inserting a bogus is not straight forward • Forecast can be very sensitive to small changes in the bogus storm • Bogus storms tend to be too resilient during Extra-tropical Transition • Bogus retains warm core too long leading to poor intensity and structure forecasts

  15. GFS vs. GFDL Initial Vertical ShearHurricane Debby August 2000

  16. Horizontal Resolution in Global Spectral Models • Horizontal fields expanded in 2-D wave function series • Associated Legendre functions in latitude • Fourier series (sines and cosines) in longitude • T indicates Triangular truncation (e.g., T400) • Latitude and longitude series contain same number of terms • Uniform resolution over the sphere • Rule of thumb for comparison to grid-point models • x = 40,000 km/3N, N=truncation number

  17. Global Model Properties

  18. The Geophysical Fluid Dynamics Laboratory (GFDL) Hurricane Model • Dynamical model capable of producing skillful intensity forecasts • Coupled with the Princeton Ocean Model (POM) (1/6° horizontal resolution with 23 vertical sigma levels) • Replaces the GFS vortex with an axisymmetric vortex spun up in a separate model simulation • Sigma vertical coordinate system with 42 vertical levels • Limited-area domain (not global) with 2 grids nested within the parent grid. • Outer grid spans 75°x75° at 1/2° resolution or approximately 30 km. • Middle grid spans 11°x11° at 1/6° resolution or approximately 15 km. • Inner grid spans 5°x5° at 1/12° resolution or approximately 9 km

  19. GFDL Model Nested Grids

  20. The Hurricane Weather Research & Forecasting (HWRF) Prediction System • Next generation non-hydrostatic weather research and hurricane prediction system • Movable, 2- way nested grid (9km; 27km/42L; ~68X68) • Coupled with Princeton Ocean Model • Atlantic only in 2008, HYCOM model for 2009 in Atlantic and East Pac • 3-D VAR data assimilation scheme • But with more advanced data assimilation for hurricane core • Plans for use of airborne and land based Doppler radar data • Became operational in 2007 • Under development since 2002 • Runs in parallel with the GFDL

  21. *HWRF *GFDL *Configurations for 2008 season

  22. “Late” versus “Early” • “Late Models” = models not available at synoptic time • Dynamical models are not usually available until 4-6 hrs after the initial synoptic time (i.e., the 12Z run is not available until as late as ~18Z in real time) • Results must be interpolated to latest NHC position (GFDL GFDI, NGPS NGPI, etc) • Late Models: All global models, GFDL, GFDN, HWRF • “Early Models” = models available shortly after synoptic time • Early Models: LBAR, BAM, CLIPER, Interpolated dynamical models

  23. Consensus Intensity Models for 2009 • Fixed • ICON: SHIPS, LGEM, GFDL-I, HWRF-I • Variable • IVCN: SHIPS, LGEM, GFDL-I, HWRF-I, GFDN-I • Smart • FSSE (Florida State Super Ensemble, experimental)

  24. Intensity Model Verification • Calculate difference between model maximum wind and best track maximum wind • Homogeneous sample • All models must be available for inclusion of a case • NHC verifies tropical and subtropical stages • Extra-tropical stage excluded • SHIFOR or D-SHIFOR model error is baseline for intensity forecast skill • Skill = 100*(ESHIFOR-Emodel)/ESHIFOR

  25. Primary Operational Atlantic Intensity Model Verification5-year Sample (2003-2007) OFCL shown for comparison Mean Absolute Error Forecast Skill *HWRF, LGEM, consensus models excluded due to sample size restrictions

  26. 2008 Intensity Guidance OFCL adds most value over guidance at shorter ranges. Modest high bias in 2008 (2007 was a low bias). Split decision between the dynamical vs statistical models. New ICON consensus, introduced this year, was very successful, beating OFCL except at 12 h.

  27. SHIFOR 1988 SHIPS 1991 LGEM 2006 GFDL 1995 (1992) GFDN 1997 HWRF 2007 Most Accurate Atlantic Intensity Model 1988-2007 (48 hr forecast) Model with Most Accurate 48 h Atlantic Intensity Forecasts

  28. Rapid Intensity Change • Max wind increase of 30 kt or more in 24 hr • Kaplan and DeMaria (2003) WF definition • 5th percentile of Atlantic tropical cyclones • Can models predict RI? • Probability of detection • False alarm ratio (probability of false detection)

  29. Example of Rapid Intensification Hurricane Wilma 2005 • Extremely rapid intensification • 65 kt at 12 UTC 18 Oct • 160 kt 24 hr later • Aircraft recon showed 4 km eye diameter near peak intensity • Challenging case for models with 9 km grid spacing

  30. GFDL FORECAST FROM 10/17/05 18Z OBSERVED Early on 19 October, Wilma deepened at a rate of ~ 10 mb/hr! GFDL MODEL DID CAPTURE SOME OF WILMA’S RAPID DEEPENING

  31. Example of Rapid Intensification Index Cont… VERIFYING: 160 KNOTS Note: SHIPS and RII output available on-line in real time ftp://ftp.tpc.ncep.noaa.gov/atcf/stext

  32. The Statistical Rapid Intensity Index (RII) • Use subset of SHIPS predictors related to RII • Input to a discriminant analysis • Provides probability of RII in next 24 hr

  33. Probabilityof detection False alarm ratio Probability of detection and false alarm ratio of independent Atlantic 2006-2007 sample

  34. Concluding Remarks on TC Intensity Model Guidance: • Intensity forecasts less skillful than track • More physical processes need to be included • Statistical intensity techniques still competitive with dynamical models • In contrast, statistical track models no longer run except CLIPER • Large effort underway to improve dynamical intensity model (HWRF)

  35. 2008 HWRF Upgrades • Upgrades to Hurricane Initialization • improve initial structure for weaker storms • improved balance • makes use of TPC observed intensity/structure • Upgrades to Model • eliminate noise over topography due to moving nest (reformulate SLP) • remove erroneous residual Turbulent Kinetic Energy TKE (now consistent w/GFS physics) • Preliminary results: • some improvement to track after day 3 • modest improvement to intensity reducing weak bias (particularly after day 2)

  36. 2008-2012 HWRF UPGRADE PLAN • Data assimilation: • Advanced initialization for hurricane core – assimilate airborne Doppler radar observations to define storm strength and storm structure • Continuous upgrades to HWRF hurricane core initialization through advanced 4-D data assimilation for winds and reflectivity • Model resolution upgrades: • Increase resolution: Horizontal resolution 1-6km, Vertical resolution ~100 levels (dependent on results of current studies). • Hurricane ensembles: High-resolution hurricane model ensembles. • Development of HWRF ensembles in progress • Model Physics: • Continuous upgrades to atmospheric/ocean boundary layer (fluxes), microphysics, deep convection (cloud-resolving scales), radiation • Ocean coupling • Replacement of POM with the HYCOM ocean model for 2009 • Coupling will be added for East/Central Pacific runs (currently Atlantic only) • Other upgrades: • Coupling to land surface model with advanced surface physics for improved rainfall forecasts at landfall. Important input to hydrology and stream flow models which will address inland flooding. • Advanced Wave Model (WAVEWATCH III) to forecast waves up to the beach, i.e. improve non-linear interactions, surf-zone shallow water physics, wave interactions with currents.

  37. Atlantic Intensity Error Trends No progress with intensity.

  38. Other Forecast Parameters in NHC Products Track Genesis (formation within 48 hr) Size (radius of 34, 50, and 64 kt) Storm Surge (within about 24 hr of landfall) Rainfall (provided by the Hydrometeorological Prediction Center) Tornadoes (provided by the Storm Prediction Center) Wave height, direction and period (provided by NHC’s Tropical Analysis and Forecast Branch)

  39. Additional Information • Overview of NHC Models: http://www.nhc.noaa.gov/aboutmodels.shtml • Operational Model Matrix (Comet Password Required): http://www.meted.ucar.edu/nwp/pcu2/ • Global Forecast System Home Page: http://www.emc.ncep.noaa.gov/ • HPC’s Subjective Model Biases Page: http://www.hpc.ncep.noaa.gov/mdlbias/biastext.shtml • HWRF’s Main Page: http://www.emc.ncep.noaa.gov/HWRF/index.html • GFDL Model Description (AMS Publication): http://ams.allenpress.com/archive/1520-0493/135/12/pdf/i1520-0493-135-12-3965.pdf • UKMET Model Technical Specifications: http://www.metoffice.gov.uk/research/nwp/numerical/unified_model/new_dynamics.html • User’s Guide to the ECMWF: http://www.ecmwf.int/products/forecasts/guide/Preface.html • NOGAPS Technical Specifications: http://www.nrlmry.navy.mil/metoc/nogaps/nogaps_char.html

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