Understanding and (possibly) rectifying historical and regional wind-pressure relationship differences By John Knaff NOAA/NESDIS Fort Collins, CO [email protected]

Why are we here? The intent of the IBTrACS project is to overcome data availability issues, and to freely disseminate this new global dataset. This was achieved by working directly with all the Regional Specialized Meteorological Centers and other international centers and individuals to create a global best track dataset, merging storm information from multiple centers into one product and archiving the data for public use. Purpose: The workshop will gather international experts in global tropical cyclone best track data to discuss a variety of topics that would serve to enhance the existing IBTrACS dataset.

IBTrACS Considerations The maximum winds/ MSLP in the best tracks Inhomogeneous by construction Satellite coverage / operational data access & tools When Dvorak intensity estimates became available Different Dvorak Tables, prior to 1978 Visible vs. EIR techniques vs. Objective Dvorak Different wind-pressure relationships Different applications of wind-pressure relationships Estimate wind Estimate pressure Different averaging periods TC structure

Outline Understanding Why wind-pressure relationships (WPRs) were developed How they have been used historically Why there are regional differences Offer Possible Solutions to Enhance IBTrACS and Reanalysis efforts. Knaff and Zehr (2007) Courtney and Knaff (2009)

Wind-Pressure Relationships were Operational (not Climatological) Tools 1940s 1970s Pressure Measurements More numerous (ships, surface, recon) Having greater precision Having greater survivability Wind is what is important! One size fits all Compromise: To improve the stability of wind estimates use pressures to estimate the wind ( better, good enough) 1970s - Present Dvorak & aircraft provide wind estimates Winds estimates now more numerous Having greater reliability Remotely sensed & survivability is not as big of an issue Wind is still most important! Pressure is estimated via WPR for legacy reasons/ model bogus input One size fits all Compromise: Pressure does not matter to the forecast (doesnt matter, good enough)

Historical & Regional Differences 1.Out of necessity Sometimes independent 2.Regional considerations Environmental pressure Mean size Mean latitude 3.Multiple formulations Cyclostophic form Statistical fit to data 4.Dvorak influences Everyone, it seems, had to have their own wind-pressure relationship

Various Wind-Pressure Relationships (WPR) Some historical WPR Atkinson & Holliday (1977;AH77)/Dvorak (1984 WPAC) RMSC La Reunion TCWC Perth (-2008) JTWC (-2007) Dvorak (1975) RMSC Miami RMSC Honolulu Koba (1990, 1991) RMSC Tokyo Crane (circa 1985) TCWC Brisbane (-2008) Love & Murphy (1985) Some cases TCWC Darwin (- 2008) V 60 vs. P

Things to consider in the IBTrACS process How the use of WPRs has resulted in inhomogeneous historical MSLP/Intensity records The possibility of assigning a more consistent MSLPs to historical best tracks (i.e., wind to MSLP) Understanding how WPRs could aid reanalysis efforts at RMSCs MSLP to Wind when MSLP available (i.e., Atlantic, N. Pacific) Consistency checks Inter-best track Versus observations

Minimum Requirements for WPRs Use existing archives of operational or easily created historical information Fixes (Aircraft, Dvorak, Drops) Best tracks Global Reanalyzes Should closely follow the physics related to the problem and account for TC wind field structure TC environment TC translation Variations of latitude

First Principles Vortex in gradient balance Steady State Azimuthal Mean Gradient form PF(lat,r env,P env, V t, V t ^2) This implies: Lower (higher) pressures occur at higher (lower) latitude, for larger (smaller) circulation systems, and for smaller (larger) radii of maximum winds given the same maximum wind.

Operational/Historical Inputs Have from the best track V 60,V 600 or MSLP estimate Location May have from advisories and fixes Storm speed (instantaneous) Pressure of Outer Closed Isobar (POCI) Radius of Outer Closed Isobar (ROCI) Radius of 34-, 50-, 64-, or 100-kt winds Dvorak, Recon fixes Radius of Maximum winds (recon, visible eye only)* Could calculate from global re-analyzes and best track archives Outer (@ 500 km) tangential winds (e.g., Knaff and Zehr) ROCI POCI Storm speeds (6/12 hourly) * RMW provide for model bogus requirements is often unreliable (goldilocks RMW)

Solutions in hand Can account for variations of structure (to some degree), environmental pressure, latitude and translation Knaff & Zehr (2007) Courtney & Knaff (2009) Holland-based models NEXT Talk

Knaff & Zehr (2007) Accounts for variations of latitude, size, environmental pressure and storm speed. Size and Latitude contribute most! Uses global re-analyses to estimate Environmental Pressure (900 km) TC Size expressed as a ratio to climatology Tangential wind at 500km from reanalyzes Climatological tangential wind at 500km (from Knaff et al. 2007) Fixes/advisories/best track provide Latitude Storm speed Intensity (V (60,600) or MSLP) Statistically fits the data (Explaining 90 % of the variance) Shortcomings Different equations for wind to-pressure/pressure-to-wind Inner-core structure (i.e., RMW, multiple wind maxima)* * RMW is often provided as part of the storm bogus, but is not always representative of the actual RMW, which is often more difficult to estimate.

Knaff & Zehr Examples PW Rozenkranz et al 1978 (JGR) Typhoon June (1975) Largest storm (Reanalysis) Size(V500/V500c) = 1.57 Leads to R34 ~ 345 nmi Environmental P (Reanalysis)= ~1008 -1005 hPa Max intensity = 160 kt (JTWC) MSLP = 875hPa

June 1975 Vmax Estimates Vmax [kt] Date/Time Best track intensity follows A&H 77

Knaff & Zehr Examples PW Typhoon Forrest (1983) One of the most intense (Reanalysis) Most rapid (~100 hPa/d) Size(V500/V500c) = 1.10 Leads to R34 ~ 300 nmi Environmental P (Reanalysis)= ~1012 -1006 hPa Max intensity = 150 kt (JTWC) MSLP = 883hPa

Forrest 1983 Vmax Estimates Vmax [kt] Date/Time Best track intensity follows A&H 77

Courtney & Knaff (2009)

The Good Blondie The Bad Angel eyes The Ugly Tuco The wind-pressure analogy When it works When it doesnt The real issues

The Good - Gustav (2008) Pressure from Wind (C&K)Wind from Pressure (C&K)

The Good - Sinlaku (2008) Pressure from Wind (C&K)Wind from Pressure (C&K)

The Bad - Ike (2008) Pressure from Wind (C&K)Wind from Pressure (C&K)

The Problem with Ike 1 km IR 9 Sept 2218 UTCH*Wind 10 Sept 130 UTC

The Ugly - Remaining Issues Inhomogeneous intensity estimates Poor/Biased intensity estimates Sparse historical records (ship, landfall, estimate methods, reconnaissance) Multiple intensity estimates (multiple agencies/ multiple methods) Lack of information about TC structure Man power for human review!

Questions?

References Courtney, J., and J. Knaff, 2009: Adapting the Knaff and Zehr Wind-Pressure Relationship for operational use in Tropical Cyclone Warning Centres. Aust. Met. Mag., in Review/Revised. Holland, G., 2008: A Revised Hurricane PressureWind Model. Mon. Wea. Rev., 136, 34323445. Knaff, J.A. and R.M. Zehr, 2008: Reply. Weather and Forecasting, 23:4, 762- 770. Knaff, J.A., and R.M. Zehr, 2007: Reexamination of Tropical Cyclone Wind- Pressure Relationships. Wea Forecasting, 22:1, 7188. Knaff, J.A., C. R. Sampson, M. DeMaria, T. P. Marchok, J. M. Gross, and C. J. McAdie, 2007: Statistical Tropical Cyclone Wind Radii Prediction Using Climatology and Persistence, Wea. Forecasting, 22:4, 781791. Landsea, C. W., C. Anderson, N. Charles, G. Clark, J. Dunion, J. Fernandez- Partagas, P. Hungerford, C. Neumann, and M. Zimmer, 2004: The Atlantic hurricane database re-analysis project: Documentation for the 1851-1910 alterations and additions to the HURDAT database. _Hurricanes and Typhoons: Past, Present and Future_, R. J. Murnane and K.-B. Liu, Eds., Columbia University Press, 177-221.

Extra Slides

Knaff & Zehr (2007) WPR based upon Environmental Pressure (Penv) Reanalysis MSLP averaged at 900 km from the center TC Size (S) TC Speed (c) Latitude () Parameterizations

Knaff & Zehr (2007) Cont Statistically fits the data (Explaining 90 % of the variance) Shortcomings Different equations for wind to-pressure/pressure-to-wind Inner-core structure (i.e., RMW, multiple wind maxima)* * RMW is often provided as part of the storm bogus, but is not always representative of the actual RMW, which is often more difficult to estimate.

Courtney & Knaff (2009) Modifies Knaff and Zehr Uses operation input Corrects for issues related to estimating MSLP of low latitude storms Advisories/best tracks provide POCI from which environmental pressure is estimated Mean radius of gales (non-zero quadrants) which provides TC size Latitude Storm Speed Intensity (V (60,600) ) Parameterizations

Courtney & Knaff (2009) cont.

Courtney & Knaff (2009)

Documents

Understanding and (possibly) rectifying historical and regional wind-pressure relationship differences By John Knaff NOAA/NESDIS Fort Collins, CO [email protected]