Revised Pressure-Wind and Radial Profile Models: Implications for Tropical Cyclone Archives Mesoscale and Microscale Meteorology Division, ESSL National Center for Atmospheric Research NCAR is Sponsored by NSF Greg Holland 1

What Use Tropical Cyclone Archives? – The single most important action we can take; – Most Important Parameters. Pressure-Wind Relationships – Role and Archive Requirements; – Pressure-Wind or Vice Versa. Reconstructing Historical Surface Winds – Approach and Archive Requirements; Summary 2Holland, IBTraCS Workshop Asheville 0509

Forecast Statistics; Statistical Forecasting; Assessing Climate Variability and Change; Insurance Premiums; Building Code Development and Engineering of Major Structures. What Use Tropical Cyclone Archives? 3Holland, IBTraCS Workshop Asheville 0509 What is the single most important action we can take?

Intensity; Overall Wind Structure; Translational Speed. What are the Most Important Damage and Engineering Factors? 4Holland, IBTraCS Workshop Asheville 0509 R 2 : Vm 3 =27%, Vt=49%, Rh -2 =70% Willis Hurricane Index α Vm 3, Vt, Rh -2

Pressure-Wind Relationships 5Holland, IBTraCS Workshop Asheville 0509 Plotted are: All known P-W Relationships (after Harper 2002); Black dashed is AH; Red triangles are Atlantic observations Generally take the form: e.g. Atkinson and Holliday: Alternative is Dvorak Look-Up Table, which can be translated to: What are the Implications for Historical Archives?

The Good: – They introduced an important level of objectivity; – They can be reproduced, provided we know what was used. The Bad: – Widespread use of different approaches without adequate documentation; – Mixing objective and subjective approaches. Further: – Historical relationships were 1-1 and could not possibly capture the true relationship; however, they were easily reproduceable; – New relationships are multi-variate, which enable capture of some of the observed scatter; however, this is at the cost of increased difficulty in reproducing what was done, especially if there is inadequate documentation. Implications of P-W Relationships 6Holland, IBTraCS Workshop Asheville 0509

AH tends to over-estimate intense hurricane winds (Knaff and Zehr 2007), and so does Dvorak (Holland 2008); Some Known Issues 7Holland, IBTraCS Workshop Asheville 0509 Ad-hoc application, including: – Lack of proper peer review of many relationships; – Variations in PW relationships; – Sometimes wind as the primary parameter, other times pressure. Fortunately, many of these can be reversed later, given adequate documentation

Issues: Western Australia 8Holland, IBTraCS Workshop Asheville 0509 Atkinson and Holliday relationship using 10 min winds (blue) and 1 min winds (black), showing the incorrect use of AH with 1 min winds. Improved documentation needed

Knaff and Zehr (2007): Holland (2008): Recent Revisions for P-W Relations 9Holland, IBTraCS Workshop Asheville 0509

Holland Model: Dependent NAT Data 10Holland, IBTraCS Workshop Asheville 0509 The variable b reproduces about half the maximum scatter in HURDAT. But the scatter experienced by most storms is captured: RMSD=3.5 m/s 85%<5 m/s Note Dvorak overestimate

The Importance of RMW 11Holland, IBTraCS Workshop Asheville 0509 Black: Holland (2008) model, left original, right including RMW Red: Hurricane model data courtesy of Yuqing Wang (f-plane, 5 km grid, no environmental flow) Should we start with pressure and derive winds, or vice versa?

Must archive both and document how they were derived; If either or both are directly observed, no issue, archive both and document this; However, >90% of archives use a combination of satellite inference and subjective judgment; Should we estimate winds and infer central pressure, or vice versa? Maximum Winds/Central Pressure? 12Holland, IBTraCS Workshop Asheville 0509

For Hurricanes, wind responds to changes in the mass field (i.e. heating); Pressures are much more conservative: – Easier to observe; – Not subject to wind definition issues; – Not subject to as much high-frequency, “random” variation; – Less impact from cyclone translation speed variations; – Not affected by supergradient wind issues. Pressure vs Winds 13Holland, IBTraCS Workshop Asheville 0509

e.g. Schubert et al (1980): Winds Respond to Mass Changes 14Holland, IBTraCS Workshop Asheville 0509 Hurricane Core Wind responds to massMass responds to wind Efficiency Inverse of Deformation Radius Deep Tropics Large, weak systems Mass adjusts to wind Wind adjusts to mass

Wind vs Pressure Variation 15Holland, IBTraCS Workshop Asheville 0509 Blue: cold SST= 26.3C Red: warm SST=30.3C Solid: dx = 1.67 km Dashed: dx = km Wind “noise” at 556 m resolution is ~10 m/s At 62 m resolution it is >20 m/s (gusts are explicitly simulated) Model results from Rotunno et al (2008)

Roll Vortices at Landfall 16Holland, IBTraCS Workshop Asheville 0509 Hurricane Dean simulation using ARW Vertical motion advects low-momentum air upward and high momentum air downward. Leads to low level wind variations of ~20 ms -1 Recommend pressure be the primary variable

Reconstructing the Surface Winds 17Holland, IBTraCS Workshop Asheville 0509 Hurricane Tracy Maximum Surface Wind Reconstruction Courtesy Bruce Harper Needs: Track, Vm, Pc, Penv, RMW Desirable: Venv

Reconstructing the Surface Winds 18Holland, IBTraCS Workshop Asheville 0509 Willoughby et al 1982 New approaches enable a good reconstruction of the surface wind field for historical storms, provided the archive includes information on: RMW, Pc, Vm, and Vne. Vne is winds in the near environment and preferably 4 obs at orthogonal directions. Also having an estimate of major secondary wind maxima would be useful, but not essential.

Examples of Profile Error Sensitivity 19Holland, IBTraCS Workshop Asheville 0509 Maximum Wind Error External Radius ErrorExternal Wind Error RMW Error Needs: Track, Vm, Pc, Penv, RMW and Venv

Summary Holland, IBTraCS Workshop Asheville 0509 Tropical Cyclone Archives need to be customer driven, Climate/CAT-Modeling are very important customers: Track, Pc, RMW, Vm, Penv, Venv, Quality Flags; Unless there are good quality direct observations, Pc is recommended as the primary archive for intensity, with wind derived from a documented P-W relation and other knowledge of wind variations; Perhaps we should include an extra section in Annual Tropical Cyclone Reports, which discusses quality issues and uncertainty assessments for the season. 20