Can Dvorak Intensity Estimates be Calibrated?

John A. Knaff

NOAA/NESDIS

Fort Collins, CO

Dvorak Technique: Overview

• The Dvorak Technique estimates tropical cyclone intensity by analyzing satellite image patterns and IR cloud top temperatures.

• Intensity is assigned with intensity units (called T-numbers ranging from 1 to 8, in 0.5 increments), where one T-number represents one day’s intensity change at an average rate.

• The T-number can be given as a maximum surface wind speed or a minimum sea-level pressure.

Dvorak Technique: Procedure

• Simplified Approach:– 1. Locate Center– 2. Assign Pattern– 3. Make

measurements (Visible or EIR)

– 4. Assign T-Number– 5. Assign CI (Current

Intensity)

Why Calibrate?

• Through a concerted WMO effort the Dvorak Technique (circa 1984) was taught to and adopted at all WMO RMSCs and Tropical Cyclone Warning Centers by the late 1980’s

• The method has been found to be relatively stable with respect to satellite sensor resolution (Zehr, Beven, DeMaria)

• Historical records exist for these estimates, thus these represent a quality climate record of global tropical cyclone intensity.

• A systematic validation vs. Aircraft estimated winds (i.e., best track) has not been done and is needed. HOW ACCURATE/PERSISE ?

Questions I Want to Answer

1. Does the Dvorak Technique have systematic biases wrt intensity, intensity trend, size, translational speed, and latitude?

2. What are the error characteristics of such estimates as a function of …. ?

3. Can CI estimates be improved for?– Operationally (for advisories)– Post-season (i.e., best tracking)– Reanalysis of historical TCs

Approach

• TC fix, advisories and best track data, 1989-2007• For each fix

– Interpolate intensity from the best track (truth)– Get Radius of Outer Closed Isobar from advisories and best tracks, interpolate to fix

time– Compare intensity (fix vs. truth)

• Homogeneous fix record from 1) Satellite Analysis Branch (SAB, Washington) and 2) Tropical Analysis and Forecast Branch (TAFB, Miami)

• Stratify by factors (composite)– Intensity – Intensity & Intensity trend (12-h) – Intensity & Latitude – Intensity & Size (ROCI)– Intensity & Speed of translation

• Regression using the composite datasets that span the range of factors.

Stratified by Intensity (~T-number)

Stratified by Intensity Trend & Intensity

TAFB SAB

Partial Correlations: Remaining Factors

TAFBFactor Partial

Correlation

Intensity Trend 0.52

Storm Speed 0.27

Sin (latitude) 0.38

ROCI -0.24

SABFactor Partial

Correlation

Intensity Trend 0.54

Storm Speed 0.39

Sin (latitude) 0.39

ROCI -0.34

Multiple Regression – Based Bias Correction

Valid for Vmax 20-140kt

A Universal Relationship

• Assume a function for biases as a fixed function of intensity

• Recalculate the regression coefficients.

Valid for Vmax 20-140kt

Conclusions

• Shown– A bias correction for the Dvorak intensity

estimates has been developed• Biases are a function of

– Intensity– Intensity change– Latitude– Size (ROCI)– Storm speed

– Can be applied in real-time, and for reanalyses• Not shown

– RMSE appear to be solely a function of intensity