1. The NHC perspective on TC wind radii analysis and forecasting: An exercise in uncertainty
Michael J. Brennan National Hurricane Center
2012 CSTAR Workshop 16 November 2012

2. Wind Radii Analysis
TC wind field structure is by far the most uncertain analysis and forecast parameter
Wind radii are defined to be the maximum extent of that wind threshold in a given quadrant of the storm (does not have to be continuous)
In the open ocean with very limited surface wind observations, analyzed wind radii are often based on
Occasional scatterometer passes and sporadic ship, buoy and island observations
Empirical/climatological relationships between TC intensity and the radii
65-kt hurricane would typically have 34-kt wind radii of 60 nm
Satellite appearance
Edge of the CDO is somewhere in between the 34-kt and 50-kt radii
Bulk parameters
Westward moving TC would have asymmetries favoring the northern semicircle
Radii generally increase with intensity and latitude

3. Wind Radii Analysis
Even in relatively “data rich” environments with aircraft, SFMR and flight-level winds extend only along radials along the aircraft track and provide a single value of the wind radii for that quadrant
One “alpha pattern” by the aircraft samples only about 2% of the wind field
Radii are constructed using a blend of flight level, SFMR and dropsonde observations, and other data (satellite and in situ)
Ships, buoys, land stations, scatterometer and AMSU can provide information on 34-kt radii
Small scale of 64-kt radii and platform limitations make them much more difficult to sample

4. Analysis Uncertainty Estimates
Based on an survey of NHC’s hurricane specialists, who conduct the post-storm analysis of tropical cyclones, subjective uncertainty estimates for were obtained for all best-tracked parameters
Below are the wind radii uncertainty estimates for cyclones making U.S. landfall, assuming that aircraft, radar, and other in- situ surface observations would be available:
Storm Type
34-kt Uncertainty (nm)
50-kt Uncertainty (nm)
64-kt Uncertainty (nm)
Tropical Storm
24.1
16.6
N/A
Cat 1-2 Hurricane
23.8
19.3
12.9
Cat 3-5 Hurricane
24.5
19.1
13.4

5. Wind Radii Forecasting
Radii are tied to the intensity forecast, which has an average error of about 20 kt by h
We typically lean heavily toward the climatology/persistence models when making radii forecasts
For stronger/larger TCs, we also get guidance from the GFDL and HWRF that can be useful, especially for the 64-kt radii
Global models (GFS, ECMWF) can be helpful with the 34- and occasionally 50-kt radii for larger TCs and systems undergoing ET (e.g., Sandy)
There may be gaps between winds of a given threshold in the quadrant
Radii Forecast
Guidance

6. Complications Near Land
When a cyclone approaches land, additional complications are added to the radii forecasts
Track forecast error (on average n mi at 72 h) can dominate errors in the radii forecasts
This is especially true when there is uncertainty about whether a quadrant will be entirely or partially over land, which can result in large over or under estimations of the 34-kt radii
Radii forecasts try to account for stronger winds over bays and sounds, but can overdo the wind field over adjacent land areas in those quadrants

7. Wind Radii Forecast Errors
Below are average NHC wind radii forecast errors (n mi) for forecasts made when U.S. watches or warnings were in effect from
Quadrant
Forecast hour 12 24 36 48 72
Northeast
34-kt
27.2 34
40.8
57.1
59.2
50-kt
9.7
17.7
16.7
21.4
43.3
64-kt
2.3
6.7 10
Southeast
21.3
31.6 42
53.1
44.2
10.8
19.6
26.7
24.1
7.8
12.9
Southwest
40.1
52.9
54.3
57.5
3.3
19.4
17.9 20
7.7
17.8
22.1
Northwest
22.6
32.6
27.9 55
10.3
9.6
11.1
14.3 45
5.9
8.9
13.6
Average errors tend to start off pretty large for 34-kt and grow with time
Some error growth also seen with 50 and 64 kt radii, although these are more constrained by size and close to the analysis uncertainty estimates
Interestingly, the NW quadrant has some of the smallest average errors
Bolded values show biases larger than the subjective analysis uncertainty estimates
Sample Size
Radii 12 24 36 48 72 34 54 44 33 21 6 50 18 13 9 7 3 64 11

8. Wind Radii Forecast Biases
Below are average NHC wind radii forecast biases (n mi) for forecasts made when U.S. watches or warnings were in effect from
Quadrant
Forecast hour 12 24 36 48 72
Northeast
34-kt
13.1
9.9
17.4
34.3
59.2
50-kt
2.5
-4.6
-16.7
-21.4
-43.3
64-kt
-1.4
-4.4
-7.1
Southeast
-9.3
-24.3
-34.1
-38.8
-44.2
-7.3
-26.7
-5.6
-10
Southwest
-9.7
-12.2
-20.5
-6.2
-20.8
-2.2
-7.7
-19.4
-17.9
-20
3.2
1.1
-0.7
Northwest
11.3
19.8 26 55
6.9
8.1
8.9
14.3 45
5.9
13.6
34-kt radii tend to be too large in the northern semicircle, especially at longer lead times, although the sample is quite small
Under forecast bias noted for most radii in the southern semicircle
Bolded values show biases larger than the subjective analysis uncertainty estimates
Sample Size
Radii 12 24 36 48 72 34 54 44 33 21 6 50 18 13 9 7 3 64 11

9. Concluding Remarks
NHC wind radii analyses and forecasts have a high degree of uncertainty
Analyses are hampered by limited data even in so-called “data rich” environment
Forecasts generally try to capture bulk aspects of the wind field evolution consistent with the track and intensity forecasts
Complications occur due to errors in track and intensity forecasts, proximity to land, and bays or sounds that may distort radii
Until dynamical models can accurately analyze and forecast the wind field structure, improvements in wind radii forecasts will likely be small