As discussed in previous posts, the TCMWindTool used to develop wind speed and gust forecasts at the NWS offices currently implements the modified Rankine vortex for wind interpolation between the various wind radii. After a literature review, a new Holland et al. (2010) model provides some promise to improved wind interpolation. As mentioned in the paper, the new method is less sensitive to wind radii forecast errors.
TCM best track data was acquired from NHC for Irene (2011) at 0600 UTC 26 August. Below are plots of the H*Wind surface analyzed wind speeds, modified Rankine vortex, and Holland et al. (2010) inteprolated wind speeds plotted out to the best track analyzed 34 knot maximum wind radii for the various quadrants. The H*Wind analyzed wind speeds is also shown.
In quadrants one and two, there is very good agreement between the H*Wind analyzed wind speeds and the Holland et al. (2010) interpolated wind speeds. This is especially apparent outside of the radius of maximum winds to the 34 knot maximum radius. The modified Rankine vortex method leads to a positive bias when compared to the H*Wind analysis at this time. Both models over predict the wind speeds up to the radius of maximum winds, likely due to the way this is specified (take as 1/2 the default eye radius of 25 nm + 8 nm).
There is much less agreement when looking at quadrants 3 and 4. This is likely due to the much weaker wind speeds observed in these quadrants, relative to the maximum sustained winds estimated by NHC and the rapid decay of wind speeds within these quadrants in the H*Wind analyses. The documentation in the Holland et al. (2010) model, does suggest reduced sensitivity to these best track wind radii errors. A senstivity analysis is currently being to conducted to validate this claim and will be presented in a later post.
In summary, the Holland et al. (2010) model shows marked improvement over the modified Rankine vortex, at least when the various wind radii are correctly forecasted and verified in the analysis. I will be looking at additional storms as well as different analysis times to continue with this verification.
(Order of images: H*Wind Analysis, Quadrant 1 Plot, Quadrant 2 Plot, Quadrant 3 Plot, Quadrant 4 Plot)