March 24, 2011 Meeting Summary
Eric Rappin gave a presentation as part of a joint seminar with AOML titled, "A Highly Configurable Vortex Initialization Method for Tropical Cyclones." Eric began by discussing the motivation for his work which included providing an alternative to the existing bogussing methods. Specifically, GFDL's use of an axisymmetric spin-up was too complex while WRF's use of an idealized Rankine was too simple. Eric's new bogussing method is highly configurable so that it can match any vortex shape and allows for the specification of a full 3-D wind field. This will minimize the adjustment time. This new bogussing method used an algorithm following Kurihara et al. (1995) for vortex removal. For vortex addition, a modified Rankine vortex or a Willoughby vortex (from Willoughby et al. 2006) was used. For the modified Rankine, only two conditions are used, when r is less than Rmax and when r is greater than Rmax. The Willoughby vortex uses a piecewise specification with three conditions: r less than or equal to R1, R1 less than or equal to r which is less than or equal to R2, and R2 less than or equal to r.
Eric next discussed the two components of the vertical structure in his method, the boundary layer and the free atmosphere. The boundary layer is that from Foster (2009) and the configurable parameters are boundary layer height and constant eddy diffusivity. For the free atmosphere, Gaussian decay method can be used with configurable parameters altitude of max tangential wind and decay parameters. The Emanuel Theory (1986) can also be used where configurable parameters are the boundary layer height and outflow temperature. Eric then showed some results from his real test using Hurricane Bill (2009) and an idealized case.
In the real test, two configurations were compared, a Modified Rankine (ModRank) with no boundary flow or secondary circulation (called "No SC" condition) and Willoughby configuration with boundary flow and secondary circulation (called "SC" condition). In the intensity graph, the Willoughby configuration intensified more quickly than the ModRank. Plots showing the u-, v-wind, and vertical motion, w, cross sections were also shown at hours 0, 2, 4, 6, 9, 12, and 24. By hour 2, strong decay was seen in the boundary layer for both configurations, but especially so for ModRank. By hour 6, ModRank looked very dissipated. By hour 12, Willoughby showed development in the boundary layer due to the SC structure being more advanced, and by hour 24, Willoughby, with SC, was definitely more developed than ModRank.
Eric then described the idealized test which used a constant SST of 28.5 degrees Celcius and no environmental flow. Again the ModRank configuration was No SC while the Willoughby configuration was SC. In this case, the ModRank configuration developed more quickly than Willoughby. At hour 2 of the simulation, both configurations are very similar in the tangential wind field. By hour 6, the ModRank starts to fall apart while Willoughby keeps strengthening. By hour 12, Willoughby has started to weaken while ModRank is growing stronger and by hour 24, they are similar and stronger. The next experiment involved comparing the Willoughby configuration with SC to that with No SC. Plots of max wind and min. pressure show both configurations very similar early on with the No SC configuration slightly stronger. Cross sections of u, v, and w show both configurations very similar through hour 6. By hour 12, No SC configuration sees a radial expansion allowing it to intensify a little faster than the SC configuration. By hour 24, they are very similar.
The last experiment Eric discussed involved comparing the Willoughby SC configuration to the Willoughby SC with no mass perturbation configuration (SC-No Mass). Here, the SC-No Mass configuration intensified much faster than SC. The vertical wind values for the SC-No Mass configuration were also very large at hour 2 and hour 4 of the simulation. Overall, the SC-No Mass configuration yielded a strong storm compared to SC. Eric concluded by saying that the test cases he showed in his presentation demonstrated that the initial spin-down of the vortex is reduced when the full 3-D wind field is taken into account, at least from a structural perspective.