July 2, 2009 Meeting Summary
Young Kwon presented part III of his work with sensitivity of the air-sea exchange coefficients on hurricane size and intensity, which focused on namelist changes, new Cd/Ch profiles, and results. Young mentioned that the Cd/Ch options were added to the namelist options in the surface physics code. There were 5 options each for Cd and Ch for a total of 25 different combinations of Cd and Ch. These options for Cd were: opt_1 was the current operational Cd for 2009 HWRF, opt_2 was an artificial value for Cd which was linearly increasing with wind speed, opt_3 was an artificial Cd value where Cd was increasing with wind speed more so than linearly, opt_4 was where Cd decreases with wind speed (as mentioned in an Emanuel paper), and opt_5 used Powell's value for Cd. Young's goal was to keep the different options for Cd similar for wind speeds less than 30 m/s, so that effects were only seen at higher wind speeds, as shown by the graph showing all the different Cd profiles. For Ch, options 1-4 were similar to those used for Cd and opt_5 used a Ch value from Jun's paper using CBLAST work. Opt_5 for Ch kept the Ch at a constant value.
Young ran the HWRF for Hanna using the Cd and Ch options mentioned above. When, Cd was varied, Ch was kept constant, and vice versa when Ch was varied. The HWRF was only run for one time using each option and not cycled. Young also mentioned that the initial conditions and boundary conditions were the same for every run. The tracks for Hanna using Cd options are all very similar. The intensity plot using different Cd options shows a different picture. The intensities for each Cd option are varied with opt_5 showing the lowest intensity values. Young mentioned that this was very counterintuitive because opt_5 had the lowest Cd values. The track plot showing the different Ch options is fairly similar to the track plot for Cd with the operational Cd value being a slight outlier. The intensity plot for the Ch options shows more variance than the track plot with opt_5 and opt_1 having lower intensities.
Young next explained his hypothesis for the lower intensities with lower Cd values. He mentioned that surface friction makes flow cross-isobar toward the low pressure center. This means that if the Cd becomes small, then there is less surface friction, and flow converges less toward the storm center. Thus, we see a larger-sized storm with a weaker maximum wind speed due to the conservation of angular momentum. Young then showed plots of storm size. The first set of plots showed the storm size for each Cd option at 24 hours. There was not much difference in the storm size. At 96 hours, opt_5 produced a larger storm than the other Cd options, which supported Young's hypothesis. Young concluded by saying that he had only chosen and run one case, and he wanted to run more cases to examine their results. Future work would also include running a storm in the East Pacific and analyzing SST.