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August 27, 2009 Meeting Summary

At today's meeting Young Kwon gave a presentation titled "Sensitivity of Air-Sea Exchange Coefficients (Cd and Ch) on Hurricane Size and Intensity: Part IV - Sensitivity Test Results". For his tests, Young mentioned that he used only cd_opt=1 (operational HWRF Cd), cd_opt=5 (Powell's Cd values), ch_opt=1 (operational HWRF Ch), and ch_opt=5 (Jun's Ch value) in different combinations. These combinations yielded four different experiments: HOPR (red) using operational HWRF Cd/Ch, CD_5 (blue) using Powell's Cd and operational Ch, CH_5 (green) using operational Cd and Jun's Ch, and H5_5 (yellow) using Powell's Cd and Jun's Ch values. Four cases were then selected which included the 2008070918 run of Bertha, 2008082700 run of Gustav, 2008083012 run of Hanna, and the 2008090118 run of Ike. A run was made using each experiment for each case for a total of 16 runs total. Young mentioned that he tried to select cases where the storms stayed mostly over the ocean and where the HWRF track forecasts were close to the observed tracks since it is not expected that Cd/Ch will greatly affect the tracks. Young also said that he tried to select cases of both weak and strong storms and both positive and negative biases.

As expected, the track plots for all four storms show experiment tracks very similar to one another and close to what was observed. For a reference point, Young presented a plot of Ch/Cd profiles versus wind speed. In this figure, CD_5 (blue) has the highest ratio value followed by HOPR (red) then H5_5 (yellow) and finally CH_5 (green). Thus, the theoretical order, by color, should be blue, red, yellow, and green from highest to lowest Ch/Cd ratio. Next a plot of maximum 10m wind intensity was shown for all four storms. Here, we saw that most of the storms follow what was outlined by the theoretical values except for Hanna where the intensities for all experiments were very similar. Next Young presented track and intensity error for all storms separated by experiment. The track errors show very similar values for all experiments, as expected. Intensity errors are lowest for the H5_5 experiment early on, then degrade slightly. For most of the forecast period, the CD_5 experiment has the highest error values. Young then presented a bar graph showing intensity bias for each of the four experiments. HOPR and CD_5 have the highest bias values for the entire forecast period with CH_5 and H5_5 having the lowest bias values. These low biases correspond to better results.

Next a plot of maximum wind - central pressure relationship was shown. Here we see that for all experiments, pressure-wind relationship values degrade as pressure drops and wind speed increases. Young mentioned cycling his future runs to see what effect that might have on this figure. Plots of Cd and Ch sensitivity to MSLP and 10m wind, sensible heat flux, latent heat flux, and surface stress were shown for Hurricane IKE at 114h in the forecast for all four experiments. The bottom two panels for each of these sensitivity plots correspond to smaller Ch values and thus weaker storms, which can be seen for all sensitivity parameters. Young concluded with his future work plan. Young plans to run Ike and Gustav using cycled HWRF runs with the CH_5 and H5_5 configurations to see the impact on the wind-pressure relationship, since this relationship affects vortex initialization. Next, he planned to combine the modified SAS convection code from Qingfu and the modified surface flux code and test some cases. Finally, Young planned to keep tuning his gravity wave drag (GWD) parameterization scheme for the 2010 upgrade of HWRF.

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