March 26, 2009 Meeting Summary
Young Kwon presented his 2009 HFIP work plan on the sensitivity of air-sea exchange coefficients (specifically Cd and Ch) on hurricane size and intensity, part I: plan and introduction. Young mentioned that depending on the results, this work might be included in the 2010 HWRF implementation. Motivation for this work included the fact that HWRF intensity forecast skill is not as good as track forecast skill, which is an issue for many dynamical models. However, the lack of intensity forecast skill in HWRF could be due to an incorrect wind-pressure relationship in the model. The HWRF has a tendency to produce a larger storm with time which, in turn, causes the wrong wind-pressure relationship. Thus, the intensity forecast skill of HWRF could be improved by correcting storm size and the wind-pressure relationship through the tuning of Cd and Ch.
Looking at the wind-pressure relationship for Hurricane Katrina (2005082600), the GFDL trend (in light blue) is closer to observations (in navy blue) than the trend for HWRF (in pink). While the HWRF starts off closer to observations when wind speed is low and pressure is high, it diverges as wind speed increases and pressure decreases. In fact, for the same pressure value (at 920 hPa), the HWRF has a much lower wind speed compared to GFDL and OBS. The HWRF storm size (for 2005082600) is also much larger and smoother than that for GFDL. HWRF also has a lower pressure and lower wind speed for its larger size compared to GFDL's higher pressure and higher wind speed for its smaller size.
Young mentioned that it is important to tune Cd and Ch first because, based on Emanuel (1995), hurricane intensity is proportional to (Ch/Cd)1/2. Thus, storm size also depends on surface exchange coefficients. A hurricane schematic shows that low level inflow of air creates an energy gain from the sea surface (Ch) but an energy loss by surface friction (Cd). If Cd is reduced, we might get more air penetration to the storm center which could lead to a smaller storm. Young's plan and progress included gathering information on enthalpy (Ch) and momentum (Cd) exchange coefficients over the ocean at high wind speeds from observations, lab experiments, and theory (which was already done). Next, the HWRF surface physics code must be changed to enable the use of a prescribed Cd and Ch separately (which is in progress). Then, experiments will be conducted using a fixed Ch and various Cd values to examine the sensitivity of Cd on the storm size and intensity skill. Experiments will also be conducted with a fixed Cd and various Ch values. All of these experiments will lead to an optimal Cd and Ch combination. Finally, Cd and Ch must be tuned with the inclusion of sea spray parameterization. This is because sea spray alters the Cd and Ch profiles at higher wind speeds. It should be noted that horizontal diffusion is being excluded for these immediate experiments.
For Young's next presentation, he plans to present the code structure of the surface and boundary layer physics. He also plans to show how to only change Cd and Ch without altering other variables. Finally some sensitivity experiment results, using a modified Cd, may be presented.