April 30, 2009 Meeting Summary
Qingfu Liu presented his recent work on the composite storm in HWRF. The composite storm comes directly from the HWRF model forecast with the environmental field removed. It's a 2-D, axisymmetric storm with hurricane perturbations for the total wind, temperature, mixing ratio and surface pressure. For weak storms, the initialization has more weight from the composite storm. For stronger storms, the initialization uses the model guess while the composite storm has a small weight.
Using observations from Franklin et al. (2000) and Powell and Black (1990), the ratio of the surface wind to the 700 mb wind is 0.88-0.91 in the eyewall. In the outer vortex, defined as within 200 miles of the storm center, this same ratio is 0.73-0.78. Qingfu noted that individual storms have large variations from these values. A plot of the total wind speed (m/s) for the composite storm shows the highest values in the eyewall from about the surface to roughly 700 mb. A plot of the temperature perturbation shows the highest values in the center of the storm from about 200-350 mb. The mixing ratio perturbation plot shows the highest values in the storm's center from about 650-850 mb. For a composite storm, the ratio of the surface wind to the 700 mb wind was 0.79 in the eyewall, which was 10% lower than observations. In the outer vortex of the composite storm, this ratio was 0.67-0.72, or 5% lower than observations.
Qingfu suggested that the smaller ratio values might be due to stronger surface drag used in the model. To test this, two new runs using Hurricane Ike with a different drag coefficient value were carried out. Using Powell's values for the drag coefficient, the model produced values for the surface to 700 mb ratio similar to observations. Qingfu noted that the composite storm and Hurricane Ike were annular-averaged storms. While the ratio of surface to 700 mb wind is close to the individual grid point value outside the eyewall, it is slightly lower inside the eyewall. An example of this is that 3-D Ike has a ratio value of 0.75-0.85 in the eyewall using the 2009 operational HWRF while this ratio is 0.82-0.91 in the eyewall if Powell's observed drag coefficient is used.
Looking at plots of these ratios for Hurricane Ike shows smaller values in the northern half of the storms compared to larger values of this ratio in the southern half of the storm. Qingfu speculated that this was due to the air parcel rising with less mixing from the upper level in the northern half of the storm. Overall, Qingfu pointed out that the composite storm would have a large impact on the model forecast for weaker storms and a small impact for stronger storms. Future work on this topic includes studying the structure of weak storms, creating more composite storms based on storm intensity (if necessary), and continuing work on model physics.