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August 26, 2010 Meeting Summary

Sun-Hee Kim presented her work on "Sensitivity Experiments for the HWRF PBL Parameterizations". Currently, the HWRF uses a non-local PBL scheme developed by Hong and Pan in 1996. Sun-Hee noted that in the boundary layer height equation, the critical bulk Richardson number (RiBc) for HWRF is equal to 0.5 compared to 0.25 used in the current GFS. Also, in the vertical diffusivity equation, the HWRF uses 150m for the asymptotic length scale (lo) while GFS uses 150m for an unstable atmosphere and 30m for stable atmosphere. Focusing on RiBc and lo, Sun-Hee conducted 5 experiments: the CTL (in blue) uses the current HWRF PBL scheme; EXP1 (in red) uses non-local diffusion with RiBc=0.25 and a mixed length of 150m; EXP2 (in green) uses non-local diffusion with RiBc=0.5 and mixed length of 30m; EXP3 (in purple) uses the current GFS PBL scheme; and EXP4 (in orange) uses local diffusion applied to all levels and mixed length of 30m. Sun-Hee explained that she conducted EXP4 to see the effect it would have and because local diffusion is used in the GFS for stable conditions. For this work, a Hurricane Ike case was used from September 3rd.

First, an analysis of the CTL experiment was presented at 48h and azimuthally averaged. The RMW (radius of maximum winds) was at 70km with inflow in the lower levels up to 2km and outflow in the upper levels. Sun-Hee pointed out that in a near neutral hurricane environment, the diffusivity and PBL height are proportional to the wind speed. Near the eye, the lowest PBL height was observed while at the RMW, the highest PBL height was observed.

Next, experiment results were shown. A vertical profile of the momentum diffusivity coefficient at the RMW showed smaller RiBc values (equal to 0.25 like in EXP 1 and 3) made the PBL height lower. Also, smaller mixed length (equal to 30m like in EXP 2 and 3) made the diffusivity coefficient weaker. In a plot showing PBL height, lower RiBc value again showed lower PBL heights (EXP 1 and 3) while lower mixed length (EXP 2 and 3) also showed lower PBL heights but with a smaller effect than RiBc values had. In the vertical profile of radial wind plot, lower RiBc values again showed lower PBL heights while the local diffusion scheme (EXP4) showed stronger inflow at lower levels compared to the other experiments. In plots showing wind distribution from the storm center, the strongest values were observed at 950 hPa for the local diffusion experiment (EXP 4) compared to the surface and 850 hPa levels.

Sun-Hee then showed plots for track and intensity from the different experiments. For central pressure, the lowest values were for EXP4 (local diffusion) while CTL (the current HWRF) has stronger diffusion values but produced a weaker storm pressure. The maximum wind plot showed EXP4 (local diffusion) wind values similar to the other experiments, despite the fact that pressure values were much lower. These results were not exactly consistent with what was observed for pressure. Sun-Hee then showed tracks for the various experiments and noted that PBL schemes had little impact on tracks as they were tightly clustered together.

Sun-Hee concluded by presenting her future work. She plans to test the new GFS PBL scheme in the HWRF as well as the Mellor-Yamada-Janjic (MYJ) scheme in HWRF. She also plans to test more cases and do sensitivity tests for background diffusivity.

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