The sensitivity of a Katrina simulation to the distribution of vertical sigma levels

A study completed for an idealized hurricane simulation over water at a constant surface temperature showed that the storm developed very differently as a function of the vertical spacing of the model sigma levels. The distribution of vertical levels in the inflow, outflow and middle layers of the atmosphere clearly affected the intensity, size, and structure of the storms, causing certain processes to be under or over resolved. A strong outflow was found to be necessary for proper storm intensification, while a strong inflow layer did not correspond to an intense storm. In fact, when a strong inflow layer was coupled with a weak outflow layer, a particularly weak storm was the result. When too few levels were assigned to the outflow layer, convection was confined to low and mid-tropospheric levels, and too little latent heating occurred at the middle levels to facilitate the formulation of a strong secondary circulation. However, too few levels in the planetary boundary level would cause a storm to intensify beyond its theoretically calculated maximum potential intensity. Before better guidelines for vertical level distribution could be developed, however, a second study of the sensitivity to vertical sigma levels for more realistic storm simulations was necessary.

This new study uses Hurricane Katrina as it made landfall in Louisiana as the test case for varying the distribution of sigma levels in the PSU/NCAR mesoscale model version 5, MM5. In the idealized simulation, varying sea surface temperatures and upper level environmental flows were not included, yet they may have an impact of the development of the storm as a function of the sigma level distribution. In addition, the physics parameterizations may also be a function of the number of sigma levels in a particular region. Several of the distributions used in the ideal study are used with this land-falling hurricane. The results are compared with the trends identified with the idealized results and with observations.

The ultimate goal of these studies is to provide recommendations for an optimal number of sigma levels and their distribution. These results have important implications to hurricane forecasting; models such as MM5 are used operationally to forecast hurricane track and intensity. If such models are sensitive to the distribution and number of vertical levels, then forecasts may not accurately represent the real situation with serious consequences to life and property.