Effects of Turbulence Parameterization on Hurricane Simulations

The objective of this study is to understand and evaluate NRL COAMPS simulated atmospheric boundary layer structure of Hurricane Isabel (2003) using in-situ observations obtained during CBLAST field experiment. A tropical cyclone (TC) version of COAMPS is configured to include three nested grids (45km, 15km, and 5km) with 15/5km nests moving with TC. The COAMPS features relevant to TC boundary layers include a turbulent-kinetic energy (TKE) based dissipative heating parameterization; sea-spray representation; and options of different mixing length formulation for a prognostic TKE turbulence scheme.

The complete TKE budget from COAMPS is derived. The results show that shear production dominates the TKE production and its values are one order of magnitude larger than the buoyancy production. The TKE advection contributes considerably to the budget near the eye wall due to the strong winds and horizontal gradients, indicating that traditional one-dimensional TKE budget may not be sufficient for TC boundary layers.

A mixing length calculation based on the Bougeault convective free-path concept is implemented. Sensitivity simulations are also performed to evaluate the sensitivity of TC intensity and boundary layer structure to different turbulence mixing length formulations. It is found that this sensitivity is significant.

We further applied the Bougeault mixing length for the convective region above 2km and used standard Mellor-Yamada mixing length within the boundary layer. This implementation recognizes the strong wind shear effect near the PBL top and buoyancy contribution within deep convection region. We are currently analyzing the results.