THE EFFECTS OF NUMERICAL DISSIPATION ON SIMULATING HURRICANE INTENSIFICATION IN A REALISTIC REGIME

Abstract:

The vortex response to heating in convective clouds, provided by warm ocean waters, is the fundamental physics controlling the energy input to tropical cyclones. Dissipation of energy occurs at the surface and through turbulent eddies of various scales most predominantly in the eyewall and boundary layer region. In numerical models, the dissipation of energy can also occur from the dynamic core with recent work showing this effect can be significant in theoretical studies with the Weather Research and Forecasting (WRF) model showing anomalously high dissipation relative to research codes (Guimond et al. 2016).

In this work, we extend the results of the above study to a more realistic regime characterized by 4-D heating sources calculated from airborne Doppler radar observations and the inclusion of additional physics (surface layer and moisture). The initial condition is a balanced, tropical storm-like vortex with forcing from the observational heating. Two numerical models are examined: The Weather Research and Forecasting (WRF) model and the Nonhydrostatic Unified Model of the Atmosphere (NUMA). Simple, constant explicit diffusion for all prognostic variables and localized diffusion based on the heating are used to examine sensitivities.

The results show that the WRF wind field is significantly spread out and diffused relative to that in NUMA. Sensitivity tests show that this spreading of the wind field is not due to the default upwind biased advection scheme or the large/small timesteps. Spectral kinetic energy budget analysis will be presented to examine the causes of this large, diffuse wind field in WRF. In addition, standard metrics for the vortex intensity will also be presented to highlight the differences between the models.