The Effects of Numerical Dissipation on Hurricane Rapid Intensification
with Observational Heating
Abstract
The computational fluid dynamics of hurricane rapid intensification (RI)
is examined through idealized simulations using two codes: a
community-based, finite-difference/split-explicit model (WRF) and a
spectral-element/semi-implicit model (NUMA). The focus of the analysis
is on the effects of implicit numerical dissipation (IND) in the
energetics of the vortex response to heating, which embodies the
fundamental dynamics in the hurricane RI process. The heating considered
here is derived from observations: four-dimensional, fully nonlinear,
latent heating/cooling rates calculated from airborne Doppler radar
measurements collected in a hurricane undergoing RI. The results
continue to show significant IND in WRF relative to NUMA with a
reduction in various intensity metrics: (1) time-integrated, mean
kinetic energy values in WRF are ~20% lower than NUMA
and (2) peak, localized wind speeds in WRF are ~12m/s
lower than NUMA. Values of the eddy diffusivity in WRF need to be
reduced by ~50% from those in NUMA to produce a similar
intensity time series.
Kinetic energy budgets
demonstrate that the pressure contribution is the main factor in the
model differences with WRF producing smaller energy input to the vortex
by ~23%, on average. The low-order spatial
discretization of the pressure gradient in WRF is implicated in the IND.
In addition, the eddy transport term is found to have a largely positive
impact on the vortex intensification with a mean contribution of
~20%. Overall, these results have important
implications for the research and operational forecasting communities
that use WRF and WRF-like numerical models.