High resolution applications of the WRF NMM

The Nonhydrostatic Mesoscale Model (NMM) WRF core has been developed at NCEP building on NWP experience. The model formulation successfully reproduced the classical two-dimensional nonhydrostatic solutions. The model nonlinear dynamics demonstrated the ability to develop the observed –3 and –5/3 spectral slopes, which were not induced by computational errors. In a decaying turbulence case on the cloud scales, the model dynamics developed the –5/3 spectrum consistent with the 3D turbulence theory. The computational efficiency of the model has been significantly higher than the computational efficiency of most established nonhydrostatic models.

In addition to operational forecasting the model has been tested in many case studies and several validation campaigns. The model demonstrated the ability to predict tropical storms realistically, and efforts are under way to implement it as the Hurricane WRF. However, particularly interesting results were obtained in the SPC/NSSL Spring Program 2004 in which the model was run with the resolution of 4.5 km and without parameterized convection. The model demonstrated the ability to spin-up severe convective systems more frequently, and with a stronger signal, than if this were happening only by chance. The high resolution WRF NMM forecasts for the first time clearly outperformed the NCEP mesoscale forecasts with parameterized convection. This result also suggests that further improvements in deterministic forecasting of severe weather phenomena can be achieved with increased resolution.