Fine-Scale Precision NWP: WRF-RTFDDA-LES

BACKGROUND

Fig. 1. Simultaneous multi-scale WRF-RTFDDA-LES simulations with six-nested-grid domains with model grid intervals varying from 33m to 8.1km.
Fig. 1. Simultaneous multi-scale WRF-RTFDDA-LES simulations with six-nested-grid domains with model grid intervals varying from 33m to 8.1km.

Demands on precision weather forecasts for weather-sensitive applications are rapidly increasing. To meet these rising needs, RAL developed numerical weather prediction technologies with sub-kilometer grid very-varge-eddy simulations (VLES) and large-eddy simulations (LES) on grids with intervals of 10s of meters. The VLES/LES NWP model is built around the NCAR Weather Research and Forecasting (WRF) model using real-time four-dimensional data assimilation (RTFDDA). The technology allows the VLES and LES models to be directly nested inside a parent mesoscale model, rendering simultaneous multi-scale simulations with full physics. The RTFDDA-V/LES system has been proven to provide valuable results for boundary-layer weather simulations in regions with complex terrain and areas with sharp transitional land use types, such as coastal regions with water-land contrasts and urban environments. The V/LES model also can improve the simulation of severe convective systems. Fig. 1 shows an example of RTFDDA-V/LES model configuration with six nested domains for the US Army Dugway Proving Ground.  

FY2019 ACCOMPLISHMENTS

Research on WRF-RTFDDA-LES in 2019 continued, with evaluations of the real-time modeling systems for the US Army Dugway Proving Ground (DPG) in Utah, and Aberdeen Test Center (ATC) in Maryland. A WRF-RTFDDA-LES model has also been set up to simulate fine-scale weather flows and mountain convection over the White Sand Missile Range in New Mexico.

Figure 2. Snapshots of WRF-RTFDDA-LES simulation of early morning (left panel; valid at 11:00 UTC, May 4, 2012 with stable boundary layer) and around-noon (right; valid at 17:32 UTC, May 4, 2012 with convective boundary layer) at 33m grid intervals. The field shown is the model vertical velocity at 200m Above Ground Level (m/s).
Figure 2. Snapshots of WRF-RTFDDA-LES simulation of early morning (left panel; valid at 11:00 UTC, May 4, 2012 with stable boundary layer) and around noon (right; valid at 17:32 UTC, May 4, 2012 with convective boundary layer) at 33-m grid interval. The field shown is the vertical velocity (m/s) at 200 m AGL..

RAL is studying the impact of grid resolution on the multi-scale flow interactions at Granite Mountain at DPG. RTFDDA-LES was configured for four nested domains, with grid intervals of 8.1, 2.7, 0.9 and 0.3 km, respectively (Fig. 1). The system assimilates all available observations, including the dense network of observations at DPG. Verification of the real-time analyses and forecasts shows the benefits of the ultra-high-resolution NWP system in resolving realistic sub-mesoscale flows; it also exposes artificially amplified turbulence over broad spatiotemporal scales. Modeling studies were conducted with six nested domains (two extra nested domains with grid sizes of 100 m and 33 m, respectively [Fig. 1]) to study microscale flows associated with the Granite Mountain (~60 km2). The modeling results show increasing ability of the VLES and LES model over the mesoscale model in resolving the fine-scale flow features, especially wind ramps (e.g. Fig. 2). 

Two approaches are being developed to constrain the artificially amplified turbulence in the VLES model. One approach is to add a TKE-based boundary-layer scheme on top of the LES sub-grid-filter, and the other is to adjust the sub-grid-filter mixing parameters. Both approaches mitigate, but do not remove, the artificially amplified turbulence. For the end user’s benefit, a wavelet-based scale separation strategy is being developed to post-process the VLES meteograms and remove the artificially amplified turbulences.


 

Figure 3. A snapshot view of RTFDDA-VLES simulation of summer monsoon convection initiation over mountain ranges around the army White Sam Missile Range.
Figure 3. A snapshot view of RTFDDA-VLES simulation of summer monsoon convection over mountain ranges around White Sands Missile Range. 

RTFDDA-VLES/LES has been employed to study the orographic convection over the mountain ranges surrounding White Sands Missile Range (WSMR) as well. Simulation experiments with seven cases show that with 300-m and 100-m grids, RTFDDA-VLES was able to forecast the initiation of moist convection over the complex mountain ranges.  

FY2020 PLANS

The improved ability of WRF-RTFDDA-V/LES is encouraging for microscale weather forecasting, but many challenges remain before it can be deployed for real-time operational forecasting. First, each V/LES NWP system should be formulated to address the specific needs of an application, and tools should be developed to improve the use and visualization of VLES forecast output for end users. Second, V/LES forecast verification and special observations that characterize microscale weather information should be explored to help understand when V/LES NWP is valuable and when it is not. This will help modelers to improve the modeling technology and to provide product guidance to end users. Lastly, data assimilation is critical for real-time operational forecasting with V/LES NWP models, and the assimilation schemes should be adapted for the V/LES-scale models.

In 2020, modeling studies with RTFDDA-V/LES will be carried out to understand small and microscale severe weather processes including strong winds, icing, thunderstorms, and extreme temperatures over small-scale complex terrain. RTFDDA-V/LES will be further assessed for modeling the high-impact local weather phenomena at different army test ranges and prepared for real-time operations at ATC, DPG, and WSMR in the next two years.