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 refined-scale precision weather forecasts from weather-sensitive applications are rapidly increasing. To meet their needs, NCAR-RAL developed numerical weather prediction technologies with sub-kilometer grid Very-Large-Eddy Simulation (VLES) and 10s-meter grid Large Eddy Simulation (LES). The VLES/LES NWP model is built around the NCAR Weather Research and Forecasting (WRF) based real-time four-dimensional data assimilation (RTFDDA) and forecasting system. The technology allows the VLES and LES models to be directly nested inside a parent mesoscale system, rendering simultaneous multi-scale simulation with full-physics capability. RTFDDA-V/LES system has been proven to provide valuable results for boundary-layer weather simulation in the regions with complex terrain and the areas with sharp transitional land use types such as coastal regions with water-land contrast and urban weather modeling. The V/LES model also can improve the simulation of severe convection systems. Fig.1 shows an example of RTFDDA-V/LES model configuration with six nested-grid domains for the U.S. Army Dugway Proving Ground.  

FY2018 ACCOMPLISHMENTS

Research on WRF-RTFDDA-LES in 2018 was mainly focused on an evaluation study of a real-time modeling system for the US Army’s Dugway Proving Ground (DPG) in Utah, and a real-time modeling system for the US Army’s Aberdeen Test Center (ATC) in Maryland. A WRF-RTFDDA-LES model has also been set up to simulate fine scale weather flows and mountain convections 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 33m grid intervals. The field shown is the model vertical velocity at 200m Above Ground Level (m/s).

To study the impact of WRF model grid resolution on the multi-scale flow interactions at Granite Mountain in DPG with a LES-scale system, RTFDDA-LES was implemented for DPG. It was configured for four nested-grid domains, with grid sizes 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-grid domains (two extra nested domains with grid sizes of 100m and 33m, respectively [Fig. 1]) to study microscale flows associated with the Granite Mountain (~60 km2) at DPG. 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 contain the artificially amplified turbulence in the VLES model. One approach is to add a TKE-based boundary layer scheme on the 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 initiation over mountain ranges around the army White Sam Missile Range.  

RTFDDA-VLES/LES bas been employed to study the orographic convection over the mountain ranges surrounding the Army’s White Sand Missile Range (WSMR) as well. Simulation experiments with seven cases show that with 300m and 100m grid VLES fine mesh grid, RTFDDA-VLES displayed a dramatic ability in forecasting convection initiation over the complex mountain ranges. 

FY2019 PLAN

Although much encouraging ability of WRF-RTFDDA-V/LES have been demonstrated for real-data microscale weather forecasting, there are many challenges remained in order to practically deploy it for real-time operational forecasting. Firstly, 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 outputs for the end users. Secondly, V/LES forecast verification and special observations that contain 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 provide product guidance to end users. Thirdly, data assimilation is critical for real-time operational forecasting of V/LES NWP models and the mesoscale data assimilation schemes should be adapted for the V/LES-scale models.

In 2019, 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.