Fine-Scale Precision NWP: WRF-RTFDDA-LES

BACKGROUND

Demands for fine-scale precision weather forecasts from weather-sensitive organizations are rapidly increasing. To meet their needs, NCAR-RAL takes advantage of its advanced real-time four-dimensional data assimilation (RTFDDA) weather forecasting system and increased computing power to study the feasibility of using its numerical weather prediction (NWP) capability to model at the sub-kilometer Very-Large-Eddy Simulation (VLES) scale down to the Large Eddy Simulation (LES) scale. To accomplish this, the newly developed WRF-RTFDDA-LES system is directly nested inside a parent mesoscale system. This fine-scale forecasting system provides detailed weather information that can be integrated to improve operational and logistic effectiveness of a multi-faceted prediction system. In the course of developing this system, RAL has conducted fundamental research on various fine-scale weather scenarios including tornado storm and wind farm turbulence.   This work is currently being adapted for research and operational use in weather-critical applications.

Demands for fine-scale precision weather forecasts from weather-sensitive organizations are rapidly increasing. To meet their needs, NCAR-RAL takes advantage of its advanced real-time four-dimensional data assimilation (RTFDDA) weather forecasting system and increased computing power to study the feasibility of using its numerical weather prediction (NWP) capability to model at the sub-kilometer Very-Large-Eddy Simulation (VLES) scale down to the Large Eddy Simulation (LES) scale. To accomplish this, the newly developed WRF-RTFDDA-LES system is directly nested inside a parent mesoscale system. This fine-scale forecasting system provides detailed weather information that can be integrated to improve operational and logistic effectiveness of a multi-faceted prediction system. In the course of developing this system, RAL has conducted fundamental research on various fine-scale weather scenarios including tornado storm and wind farm turbulence.   This work is currently being adapted for research and operational use in weather-critical applications.

FY2017 ACCOMPLISHMENTS

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).

Research on WRF-RTFDDA-LES during this year 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. 

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. 

RTFDDA-VLES/LES bas been employed to study the orographic convection over the mountain ranges surrounding the Army’s White Sand Missile Range 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.

FY2018 Plans

Although much encouraging ability of WRF-RTFDDA-LES/VLES have been and demonstrated for real-world microscale weather forecasting, there are many challenges remained in order to practically realize the value of the LES/VLES NWP technology. First, each LES/VLES 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, VLES/LES forecast verification strategies and special observations contains microscale weather information should be explored to help understand when LES/VLES NWP is valuable and when it is not. This will help modelers to improve the modeling technology and provides guidance for end users. Thirdly, LES-scale data assimilation is critical for operational forecasting of VLES/LES NWP models. Meanwhile, modeling studies in FY2018 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-LES/VLES will be further assessed for modeling the high-impact local weather phenomena at different army test ranges.