Nested-grid simulation and forecasting experiments of orographic flows at US Army DPG Granite Mountain with NCAR RTFDDA-LES

Recent advances of computing capacity and numerical modeling technologies have been pushing the front of real-time weather prediction toward ultra-high resolutions, especially for focused-region local-scale weather analysis and forecasting services, such as with the ATEC (Army Test and Evaluation Command) and NCAR joint 4DWX operational RTFDDA (real-time four-dimensional data assimilation) and forecasting systems. RTFDDA has been implemented to provide rapid analysis and forecasts at eight Army test ranges over different regions of the US. RTFDDA is built upon the Weather Research and Forecasting (WRF) model and capable of effectively assimilate all available weather observations into the full-physics WRF model to produce high-accuracy multi-scale 4D weather information from synoptic scale to microscale (< 2 km). To take the advantage of the computing advances, RTFDDA has been enhanced with LES (large-eddy-simulation) and VLES (very-large-eddy simulation) modeling capabilities, referred to as RTFDDA-LES. An objective of RTFDDA-LES is to advance the ATEC-NCAR 4DWX operational NWP to LES and VLES scales with grid sizes of 10s - 100s meters by simultaneously simulating synoptic-scale, mesoscale, and small and microscale weather processes to provide ultra-high-resolution and accurate 4D weather information to support ATEC routine and mission-critical test activities.

A realtime RTFDDA-VLES system configured with four nested-grid domains, with grid sizes of 8.1, 2.7, 0.9 and 0.3km respectively, has been deployed at the Army Dugway Proving Ground (DPG) and started experimental operational forecasting since May 2012. In this paper, we will briefly introduce the RTFDDA-VLES and RTFDDA-LES model system, present the statistical verification of the RTFDDA forecasts against the special surface and tower weather observations, and depict the advantages and limitations of the ultra high resolution model. To validate the VLES NWP capability, we carried out a simulation with six nested-grid domains by extending the DPG RTFDDA-VLES with two extra nested domains with grid sizes of 100m and 33m, respectively. The study focuses on the Granite Mountain (~10x15km independent) and neighboring area at DPG, where a complex-terrain flow field project (MATERHORN) is on going. The simulation period is focused on two days of May in 2012. The model results of Domain 6 (at 33m grid intervals) provide a true LES-scale NWP reference to assess VLES (Domains 4 - 5) model. The verification result shows evident advantages of the VLES model over mesoscale model.