Recent Advances in the A2e Mesoscale to Microscale Coupling Project

The purpose of the US DOE’s Mesoscale-Microscale Coupling (MMC) Project is to develop, verify, and validate physical models and modeling techniques that bridge the most important atmospheric scales that determine wind plant performance and reliability. As part of DOE’s Atmosphere to Electrons (A2e) program, the MMC project seeks to create a new predictive numerical simulation capability that is able to represent the full range of atmospheric flow conditions impacting wind plant performance. This project, which includes the work of seven U.S. national laboratories, is now in its fourth year and focusing research on coupling models run complex terrain situations and nonstationary conditions.

The approach to the project is to choose interesting case days for which there are observational data for validation. For the complex terrain cases, the MMC team has focused on the Pacific Northwest and are leveraging observational data derived from the Wind Forecast Improvement Project 2 field project in that region. There are several science questions that form the focus of the project, including: 1) What is the impact of modeling across the so-called terra incognita, that grid resolution between about 100 m and the boundary layer depth at which numerical artifacts are often difficult to distinguish from physical boundary layer rolls? We look at the impact of including simulations using a new fully three-dimensional turbulence parameterizations. 2) How to best initialize turbulence at the microscale that was not resolved in the mesoscale model? To address this issue, the team has explored the impact of temperature perturbations and momentum perturbations, as well as modeling the canopy. 3) What is the best way to handle the surface layer parameterizations consistently at the mesoscale and the microscale? Three different explicit canopy models were tested to address this issue.

Formal assessment was accomplished for a complex terrain case from the WFIP2 observational experiment using observations at the Physics Site for November 21, 2016. This case study was characterized by topographic wake and mountain waves over the area. The MMC simulation was carried out using WRF’s nesting capability, with the parent nest run in mesoscale mode while two inner nests were run in LES mode. Turbulent sensible heat flux was predicted quite well except during a period possibly during relatively short intervals when the model did not accurately capture cloud cover. Spectral analysis shows excellent agreement between measured and simulated turbulence frequency spectra in the well-resolved portion of the inertial range. Good agreement indicates then even when the mesoscale flow is not captured accurately, the turbulent energy transfer from large turbulent production scales to smaller scales can be represented accurately in a well resolved LES. The assessment of mesoscale-to-microscale coupling within a single model confirms the feasibility and validity of the approach that relies of online coupling within the same model.