Abstract
AbstractIn recent years, new measurement systems have been deployed to monitor and quantify methane emissions from the natural gas sector. Large-eddy simulation (LES) has complemented measurement campaigns by serving as a controlled environment in which to study plume dynamics and sampling strategies. However, with few comparisons to controlled-release experiments, the accuracy of LES for modeling natural gas emissions is poorly characterized. In this paper, we evaluate LES from the Weather Research and Forecasting (WRF) model against Project Prairie Grass campaign measurements and surface layer similarity theory. Using WRF-LES, we simulate continuous emissions from 30 near-surface trace gas sources in two stability regimes: strong and weak convection. We examine the impact of grid resolutions ranging from 6.25 m to 52 m in the horizontal dimension on model results. We evaluate performance in a statistical framework, calculating fractional bias and conducting Welch’s t-tests. WRF-LES accurately simulates observed surface concentrations at 100 m and beyond under strong convection; simulated concentrations pass t-tests in this region irrespective of grid resolution. However, in weakly convective conditions with strong winds, WRF-LES substantially overpredicts concentrations – the magnitude of fractional bias often exceeds 30%, and all but one C-test fails. The good performance of WRF-LES under strong convection correlates with agreement with local free convection theory and a minimal amount of parameterized turbulent kinetic energy. The poor performance under weak convection corresponds to misalignment with Monin-Obukhov similarity theory and a significant amount of parameterized turbulent kinetic energy.
Highlights
Natural gas production within the U.S has surged in the past decade, increasing by more than50% since 2010 (EIA 2020)
We model two types of convection—a strongly convective boundary layer (SCBL) and a weakly convective boundary layer (WCBL)—and we simulate each with a coarse, moderate, and fine-resolution grid
The simulated emission height decreases from 10.5 m in the coarse simulation to 1.5 m in the fine simulation, as trace gas is released from the center of the lowest cell
Summary
Natural gas production within the U.S has surged in the past decade, increasing by more than50% since 2010 (EIA 2020). Large emissions from routine operations (Thorpe et al 2020) and malfunctioning equipment (Conley et al 2016) have spurred the development of new methane emission monitoring instruments and platforms, including satellites, piloted aircraft, unmanned aircraft, open-path lasers, and ground-based point sensors (Fox et al 2019). Source estimation techniques (SETs) are used to interpret source characteristics (e.g. emission rate) from the trace gas concentration measurements collected via these systems (Harper et al 2011). Operational source estimation techniques (OSETs) are computationally low-cost and simple to use, and they vary from instrument to instrument. Satellites and remote sensing aircraft often use the integrated mass enhancement (IME) technique (Frankenberg et al 2016; Varon et al 2018; Jongaramrungruang et al 2019). In situ aircraft measurements often use mass balance techniques (Karion et al 2013; Conley et al 2017). Many ground-based sensors employ techniques that rely on a transport and dispersion model, such as the Gaussian Plume Model (Pasquill 1972; U.S EPA 2014; Coburn et al.2018)
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