Observation and Simulation of Methane Plumes During the Morning Boundary Layer Transition

Abstract

Methane (CH₄) contributes significantly to global warming. However, accurate identification of CH₄ sources for reducing CH₄ emissions is often hampered by inadequate accuracy and spatiotemporal coverage of CH₄ detection, and lack of accurate CH₄ forward modeling used in top-down inversion systems. In this study, a field experiment was conducted in Pampa, Texas using two CH₄ sensors (LI-COR and OGI camera) to detect CH₄ releases. We investigated whether high-resolution simulations using the Weather Research and Forecasting (WRF) model with greenhouse gases (WRF-GHG) could accurately simulate the CH₄ plumes in the presence of evolving atmospheric boundary layer from sunrise to noon. CH₄ plumes showed substantial variation in time. At a release rate of ∼17.5 kg hr⁻¹, the maximum enhancement of CH₄ measured by LI-COR was 2.6 ppm at sunrise (7:36 a.m.), 250 m from the release location. Within half an hour after sunrise, this enhancement decreased to 0.3–0.4 ppm. The enhancement was 0.2 ppm by 10:00 a.m. and further dropped to less than 0.1 ppm after 11:30 a.m. Due to the low temperature at sunrise, the OGI camera failed to detect the CH₄ plume. The WRF-GHG large-eddy simulation (LES) with 32 m grid spacing successfully reproduced these CH₄ enhancements. In situ measurements together with numerical simulations illustrate the impact of the transition from a stable boundary layer in the early morning to a convective boundary layer at noon on the dispersion of CH₄ plumes. Additionally, CH₄ plumes from a cattle farm in Oklahoma are briefly examined using the same modeling approach.