WRF Simulations of Passive Tracer Transport from Biomass Burning in South America

Abstract

Highlights: What are the main findings? This is a single-event case study from 15 to 20 August 2019 on Amazon smoke. Long-range transport to SE Brazil occurred when a persistent 2–4 km lofted layer coexisted for hours with a favorable 700–600 hPa projected flow, opening a corridor for outflow. There is a PBL sensitivity in this case. MYNN 2.5 best matched the observed arrival altitude and timing over MASP, YSU produced thicker yet delayed plumes, and BouLac showed intermittent pulses. What is the implication of the main finding? There is a two-ingredient diagnostic from this case that can flag long-range smoke outflow risk for operations. The presence of a 2–4 km lofted layer together with favorable 700–600 hPa flow is reported. The evidence remains case-specific. Broader use needs multi-event and multi-season evaluation with additional validation datasets. This single high-impact case study investigates the impact of planetary boundary layer (PBL) representation on long-range transport of Amazon fire smoke that reached the Metropolitan Area of São Paulo (MASP) from 15 to 20 August 2019, using the WRF model to compare three PBL schemes (MYNN 2.5, YSU, and BouLac) and three source-tagged tracers. The simulations are evaluated against MODIS-derived aerosol optical depth (AOD), the Light Detection and Ranging (LiDAR) time–height curtain over MASP, and HYSPLIT forward trajectories. Transport is diagnosed along the source-to-MASP pathway using six-hourly cross-sections and two integrative metrics: the projected mean wind in the 700–600 hPa layer and the vertical moment of tracer mass above the boundary layer. Outflow and downwind impact are strongest when a persistent reservoir between 2 and 4 km coexists with projected winds for several hours. In this episode, MYNN maintains an elevated 2–5 km transport layer and matches the observed arrival time and altitude, YSU yields a denser but delayed column, and BouLac produces discontinuous pulses with reduced coherence over the city. A negatively tilted trough, jet coupling, and a nearly stationary front establish a northwest-to-southeast corridor consistent across model fields, trajectories, and satellite signal. Seasonal robustness should be assessed with multi-event, multi-model analyses.