Observed slump of sea land breeze in Brisbane under the effect of aerosols from remote transport during 2019 Australian mega fire events

Abstract

Abstract. The 2019 Australian mega fires were unprecedented considering their intensity and consistency. There has been much research on the environmental and ecological effects of these mega fires, most of which focused on the effect of huge aerosol loadings and the ecological devastation. Sea land breeze (SLB) is a regional thermodynamic circulation closely related to coastal pollution dispersion, yet few have looked into how it is influenced by different types of aerosols transported from either nearby or remote areas. Mega fires provide an optimal scenario of large aerosol emissions. Near the coastal site of Brisbane Archerfield during January 2020, when mega fires were the strongest, reanalysis data from Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) showed that mega fires did release huge amounts of aerosols, making aerosol optical depth (AOD) of total aerosols, black carbon (BC) and organic carbon (OC) approximately 240 %, 425 % and 630 % of the averages in other non-fire years. Using 20 years' wind observations of hourly time resolution from a global observation network managed by the National Oceanic and Atmospheric Administration (NOAA), we found that the SLB day number during that month was only 4, accounting for 33.3 % of the multi-years' average. The land wind (LW) speed and sea wind (SW) speed also decreased by 22.3 % and 14.8 % compared with their averages respectively. Surprisingly, fire spot and fire radiative power (FRP) analysis showed that heating effects and aerosol emission of the nearby fire spots were not the main causes of the local SLB anomaly, while the remote transport of aerosols from the fire centre was mainly responsible for the decrease of SW, which was partially offset by the heating effect of nearby fire spots and the warming effect of long-range transported BC and CO2. The large-scale cooling effect of aerosols on sea surface temperature (SST) and the burst of BC contributed to the slump of LW. The remote transport of total aerosols was mainly caused by free diffusion, while the large-scale wind field played a secondary role at 500 m. The large-scale wind field played a more important role in aerosol transport at 3 km than at 500 m, especially for the gathered smoke, but free diffusion remained the major contributor. The decrease of SLB speed boosted the local accumulation of aerosols, thus making SLB speed decrease further, forming a positive feedback mechanism.

Effects of Sea Land Breeze on Air‐Sea Turbulent Heat Fluxes in Different Seasons Using Platform Observation in East China Sea

“…Some months in spring and autumn have similar amount of SWR as those in summer since the in situ SWR is influenced by both solar zenith angle and cloud. Except for high aerosol emission periods, lower cloud cover offsets the effect of smaller solar zenith angle through weaker “Umbrella Effect” to ensure enough in situ SWR, which boosts the triggering of SLB (Miller et al., 2003; Shen, Zhao, Yang, Yang, & Zhou, 2022; Shen et al., 2021a). Most importantly, not all SLB days have higher level of SWR flux than the monthly mean SWR flux.…”

“…Third, only local wind signals during PTS (PTL) on SLB days are selected to further calculate the average SLB speed. With the help of these 3 filters, the reliability of this method has been proved from several perspectives, including the comparison between its outcome and objective climate (physical process) analysis (Shen, Zhao, & Yang, 2022; Shen, Zhao, Yang, Yang, & Zhou, 2022; Shen et al., 2021a). In order to make it clear, we further introduce briefly with critical points in these three steps being clarified.…”

Weakened Sea‐Land Breeze in a Coastal Megacity Driven by Urbanization and Ocean Warming

“…(2019) demonstrated that the monthly average SLBD during the summer of Shanghai from 2003 to 2010 was 5, that is, 7 days less than that during the summer in Tianjin over the same period. Only 4 SLBD in January 2020 in Brisbane were observed (Shen et al., 2022), that is, 5 days less compared to Tianjin. In Cotonou, where the West African monsoon is prevalent, a total of 417 SLBD were observed over the five years from 2006 to 2010 (Gbambie & Steyn, 2013).…”

“…The number of SLBD fluctuated at post-2011 after a sharp decline was observed before 2011. Only 4 SLBD in January 2020 in Brisbane were observed (Shen et al, 2022), that is, 5 days less compared to Tianjin. In Cotonou, where the West African monsoon is prevalent, a total of 417 SLBD were observed over the five years from 2006 to 2010 (Gbambie & Steyn, 2013).…”

“…SLB is caused by the temperature difference between land and sea (Crosman & Horel, 2010; Miller et al., 2003). The changes in the frequency, strength, and structure of SLB is attributed to the changes in land use and cover, sea surface temperature (SST) anomaly, stronger anthropogenic heat emissions, and regional extreme weather (Ding et al., 2004; Hu et al., 2022; Shen et al., 2022). The variations in SLB would contribute to the evolution of the coastal atmospheric boundary layer, regional transport of air pollutants, and local weather such as wind, temperature, and humidity (Igel et al., 2018; Kim et al., 2021; J. M. Park et al., 2020).…”