A conceptual model for the link between Central American biomass burning aerosols andsevere weather over the south central United States

Abstract

Each spring, smoke particles from fires over the Yucatan Peninsula and south Mexico crossover the Gulf of Mexico into the United States (US) under the control of moist oceanic airflow from the southwestern branch of the subtropical (Bermuda) high. Smoke can betransported deep into the south central US, where dry lines and warm conveyor belts arefrequently formed and cause deep convection and severe weather. Lyons et al (1998Science 282 77–80) and Murray et al (2000 Geophys. Res. Lett. 27 2249–52) noticed a∼50% increase of lightning along the smoke transport path over the south central US during theMay 1998 Central American smoke episode. Here we present a conceptual model of coherentmicrophysical and meteorological mechanisms through which smoke may impact convectiveclouds and subsequently result in more severe weather over the south centralUS. The conceptual model depicts a chain of processes in which smoke particlesare first activated as cloud condensation nuclei when they are entrained intothe warm conveyor belt, a convective zone formed over the south central US asa result of the encounter between the mid-latitude trough and the subtropicalBermuda high. As the convection continues with deepening of the mid-latitudetrough, the greater concentration of water cloud condensation nuclei delays thewarm rain processes, enhances the development of ice clouds, and invigoratesthe updrafts, all of which contribute to the formation of severe weather such ashail and lightning. The conceptual model is based on the reasoning of physicalmechanisms revealed in previous studies (over the tropical biomass region), andis supported here through the analysis of satellite data, ground observations,aerosol transport model results, and idealized cloud resolving simulations of aday in May 2003 when record tornado events occurred over the south central US.Further assessment of this conceptual model is discussed for future investigations.