摘要:Each spring, smoke particles from fires over the Yucatan Peninsula and south Mexico cross
over the Gulf of Mexico into the United States (US) under the control of moist oceanic air
flow from the southwestern branch of the subtropical (Bermuda) high. Smoke can be
transported deep into the south central US, where dry lines and warm conveyor belts are
frequently formed and cause deep convection and severe weather. Lyons et al (1998
Science 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 the
May 1998 Central American smoke episode. Here we present a conceptual model of coherent
microphysical and meteorological mechanisms through which smoke may impact convective
clouds and subsequently result in more severe weather over the south central
US. The conceptual model depicts a chain of processes in which smoke particles
are first activated as cloud condensation nuclei when they are entrained into
the warm conveyor belt, a convective zone formed over the south central US as
a result of the encounter between the mid-latitude trough and the subtropical
Bermuda high. As the convection continues with deepening of the mid-latitude
trough, the greater concentration of water cloud condensation nuclei delays the
warm rain processes, enhances the development of ice clouds, and invigorates
the updrafts, all of which contribute to the formation of severe weather such as
hail and lightning. The conceptual model is based on the reasoning of physical
mechanisms revealed in previous studies (over the tropical biomass region), and
is supported here through the analysis of satellite data, ground observations,
aerosol transport model results, and idealized cloud resolving simulations of a
day in May 2003 when record tornado events occurred over the south central US.
Further assessment of this conceptual model is discussed for future investigations.