Crucial roles of eastward propagating environments in the summer MCS
initiation over the U.S. Great Plains
Abstract
This study aims at improving understanding of the environments
supporting summer MCS initiation in the U.S. Great Plains. A
self-organizing map analysis is conducted to identify four types of
summer MCS initiation environments during 2004-2017: Type-1 and Type-2
feature favorable large-scale environments, Type-3 has favorable
lower-level and surface conditions but unfavorable upper-level
circulation, while Type-4 features the most unfavorable large-scale
environments. Despite the unfavorable large-scale environment,
convection-centered composites reveal the presence of favorable
sub-synoptic scale environments for MCS initiation in Type-3 and Type-4.
All four types of MCS initiation environments delineate a clear eastward
propagating feature in many meteorological fields, such as potential
vorticity, surface pressure and equivalent potential temperature,
upstream up to 25 west of and ~36 hours before MCS
initiation. While the propagating environments and local,
non-propagating low-level moisture are important to MCS initiation at
the foothill of the Rocky Mountains, MCS initiation in the Great Plains
is supported by the coupled dynamical and moisture anomalies, both
associated with eastward propagating waves. Hence, the MCSs initiated at
the plains can produce more rainfall than those initiated at the
foothill due to more abundant moisture supply. By tracking MCSs and
mid-tropospheric perturbations (MPs), a unique type of sub-synoptic
disturbances with Rocky Mountains origin, it is shown that
~30% of MPs is associated with MCS initiation, mostly
in Type-4. Although MPs are related to a small fraction of MCS
initiation, MCSs that are associated with MPs tend to produce more
rainfall in a larger area with a stronger convective intensity.