Deep convection in the tropics and midlatitudes is often organized
into mesoscale convective systems or MCSs. Over the past thirty-five
years one prominent mode of organization of MCSs has been identified
and studied extensively, the leading-line/trailing-stratiform (TS)
MCS. Generally referred to as a squall line, the TS system owes its
precipitation structure to a gust front triggering along a leading
line of deep convective cells and storm-relative front-to-rear flow
aloft transporting ice and snow rearward to form the trailing
stratiform precipitation system.
A recent study of midlatitude MCSs has shown that in addition to TS
systems, two other modes of organization are prevalent:
leading-stratiform (LS) and parallel-stratiform (PS) stratiform
precipitation systems (Parker and Johnson 2000). In a study of
nearly 100 warm-sector MCSs, Parker and Johnson found 60% to be of
the TS type; however, the LS and PS modes were not negligible,
accounting for 20% each. Furthermore, the LS and PS modes are
typically slower-moving than TS systems, thereby implicating them
with heavy rainfall and flash floods. A recent study of United
States flash-flood events has found two dominant modes of mesoscale
organization to such storms that bear some relationship to the LS and
PS systems (Schumacher and Johnson 2004), one with a line of training
cells parallel to a generally east-west, quasistationary frontal zone
with stratiform precipitation displaced to the north, and another
with a PS-type structure characterized by back-building cells along
an outflow boundary to the west and stratiform precipitation to the
east.
The way in which convection responds to shear has been recently
investigated using data from the 1998 South China Sea (SCS) Monsoon
Experiment (SCSMEX). Analysis of BMRC (Bureau of Meteorology
Research Centre) radar data from Dongsha Island reveals a wide range
of organizational modes of convection over the northern SCS.
Proximity sounding data indicate that lower and middle level vertical
wind shears exerted a dominant control over the orientation of
convective lines within mesoscale convective systems in this region,
as has been found in the Australian monsoon region and the equatorial
western Pacific. The results are consistent with the conceptual
model of LeMone et al. (1998) based on the Tropical Ocean Global
Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE),
except two new organizational modes have been identified:
shear-parallel bands for strong low-level shear and weak midlevel
shear when there is weak instability and the air is dry aloft, and
shear-parallel bands for strong shears in both layers when the shear
vectors are in the same direction. Midlatitude influences, namely,
the passage of troughs over southern China, likely contributed to
these two additional modes.
Submittal Information
Name :
Date :
Richard H. Johnson
03-Jul-04-01:59:09
Organization :
Theme :
Colorado State University
Theme 2
Address :
Presentation :
Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523