Organization of Mesoscale Convective Systems
Richard H. Johnson
Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523-1371 USA;e-mail: johnson@atmos.colostate.edu
Abstract
A recent study of linear, midlatitude mesoscale convective systems (MCSs) over the eastern two-thirds of the UnitedStates has revealed three dominant modes of MCS organization: trailing stratiform (TS), leading stratiform (LS), andparallel stratiform (PS) systems. While TS are the most common, LS and PS modes are not negligible and in some cases
contribute to flash flooding. A follow-on study of extreme-rain-producing storms over the same region indicates two
dominant modes of MCS organization of such storms, one with training convective cells and the other with back-building
cells. Study of MCSs during the 1998 GAME/SCSMEX reveals that the shear in the lower and middle troposphere plays
a dominant role in determining the orientation of convective lines over the northern South China Sea.
Keywords: mesoscale, convection, MCS, organization, heavy rainfall

1. Introduction
Convection often takes the form of mesoscale cloud systems comprised of individual convective elements at various stages of their life cycles. The cloud elements are often
organized into bands, arcs or other contiguous structures.
Such systems are often referred to as mesoscale convective
systems or MCSs. In this review, we will explore the organization of MCSs, identify specific patterns that are conducive to heavy rainfall and flash floods, and examine thefactors that determine the orientation of convective lines
during the May-June 1998 South China Sea Monsoon Experiment (SCSMEX).

2. Modes of organization of MCSs
A recent survey has been conducted of the organizationof MCSs over the eastern two-thirds of the United States using WSR-88D radar data (Parker and Johnson 2000). The
focus has been on linear MCSs, defined as MCSs that contain a convective line with a contiguous or nearly contiguous chain of convective echoes sharing a nearly common
leading edge and moving approximately in tandem. Nearly
all of the 88 cases studied for May and June, 1996 and 1997,
were classifiable into three basic modes of organization.
These classifications are based on the dominant mode observed throughout the MCS life cycle. The three modes,
shown in Fig. 1, are convective lines with trailing (TS),leading (LS), and parallel (PS) stratiform rain. The wellknown TS classification (e.g., Houze et al. 1990) accounted
for 58% of the cases. However, a new finding is the existence of an important fraction of LS and PS cases, each accounting for 19% of the total population. The existence of
the LS mode of organization at midlatitudes has been previously documented, but generally has not received muchattention. Both the LS and PS modes move more slowly
and are shorter-lived than their TS counterparts, and theirenvironmental flow structures display important differencesfrom those of each other and of TS storms. Wang (2004)
found that LS systems frequently occurred during the onsetof the summer monsoon over the northern South China Sea
during SCSMEX.
These differences are illustrated in Fig. 2, a depictionof storm-relative wind profiles for LS, PS, and TS systems
based on soundings taken in advance of the storms. A major distinction between the LS and TS systems is in the flow
aloft, where westerly storm-relative flow occurs in the LS

Fig. 1: Schema of idealized life cycles for three linear MCS
archetypes: (a) leading line trailing stratiform (TS), (b) convective line with leading stratiform (LS), and (c) convective
line with parallel stratiform (PS). Approximate time intervals between phases: for TS 3-4 h; for LS 2-3 h; for PS 2-3
h. Levels of shading roughly correspond to 20, 40, and 50
dBZ.. From Parker and Johnson (2000).

systems. This flow helps to advect hydrometeors ahead of
the convective line. PS systems are distinct from the othertwo types in that there is strong line-parallel, storm-relative
flow aloft. This flow explains the preponderance of stratiform precipitation at the northern end of these systems.
In all three cases there is strong low-level and weak midlevel line-perpendicular shear, consistent with the observed
shear-perpendicular convective line development (LeMone
et al. 1998). Presumably, the interaction between the cold
pool and the shear plays a role in organizing the convective
line (Rotunno et al. 1988).
The easterly low-level, storm-relative flow for the LS
cases is based on soundings ahead of the storms. However,there is evidence from more recent studies that in many LS
systems, there is a “rear-to-front” low-level, storm-relative
flow. These “rear-fed” LS cases often exhibit repeated
growth of new convective cells to the west of old ones, orback-building, which can lead to locally heavy rainfall inmany instances. An example of the airflow in such a system
is shown in Fig. 3 (Pettet and Johnson 2003). This figure isbased on a study of two rear-fed LS MCSs using the WSR88D Doppler radar network. In many respects, the radar re-

LS

3.7

4.2

2.4

0.9

2.1

-1.8

5 km

Leading
stratiform

4.5

PS

10 km

-8.8

8.7

3.3

8.0

-0.3

2.8

-1.9

sfc

(2004) explained this transition as a result of the LS systems(1) decreasing the line-perpendicular wind shear nearby as
they developed, and (2) gradually increasing the system’s
cold pool strength, leading to more rearward-sloping updrafts. PS systems also often transitioned to TS during theirlater stages, also likely in response to increasing cold-poolstrength with time.

10 km

5 km

Parallelstratiform

1.2

TS

-13.7

sfc

3.8

-1.3

3.3

-4.9

1.2

-7.4

-1.9

-12.1

10 km

5 km

Trailing
stratiform

sfc

Fig. 2: Vertical profiles of layer-mean storm-relative winds
for linear MCS classes. Wind vectors depicted as lineparallel and line-perpendicular components in m s−1 . Layers depicted are 0–1 km, 2–4 km, 5–8 km, and 9–10 km.
From Parker and Johnson (2000).

flectivity and airflow structure of these systems (Fig. 3) are
mirror images of TS systems, including a secondary precipitation maximum (the stratiform region) separated from theconvective line by a reflectivity minimum and a midlevel inflow jet (from the front in the case of an LS system and fromthe rear in a TS system). However, unlike many TS systems,
the low-level inflow in the LS systems was elevated, with anassociated elevated θe maximum.

Fig. 4: Illustration of evolutionary pathways for MCSs. Labels along each pathway denote the initial and final modesof stratiform precipitation production. The total numberof cases following each step is indicated. Idealized composite positions of convective elements and stratiform precipitation are depicted schematically along each pathway.
Note: some pairs of evolutionary pathways (e.g., TS→PS
and PS→TS) resulted in generally similar reflectivity patterns. From Parker and Johnson (2000).

3. Extreme-rainfall-producing MCSs

Fig. 3: Conceptual model of a rear-fed LS MCS above a stable surface layer viewed in a vertical cross section oriented
perpendicular to the convective line (i.e., parallel to its motion from left to right) during the mature stage of its life
cycle. Arrows indicate mean, storm-relative flow. Radar
reflectivity is indicated by thin contours. Cloud outline isthicker contour. Areas of enhanced reflectivity are shaded.
Surface high and low pressure centers are indicated by H
and L, respectively. From Pettet and Johnson (2003).

An important point to be made concerning TS, LS, andPS systems is that their classification was based on the dominant mode of organization over their life cycles. However,these storms frequently transitioned from one mode to another during their lifetimes, as shown in Fig. 4. TS systems
typically remained TS throughout their lifetimes, but transitioned from symmetric to asymmetric in structure during
the later stages of their life cycles (Hilgendorf and Johnson
1998). The majority of LS systems remained LS, but a number transtitioned to TS at later stages. Parker and Johnson

A study has recently been completed of 116 heavyrainfall cases over the eastern two-thirds of the UnitedStates to determine the dominant modes of MCS organization associated with extreme-rain-producing events (Schumacher and Johnson 2004). Storms were studied over a period of three years, from 1999 to 2001, including all seasons. Sixty-five percent of the total number of events wereassociated with MCSs. Of those, two prominant modes oforganization have been found, illustrated in Fig. 5. The
first type (Fig. 5a), comprising 32% of the MCSs, is characterized by a typically east-west convective line along aquasi-stationary frontal boundary with west-to-east training convective cells on the cool side of the boundary andan area of stratiform precipitation displaced to the north.
The second type (Fig. 5b), comprising 20% of the cases, istypically smaller in overall size and features back-building
cells along an outflow boundary, with repeated cell formation over the same location and an area of stratiform precipitation typically downstream. It is believed that these twopatterns of organization are also present in extreme-rainproducing MCSs in other parts of the world, including thesmall low-level shear

STRATIFORM MOTION

small mid-level shearA) TRAINING LINE -- ADJOINING STRATIFORM (TL/AS)

1large mid-level shear

Asian monsoon region.

3

large low-level shear2

2r (dry aloft)
L
L

MID-LEVEL SHEAR

CELL MOTIONSTATIONARY FRONT OR OTHER BOUNDARY

c

4
M

M
L
M

L

a

b

M

L

LOW-LEVEL SHEARB) BACKBUILDING / QUASI-STATIONARY (BB)NEW CELLS FORM HERE
OUTFLOW BOUNDARYCELL MOTIONPROPAGATION~ 150 KM

Fig. 5: Two extreme-rain-producing storm mature-stage
archetypes. (a) Training-line, adjoining-stratiform (TL/AS)
organization, and (b) Back-building (BB) organization,
based on a study of 116 such events in the eastern twothirds of the United States for the period 1999-2001. Distance scale applies to both systems. From Schumacher andJohnson (2004).

4. Organization of monsoon convection during
GAME/SCSMEX
It has long been known that the organization of tropicalconvection is influenced predominantly by the vertical shear
and convective available potential energy or CAPE (Moncrieff and Green 1972). Various observational studies inthe eastern Atlantic and northern Australia have confirmed
the strong influence of environmental winds on the structure, orientation, and propagation of convective bands (e.g.,
Barnes and Seickman 1984; Alexander and Young 1992;
Keenan and Carbone 1992). Recently, LeMone et al. (1998)
investigated the organization of convection over the western Pacific warm pool using aircraft data from the Tropical Ocean Global Atmosphere Coupled Ocean-AtmosphereResponse Experiment (TOGA COARE). In agreement with
Alexander and Young (1992) they find that vertical shear
in the low-to-midtroposphere is a key factor in determining
the orientation of convective bands, while CAPE influences
their depth and longevity. Their results have been recently
supported by numerical simulations of convection in shear
by Robe and Emanuel (2001). We have recently tested theseconcepts using BMRC C-POL radar data from SCSMEX,
and find that, in general, they can be extended to the Asianmonsoon region, with some modifications (Johnson et al.
2004).
Four primary modes of organization have emerged fromthe results of this study, which are summarized in Fig. 6.

Fig. 6: Schematic depiction from LeMone et al. (1998)
of four main categories of convective structures for given
vertical shears in the lower troposphere (1000–800 hPa)
and at middle levels (800–400 hPa) based on COARE observations, but modified to include results from SCSMEX(modes 2r and 4c added). Length of schematic convective bands is ∼100–300 km; line segments in upper-leftframe are up to 50 km length. Cutoff between “strong” and“weak” shear for lower layer (1000–800 hPa) is 4 m s −1
and for middle layer (800–400 hPa) is 5 m s−1 . Arrows
marked L and M are shear vectors for lower and middle
layers, respectively. See text for descripition of convectivemodes.

This figure, adapted from LeMone et al. (1998), is a summary of their findings from TOGA COARE, but supplemented by new results from SCSMEX (two new modes
2r, and 4c). In general, the organizational modes for SCSMEX were consistent with those determined by LeMone
et al. (1998) for the western Pacific warm pool. They foundthat when the shear in the lowest 200 hPa exceeded 4 m
s−1 and the shear from 800 to 400 hPa was less than 5 m
s−1 , the orientation of primary convective band was perpendicular to the low-level shear (Type 2 in Fig. 6). Secondary lines parallel to the low-level shear were found insome cases ahead of the primary band. In the absence ofstrong low-level shear, lines formed parallel to the 800-400hPa shear when its magnitude exceeded 5 m s−1 (3 in lowerleft frame). When the vertical shear exceeded the thresholds
in both layers and the shear vectors were not in the same direction (lower-right frame), the primary band was normal
to the low-level shear (4a or 4b). Trailing secondary bands
parallel to the midlevel shear occurred if the midlevel shear
was opposite the low-level shear (4b). When the shear inboth layers was weak, convection developed in arcs along
outflow boundaries (1). Two additional modes of convection have been identified from analysis of SCSMEX C-POL
radar data (Fig. 6): shear-parallel bands (2r) for strong lowlevel shear and weak midlevel shear when the air is dry
aloft, and shear-parallel bands (4c) for strong shears in bothlayers when the shear vectors are in the same direction.
Midlatitude influences likely contributed to these two additional modes by producing strong westerlies (in the caseof 4c) during the passage of a strong upper-level trough andmidtropospheric drying (in the case of 2r) following pas-sage of the trough. Mechanism(s) for 4c cases are uncertain,
but they may be associated with confluence lines connectedwith the upper-level troughs. In the 2r cases, the lines areprobably a manifestation of horizontal convective rolls inthe boundary layer (hence the use of r). In these cases it
is so dry aloft and the instability so weak that the convection cannot penetrate to higher levels and produce strong
downdrafts and cold pools, which would otherwise serve toreorient the lines into a shear-perpendicular direction.

4. Summary and Discussion
Recent studies of midlatitude and monsoon convective
systems (MCSs) have provided new insight regarding MCS
organization, evolution, and structural properties. Investigations using operational radar data over the United States
have revealed three dominant modes of organization: trailing stratiform (TS), leading stratiform (LS), and parallel
stratiform (PS) systems. While TS are the most common,
LS and PS modes are not negligible and, in some cases,
are associated with flash floods. Recent study of extremerain-producing storms over the eastern two-thirds of theUnited States indicates two dominant modes of MCS organization of such storms, one with training convective
cells and the other with back-building cells. Study of 1998
GAME/SCSMEX MCSs reveals that the shear in the lower
and middle troposphere plays a dominant role in determining the orientation of convective lines over the northern
South China Sea. Results are similar to findings for thetropics, but two new modes of organization are highlighted,
which are argued to be related to midlatitude influences.
ACKNOWLEDGMENTS This research has been supported by
the National Science Foundation under Grants ATM-9618684 andATM-0071371, and by the National Aeronautics and Space Administration under Grant NNG04GA22G. The following individuals are thanked for their help in this research: Matt Parker, Russ
Schumacher, Crystal Pettet, Paul Ciesielski, Tom Keenan, SteveAves, and Rick Taft.

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