330 8 Thunderstorms
Two types of long-lived line can
occur-a
line of supercell storms or a line of
ordinary multicell storms. The term ordinary here refers to the fact that multicell
storms are far more common than supercells and, not surprisingly, lines of multi-
cell storms are the typical case, while lines of supercells are relatively rare.
Nonetheless, a line of supercell storms remains of great interest, and it is easier to
explain. The model calculations show that a line of supercells can exist in an
environment of deep, strong shear (required to produce the highly rotational
supercells), where the shear vector forms a 45° angle to the orientation of the line
(Fig. 8.55). In this configuration, the storms are arranged such that their respec-
tive circulations do not interfere.s"
The more challenging problem is to account for the long-lived line of multicell
storms. Here the low-level wind shear ahead of the line appears to be the main
factor. A heuristic
argument-S is based on the two-dimensional horizontal vortic-
ity equation (2.61). Two situations are depicted in Fig. 8.56a and b. In the no-shear
case (Fig. 8.56a), a buoyant parcel rising over the gust front must follow a rear-
ward sloping path. If the buoyancy is maximum in the center of the parcel,
counter-rotating vortices on either side of the center of the parcel would develop
and the core of the parcel would rise vertically. However, the buoyancy gradient
across the gust front constitutes an additional contribution to the buoyancy gradi-
ent at low levels. Hence the vertical rise associated with the buoyancy of the
parcel is superimposed on negative vorticity generation associated with the gust
front. Thus, the flow in the vicinity
ofthe
gust front is sloped, reflecting the overall
dominance of negative vorticity. In the case of low-level shear ahead of the gust
front (Fig. 8.56b), the initial vorticity of the inflow air (i.e., the ambient vorticity
associated with the shear) is positive.
It
is then possible that the negative baro-
clinic generation by the gust front
just
neutralizes the initial positive vorticity, so
that as the parcel rises above the boundary layer it has no predisposition toward
either negative or positive vorticity, and it thus rises vertically.
There are two controversial points regarding this heuristic argument. First, it
has been pointed out
243
that in the case where the line is part of a mesoscale
convective system, the circulation of the mesoscale system provides additional
sources of vorticity for the region of the gust front. This point will be discussed
further in Chapter 9 (Sec. 9.2.2.8).
The second controversial point is concerned with the later stages of the lifetime
of the convective line. The structure shown in Fig. 8.56b (i.e., vertically erect
convection) is essential for a long-lived line to become established. Strong low-
level shear is unquestionably important to resisting the gust front, and in the early
stage of the line, parcels rise vertically as in Fig. 8.56b. Later, as the cold-pool
gust front strengthens, the line structure evolves toward a structure like that in
Fig. 8.56c, with parcels moving along sloping rearward trajectories, which are the
result of the cold pool becoming stronger, the circulation across the gust front
24\ This form of line organization was suggested by Lilly (1979).
242 Offered by Rotunno et af. (1988).
243 By Lafore and Moncrieff (1989).